LG Refrigerator

Here I post LG Fridge Price list of the month July 2015 , August 2015 , September 2015, October 2015

Smart card, Chip card, Integrated circuit card

Applications and Working Methods Smart card, Credit card, Debit card, SIM card, ATM card, Magnetic stripe card

A smart card, chip card, or integrated circuit card (ICC), is any pocket-sized card with embedded integrated circuits. A smart card or microprocessor cards contain volatile memory and microprocessor components. The card is made of plastic, generally polyvinyl chloride, but sometimes acrylonitrile butadiene styrene or polycarbonate . Smart cards may also provide strong security authentication for single sign-on (SSO) within large organizations.

Applications

Computer security

The Mozilla Firefox web browser can use smart cards to store certificates for use in secure web browsing.
Some disk encryption systems, such as FreeOTFE, TrueCrypt and Microsoft Windows 7 BitLocker, can use smart cards to securely hold encryption keys, and also to add another layer of encryption to critical parts of the secured disk.
Smart cards are also used for single sign-on to log on to computers.
Smart card support functionality has been added to Windows Live passports.

Credit cards

Main articles: Contactless smart card and Credit card

These are the best known payment cards (classic plastic card):

• Visa: Visa Contactless, Quick VSDC—"qVSDC", Visa Wave, MSD, payWave
• MasterCard: PayPass Magstripe, PayPass MChip
• American Express: ExpressPay
• Discover: Zip

Roll-outs started in 2005 in USA. Asia and Europe followed in 2006. Contactless (non PIN) transactions cover a payment range of ~$5–50. There is an ISO/IEC 14443 PayPass implementation. Some, but not all PayPass implementations conform to EMV.
Non-EMV cards work like magnetic stripe cards. This is a typical USA card technology (PayPass Magstripe and VISA MSD). The cards do not hold/maintain the account balance. All payment passes without a PIN, usually in off-line mode. The security of such a transaction is no greater than with a magnetic stripe card transaction.
EMV cards have contact and contactless interfaces. They work as a normal EMV card via contact interface. Via contactless interface they work somewhat differently in that the card command sequence adopts contactless features such as low power and short transaction time.

Cryptographic smart cards

Cryptographic smart cards are often used for single sign-on. Most advanced smart cards include specialized cryptographic hardware that uses algorithms such as RSA and DSA. Today's cryptographic smart cards generate key pairs on board, to avoid the risk from having more than one copy of the key (since by design there usually isn't a way to extract private keys from a smart card). Such smart cards are mainly used for digital signature and secure identification, (see applications section).
The most common way to access cryptographic smart card functions on a computer is to use a vendor-provided PKCS#11 library. On Microsoft Windows the CSP API is also supported.
The most widely used cryptographic algorithms in smart cards (excluding the GSM so-called "crypto algorithm") are Triple DES and RSA. The key set is usually loaded (DES) or generated (RSA) on the card at the personalization stage.
Some of these smart cards are also made to support the NIST standard for Personal Identity Verification, FIPS 201.

Financial

Smart cards serve as credit or ATM cards, fuel cards, mobile phone SIMs, authorization cards for pay television, household utility pre-payment cards, high-security identification and access-control cards, and public transport and public phone payment cards.
Smart cards may also be used as electronic wallets. The smart card chip can be "loaded" with funds to pay parking meters and vending machines or at various merchants. Cryptographic protocols protect the exchange of money between the smart card and the accepting machine. No connection to the issuing bank is necessary, so the holder of the card can use it even if not the owner. Examples are Proton, Geldkarte, Chipknip and Mon€o. The German Geldkarte is also used to validate customer age at vending machines for cigarettes.

Sim Card

Health care (medical)

Smart health cards can improve the security and privacy of patient information, provide a secure carrier for portable medical records, reduce health care fraud, support new processes for portable medical records, provide secure access to emergency medical information, enable compliance with government initiatives and mandates, and provide the platform to implement other applications as needed by the health care organization.

Identification

A quickly growing application is in digital identification. In this application, the cards authenticate identity. The most common example employs Public key infrastructure (PKI). The card stores an encrypted digital certificate issued from the PKI provider along with other relevant information. Examples include the U.S. Department of Defense (DoD) Common Access Card (CAC), and various identification cards used by many governments for their citizens. Combined with biometrics, cards can provide two- or three-factor authentication. Smart cards are not always privacy-enhancing, because the subject carries possibly incriminating information on the card. Contactless smart cards that can be read from within a wallet or even a garment simplify authentication.
The first smart card driver's license system in the world was implemented in 1987 in Turkey. Turkey had a high level of road accidents and decided to develop and use digital tachograph devices on heavy vehicles, instead of the existing mechanical ones, to reduce speed violations. Since 1987, the professional driver's licenses in Turkey are issued as smart cards and the driver is required to insert his driver's license into the digital tachograph before starting to drive. The tachograph unit records speed violations for each driver and gives a printed report. The driving hours for each driver is also being monitored and reported. In 1990 the European Union conducted a feasibility study through BEVAC Consulting Engineers, titled "Feasibility study with respect to a European electronic drivers licence (based on a smart-card) on behalf of Directorate General VII". In this study, chapter seven is dedicated to the experience in Turkey, stating that the electronic driver's license application, in the form of smart cards, was first implemented in Turkey in 1987.
A smart card driver's license system was later issued in 1995 in Mendoza province of Argentina. Mendoza had a high level of road accidents, driving offenses, and a poor record of recovering outstanding fines. Smart licenses hold up-to-date records of driving offenses and unpaid fines. They also store personal information, license type and number, and a photograph. Emergency medical information such as blood type, allergies, and biometrics (fingerprints) can be stored on the chip if the card holder wishes. The Argentina government anticipates that this system will help to collect more than $10 million per year in fines.
In 1999 Gujarat was the first Indian state to introduce a smart card license system. To date  it has issued 5 million smart card driving licenses to its people.

“

a national ID card, protected by a 1,024-bit key code, is impossible to break without a supercomputer working away for a hundred years

”

In 2002, the Estonian government started to issue smart cards named ID Kaart as primary identification for citizens to replace the usual passport in domestic and EU use. As of 2010 about 1 million smart cards have been issued (total population is about 1.3 million) and they are widely used in internet banking, buying public transport tickets, authorization on various websites etc.
By the start of 2009 the entire population of Spain and Belgium will have an eID card that is used for identification. These cards contain two certificates: one for authentication and one for signature. This signature is legally enforceable. More and more services in these countries use eID for authorization.
Smart cards are also beginning to be used in emergency situations. In 2004, The Smart Card Alliance issued a statement expressing the need to "to enhance security, increase Government efficiency, reduce identity fraud, and protect personal privacy by establishing a mandatory, Government-wide standard for secure and reliable forms of identification". In light of this, emergency response personnel have now begun to carry these cards so that they can be positively identified in emergency situations. WidePoint Corporation, a smart card provider to FEMA, produces cards that contain additional personal information, such as medical records and skill sets. Cards like these provide immediate access to information, which allows first responders to bypass organizational paperwork and focus more time on the emergency resolution.

Schools

Smart cards are being provided to students at schools and colleges. Usage includes:

• Tracking student attendance
• As an electronic purse, to pay for items at canteens, vending machines etc
• Tracking and monitoring food choices at the canteen, to help the student maintain a healthy diet
• Tracking loans from the school library

Public transit

Smart cards and integrated ticketing have become widely used by public transit operators around the world. Card users may use their cards for other purposes than for transit, such as small purchases. Some operators offer points for usage, exchanged at retailers or for other benefits. Example include the Octopus Card used in Hong Kong, London's Oyster Card, and San Francisco's Clipper card. However, they have been criticized for presenting a privacy risk because it can allow the mass transit operator (and the government) to track an individual's movement. In Finland, for example, the Data Protection Ombudsman prohibited the transport operator Helsinki Metropolitan Area Council (YTV) from collecting such information, despite YTV's argument that the card owner has the right to a list of trips paid with the card. Earlier, such information was used in the investigation of the Myyrmanni bombing.^{[citation needed]}

Concessionary travel

A highly successful use for smart cards within the UK is in concessionary travel schemes. Mandated by the Department for Transport, travel entitlements for elderly and disabled residents are administered by local authorities and passenger transport executives. Smart cards have been issued as bus passes to qualifying residents; however these smart cards can instead now be used by elderly and disabled people who qualify for concessionary taxi travel. These schemes are part of an additional service offered by some local authorities as an alternative for residents unable to make use of their bus pass. One example is the "Smartcare go" scheme provided by Ecebs.

Other

Smart cards are widely used to protect digital television streams. VideoGuard is a specific example of how smart card security worked (and was cracked).
The Malaysian government uses smart identity cards carried by all citizens and resident non-citizens. The personal information inside the MYKAD card can be read using special APDU commands.
Since April 2009, Toppan Printing Company (Toppan insatsu^?) has manufactured reusable smart cards for money transfer and made from paper instead of plastic.

Voltage stabilizer

  What is Voltage stabilizer?

Stabilizer is the Automatic Voltage regulator. It regulates the output power. It produces the output 230 V ± 1% (Variable 5%) OR 415 ± 1% (Variable 5%) OR As per customer demand for your appliance .

  What is the function of voltage stabilizer?

Voltage stabilizers are an effective solution to voltage fluctuation problems. They are designed to current a wide range of Input Fluctuations to maintain specified output voltage. The output voltage waveform is completely distortion free and the regulation is unaffected by the load power factor.

  How can I protect the appliance from high voltage risk?

Voltage stabilizer should have the high voltag protection. If the Input is going beyond the limit and Output is going beyond the limit in worst cases The stabilizer should sense Input and Output condition and cutoff the output supply of the stabilizer. So In this way stabilizer protects the connected appliances.

  How will I select stabilizer rating?

Before selection the stabilizer you need to know how much power you use. By taking an inventory of all the essential electrical loads and doing a basic electrical load evaluation, you can get a good idea how much power your system needs to produce. Second, you have to know about the power Fluctuations situations also that mean what voltage minimum / maximum you are getting from the main A.C supply. In brief, you have to select the Input Voltage window and the power consumption of your appliance.

Digital Basics

What are the digital basics?

Digital circuits work on the basis of a transistor being used as a switch. Consider a light switch, a transistor can be considered almost the same and in some circuits transistors are used to control large amounts of power with very little input power being used.
Look at figure 1 below. Here are two crude transistor switch circuits. In the first circuit if there is no voltage applied to the base of Q1 then it is not switched "on" and accordingly the + 5V passing through the 10K load resistor from our + 5V supply appears at both the collector of the transistor and also at output 1.
If we apply + 5V to the base of Q1 then because it is greater than 0.7 V than the grounded emitter, see the topic "transistors" for much greater detail on that operation, Q1 will switch on just like a light switch causing the + 5V from our supply to drop entirely across the 10K load resistor. This load could also be replaced by a small light bulb, relay or LED in conjunction with a resistor of suitable value. In any event the bulb or led would light or the relay would close.


Figure 1. - transistors as digital switches in digital basics The basic principle in digital basics is that we have just created an "electronic switch" where the positive voltage on the base produces zero voltage at the output and zero voltage on the input produces the + 5V on the output.
The output is always the opposite to the input and in digital basics terms this is called an "inverter" a very important property. Now looking at Q2 and Q3 to the right of the schematic we simply have two inverters chained one after the other. Here if you think it through the final output 2 from Q3 will always follow the input given to Q2. This in digital basics is your basic transistor switch.

Logic Blocks in Digital Basics

Depending upon how these "switches" and "inverters" are arranged in integrated circuits we are able to obtain "logic blocks" to perform various tasks. In figure 2 we look at some of the most basic logic blocks.


Figure 2. - digital switches in digital basics In the first set of switches A, B, and C they are arranged in "series" so that for the input to reach the output all the switches must be closed. This may be considered an "AND-GATE".
In the second set of switches A, B, and C they are arranged in "parallel" so that for any input to reach the output any one of the switches may be closed. This may be considered an "OR-GATE".
These are considered the basic building blocks in digital logic. If we added "inverters" to either of those blocks, called "gates", then we achieve a "NAND-GATE" and a "NOR-GATE" respectively.
Here in figure 3 we examine the digital basics in schematic form.


Figure 3. - digital basics in schematic form Now here we have depicted four major logic blocks AND-GATE, NAND-GATE, OR-GATE and NOR-GATE plus the inverter. Firstly the "1's" and the "0's" or otherwise known as the "ones" and "zeros". A "1" is a HIGH voltage (usually the voltage supply) and the "0" is no voltage or ground potential. Other people prefer designating "H" and "L" for high and low instead of the "1's" and the "0's". Stick with which system you feel most comfortable.
Several interesting points emerge here. Of interest to the next section on binary numbers is the pattern of all the inputs for each logic block. Not only are they identical but, for only two inputs A and B there are four possible output situations which are called "states". These are digital basics. There actually can be many numbers of inputs. An eight input NAND-GATE is a common and quite useful digital logic block.
Next of particular interest is if you study them very carefully, that for the very identical inputs, each of these logic blocks gives us a totally different output result. Compare them.
Finally for the same inputs the NOR-GATE outputs are the direct opposite to the AND-GATE outputs while the OR-GATE outputs are the direct opposite to the NAND-GATE outputs.

Binary Numbers and Hex-Decimal in Digital Basics

If you have a single switch or input you can have two possible input states, it is either on or off. With two switches or inputs you have four possible input states as shown above. If you go to three inputs you have eight possible states and four inputs give you sixteen states. Again digital basics.
By adding another input you double the previous number of states. Doubling the inputs gives you the square of the states.
We say four inputs gives sixteen states so doubling that gives us eight inputs so the number of states should be 16 X 16 or 256.
Consider this. If I offered you a job and I made you two alternative offers for monthly payment - Offer No. 1 is to pay you a most generous $10,000.00 for the month. Offer No. 2 is to pay you one cent for the first day you work for me, two cents the next day and doubling each day thereafter for the whole 30 day month. Which offer would you accept? Answer at the very bottom of this page.

Binary Coded Decimal To the right we have provided a table of BCD data which is all based upon the old "1's" and "0's". If at first it looks a bit intimidating don't worry you will very quickly get the hang of it. Notice first of all we have in the extreme right hand column the numbers 0 - 9 and the letters A to F. The first four columns are headed 8 - 4 -2 - 1 We explained earlier by adding switches you double the previous capacity for numbering in binary. Notice the pattern of our 0's and 1's. Under the column 1 we get a succession of 0, 1, 0, 1.....  Under the column 2 we get a succession of 0, 0, 1, 1..... etc. In fact under every column heading you have exactly an equal number of zeros first  followed by the same number of ones. Look at column 8 for example. Eight zeros followed by eight ones. Now look at the far right column and look up number seven, follow that row reading across right to left and you will see the sequence 0 - 1 - 1 - 1. Okay if a one means a turned on switch with the value of that column what does 4 + 2 + 1 =?

Binary Coded Decimal - BCD 8 4 2 1   0 0 0 0 0 0 0 0 1 1 0 0 1 0 2 0 0 1 1 3 0 1 0 0 4 0 1 0 1 5 0 1 1 0 6 0 1 1 1 7 1 0 0 0 8 1 0 0 1 9 1 0 1 0 A 1 0 1 1 B 1 1 0 0 C 1 1 0 1 D 1 1 1 0 E 1 1 1 1 F

Of course the answer was seven. Try it with any number you like. Alright what's this A to F stuff? Look at a digit on a digital clock or watch for example. For those numbers to be represented in digital format requires four switches but now we will start using the correct terms. The word is "bits", heard that before? Now we're right into digital basics.
Four bits are called "a nibble" and guess what?, eight bits are called "a byte". Bet you've heard that one for sure unless you live under a rock.
You should know by now that four switches (OK bits right!) can represent sixteen states and with a digital clock you only go 0 to 9 and don't need anything else so that was called BCD or Binary Coded Decimal. The last word is because we humans count in decimal format or decades. Digital devices including computers DON'T, they can't. All they see are ones and zeros, nothing else.

 

Introduction to transistors?

Transistors, I was once told, "were the fastest acting fuse known to mankind". This of course was a reference to the fact an early transistor was intolerant of fault conditions whereas in years gone by, vacuum tubes (valves) would cop a lot of abuse. Just remember that fact. [one of "murphy's laws" - The component exists to protect the fuse]
Generally transistors fall into the category of bipolar transistor, either the more common NPN bipolar transistors or the less common PNP transistor types. There is a further type known as a FET transistor which is an inherently high input impedance transistor with behaviour somewhat comparable to valves. Modern field effect transistors or FET's including JFETS and MOSFETS now have some very rugged transistor devices. I am often asked about the term "bipolar" - see later.

History of Transistors

The transistor was developed at Bell Laboratories in 1948. Large scale commercial use didn't come until much later owing to slow development. Transistors used in most early entertainment equipment were the germanium types. When the silicon transistor was developed it took off dramatically. The first advantages of the transistor were relatively low power consumption at low voltage levels which made large scale production of portable entertainment devices feasible. Interestingly the growth of the battery industry has paralleled the growth of the transistor industry. In this context I include integrated circuits which of course are simply a collection of transistors grown on the one silicon substrate.

How do transistors work?

Transistors work on the principle that certain materials e.g. silicon, can after processing be made to perform as "solid state" devices. Any material is only conductive in proportion to the number of "free" electrons that are available. Silicon crystals for example have very few free electrons. However if "impurities" (different atomic structure - e.g. arsenic) are introduced in a controlled manner then the free electrons or conductivity is increased. By adding other impurities such as gallium, an electron deficiency or hole is created. As with free electrons, the holes also encourage conductivity and the material is called a semi-conductor. Semiconductor material which conducts by free electrons is called n-type material while material which conducts by virtue of electron deficiency is called p-type material.

How do holes and electrons conduct in transistors?

If we take a piece of the p-type material and connect it to a piece of n-type material and apply voltage as in figure 1 then current will flow. Electrons will be attracted across the junction of the p and n materials. Current flows by means of electrons going one way and holes going in the other direction. If the battery polarity were reversed then current flow would cease.

Figure 1. - electron flow in a p-n juction of a diode

Some very interesting points emerge here. As depicted in figure 1 above a junction of p and n types constitutes a rectifier diode. Indeed a transistor can be configured as a diode and often are in certain projects, especially to adjust for thermal variations. Another behaviour which is often a limitation and at other times an asset is the fact that with zero spacing between the p and n junctions we have a relatively high value capacitor.
This type of construction places an upper frequency limit at which the device will operate. This was a severe early limitation on transistors at radio frequencies. Modern techniques have of course overcome these limitations with some bipolar transistors having Ft's beyond 1 Ghz. The capacitance at the junction of a diode is often taken advantage of in the form of varactor diodes. See the tutorial on diodes for further details. The capacitance may be reduced by making the junction area of connection as small as possible. This is called a "point contact".
Now a transistor is merely a "sandwich" of these devices. A PNP transistor is depicted in figure 2 below.

Figure 2. - sandwich construction of a PNP transistor

Actually it would be two p-layers with a "thin" n-layer in between. What we have here are two p-n diodes back to back. If a positive voltage (as depicted) is applied to the emitter, current will flow through the p-n junction with "holes" moving to the right and "electrons moving to the left. Some "holes" moving into the n-layer will be neutralised by combining with the electrons. See electron theory and atoms. Some "holes" will also travel toward the right hand region.
The fact that there are two junctions leads to the term "bipolar transistor".
If a negative voltage (as depicted) is applied to the collector of the transistor, then ordinarily no current flows BUT there are now additional holes at the junction to travel toward point 2 and elctrons can travel to point 1, so that a current can flow, even though this section is biased to prevent conduction.
It can be shown that most of the current flows between points 1 and 2. In fact the amplitude (magnitude) of the collector current in a transistor is determined mainly by the emitter current which in turn is determined by current flowing into the base of the transistor. Consider the base to be a bit like a tap or faucet handle.

Transistor amplification

Because the collector current (where the voltage is relatively high) is pretty much the same as the emitter current and also controlled by the emitter current (where the voltage is usually much lower) it can be shown by ohms law

P = I ² X R

that amplification occurs. See small signal amplifiers.

The NPN transistor

We discussed a PNP transistor above. The only differences between PNP and NPN transistors are in manufacturing (i.e. location of the p-layers and n-layers) and of much importance in the biasing. The schematic symbols for PNP and NPN transistors, (the work horse is the NPN) are shown in figure 3 below. A silicon NPN transistor needs to be forward biased by about 0.65V for it to turn on.

Historical Footnote on Transistors - [added 1st May, 2000]

This is an interesting excerpt from a post by a friend to a list I subscribe to:
"The more I think about Tesla the more it brings to mind another bright guy that got off track and missed out. His name was Shockley. I worked for him in the early days of Silicon Valley".
"He had the technology and the people to put the 'silicon transistor' on the market, BUT he was obsessed with a thing he called the "four layer diode" to be used for telephone switching. That product finally went nowhere and the guys that left Shockley and started Fairchild Semiconductor were the guys that marketed the transistor in it's first commercial silicon form".
"And the four layer diode?, finally turned out to be the SCR, a good product but not the world beater that the silicon transistor was".
[end historical footnote]
Meanwhile back to our transistor tutorial and figure 3 depicting a schematic of a PNP transistor and an NPN transistor.

Figure 3. - schematic of PNP transistor and NPN transistor

Notice the only difference is the location and direction of the arrows in the emitter. This denotes direction of current flow in the emitter. Note: that is not a topic I will enter into discussion as I've seen too many discussions already - I have no opinion .
Also see small signal amplifiers.
Download PDF data sheet P2N2222A - plastic bipolar transistor 238K

FET's as transistors

In figure 4 below I have depicted the schematics of the two most popular types. A J-FET and a dual gate mosfet. Typical types might be MPF-102 for a J-FET and the old RCA 40673 for the dual gate.

Figure 4. - schematic of J-FET transistor and dual gate mosfet transistor

The FET of course is characterised by its extremely high input impedance. Some people claim the FET is a superior device to a bipolar transistor. I consider that to be a subjective opinion with the proviso that FET development has led to some amazing developments, particularly with power-fets.
I won't go into any length about how FETS operate except to point out the principal differences to NPN and PNP transistors. A bipolar transistor has moderate input impedance (depending on configuration) while some FETs can and do have input impedances measured in megohms. Bipolar transistors are essentially "current" amplifiers while FETS could be considered voltage amplifiers.

What are transformers?

The name transformers is derived from the fact that when two coils are placed in close inductive proximity to one another the lines of force from one cut across the the turns of the other inducing an ac current, energy is transformed from one winding to another and this is called transformer action.
There are a great variety of transformers for a variety of applications including power transformers, audio transformers and rf transformers among others. All work on the above principle.

Power transformers

As the name implies a power transformer is designed to usually translate voltage from one level to another. Another type called a current transformer will not be discussed here. The schematic of a transformer is depected in figure 1 below. Consider also the topics covered under power supplies where power transformers are used.

Figure 1. - general transformer schematic

Some power transformers have a centre tap on the secondary side. Note in figure 1 above the left hand side is usually denoted the "primary" whilst the right hand side is denoted the power transformers "secondary" side. Most power transformers are designed for frequencies in the region of 50 / 60 Hz which are the principle mains frquencies around the world.
Some examples of power transformers are shown in figure 2 below.

Figure 1. - photograph of different types of power transformers

The transformer on the upper left has "flying leads" for, in the case of all these transformers, the incoming voltage is the Australian standard 240V AC. The secondary side has three "lugs" to connect to and this is a 240V - 6.3V CT transformer.
The transformer on the upper right hand side is a multi-tap type with "flying leads" on both sides. The output allows you to select 6.3V, 9V, 12V and 15V depending upon your requirements, maximum current is 1A.
The transformer on the bottom left is called a "plug pack" in Australia. This one plugs directly into a power point and because a rectifier is included within the plug pack it produces 12DC @ 1A on its output. Note the four way connector shown at the centre, very bottom of the picture. This connector is designed to connect to the four sockets the manufacturer considers most popular.
The power transformer on the bottom right has a bit more "grunt" but only providing "lugs" for connecting leads. It is also a "multi-tap" type but designed to provide 2 amps.
Modern power transformers are wound on a "bobbin" which fits a core manufactured of materials to suit mains frequencies. The power handling capacity of a power transformer is determined by the physical size of the core and its properties. Design information is available from manufacturers. Ultimately the design information will provide the number of turns per volt. It is important to note that "toroid" power transformers are becoming increasingly popular and can handle larger amounts of power for the same physical dimensions and are thought by many "experts" to offer superior performance, particularly in higher power audio amplifiers.
The relationship of turns per volt holds good for both primary and secondary. A transformer designed for a nominal 250V AC input and a nominal 6.3V secondary output has a turns ratio of 250 / 6.3 or about 40:1
"Good design" usually leads to the cross-sectional "copper" areas of both the primary and the secondary being equal. Purely by way of illustration and not necessarily related to the real world, if the primary consisted of 2,400 turns of #34 gauge wire which is 0.16mm dia we would have a total cross sectional area of:
2,400 * [(0.16 ² * pi) / 4]. Where [(pi X D ²) / 4] is the customary formula to determine the area of a circular object.
Here you should get 2400 X 0.0201 = 48.24 mm ²
Therefore if our turns ratio was 20:1 for a 12V secondary, we would get 200 turns secondary still occupying approximately 48.24 mm ². With a little high school algebra we determine this gives us a secondary diameter of 0.55 mm diameter which is around about #24 gauge wire.
CAUTION: Of necessity I have greatly simplified the above to give a broad overview of how power transformers are designed. Always consult manufacturers such as Magnetics for tables and correct design information.
Remember "electricity KILLS!". Home construction of power transformers is a lost cause because the cost of component parts will always greatly exceed the cost of an off-the-shelf transformer.

Audio transformers

Essentially the main purpose of an interstage audio transformer is to isolate the DC and couple the signal, with minimal loss. The transformer windings look like short circuits to DC, yet are seen as complex impedances to the AC signal. Much which is contained on the topic of audio transformers is of necessity somewhat over simplified to give a general overview.
Go to: Audio Transformers

RF transformers

RF transformers generally fall into two categories, band pass filters and broad band transformers. Bandpass filters might well fall into the category of those used in IF amplifier filters while broad band transformers are generally used for impedance matching.

Figure 3. - schematic of an rf transformer

The type of broad band transformer depicted in figure 3 is often wound on a ferrite toroid of sufficient permeability to give a reactance of about 5 tomes the highest impedance at the lowest frequency of interest.

DIODES

What are Diodes?

Diodes are semiconductor devices which might be described as passing current in one direction only. The latter part of that statement applies equally to vacuum tube diodes. Diodes however are far more versatile devices than that. They are extremely versatile in fact. It might pay you to review the topic of Electron theory and atoms
Diodes can be used as voltage regulators, tuning devices in rf tuned circuits, frequency multiplying devices in rf circuits, mixing devices in rf circuits, switching applications or can be used to make logic decisions in digital circuits. There are also diodes which emit "light", of course these are known as light-emitting-diodes or LED's. As we say diodes are extremely versatile.

Schematic symbols for Diodes

A few schematic symbols for diodes are:

Figure 1 - schematic symbols for diodes

Types of Diodes

The first diode in figure 1 is a semiconductor diode which could be a small signal diode of the 1N914 type commonly used in switching applications, a rectifying diode of the 1N4004 (400V 1A) type or even one of the high power, high current stud mounting types. You will notice the straight bar end has the letter "k", this denotes the "cathode" while the "a" denotes anode. Current can only flow from anode to cathode and not in the reverse direction, hence the "arrow" appearance. This is one very important property of diodes.
The second of the diodes is a zener diode which are fairly popular for the voltage regulation of low current power supplies. Whilst it is possible to obtain high current zener diodes, most regulation today is done electronically with the use of dedicated integrated circuits and pass transistors.
The next of the diodes in the schematic is a varactor or tuning diode. Depicted here is actually two varactor diodes mounted back to back with the DC control voltage applied at the common junction of the cathodes. These cathodes have the double bar appearance of capacitors to indicate a varactor diode. When a DC control voltage is applied to the common junction of the cathodes, the capacitance exhibited by the diodes (all diodes and transistors exhibit some degree of capacitance) will vary in accordance with the applied voltage. A typical example of a varactor diode would be the Philips BB204G tuning diodes of which there are two enscapsulated in a TO-92 transistor package. At a reverse voltage Vr (cathode to anode) of 20V each diode has a capacitance of about 16 pF and at Vr of 3V this capacitance has altered to about 36 pF. Being low cost diodes, tuning diodes have virtually replaced air variable capacitors in radio applications today.
The next diode is the simplest form of vacuum tube or valve. It simply has the old cathode and anode. These terms were passed on to modern solid state devices. Vacuum tube diodes are mainly only of interest to restorers and tube enthusiasts.
The last diode depicted is of course a light emitting diode or LED. A led actually doesn't emit as much light as it first appears, a single LED has a plastic lens installed over it and this concentrates the amount of light. Seven LED's can be arranged in a bar fashion called a seven segment LED display and when decoded properly can display the numbers 0 - 9 as well as the letters A to F.

VOLTAGE

Voltage should be more correctly called "potential difference". It is actually the electron moving force in electricity (emf) and the potential difference is responsible for the pushing and pulling of electrons or electric current through a circuit.

Sources of electromotive force (EMF) or voltage

To produce a drift of electrons, or electric current, along a wire it is necessary that there be a difference in "pressure" or potential between the two ends of the wire. This potential difference can be produced by connecting a source of electrical potential to the ends of the wire.
As I will explain later, there is an excess of electrons at the negative terminal of a battery and a deficiency of electrons at the positive terminal, due to chemical action.
Then it can be seen that a potential difference is the result of the difference in the number of electrons between the terminals. The force or pressure due to a potential difference is termed e.m.f. or voltage.
See: electron theory
An emf also exists between two objects whenever there is a difference in the number of free electrons per unit volume of the object. If the two objects are both negative, current will flow from the more negatively charged to the less negatively charged when they are connected together. There will also be an electron flow from a less positively charged object to a more positively charged object.
The electrostatic field, i.e. the strain of the electrons trying to reach a positive charge or from a more highly negative charge is emf or voltage.
It is expressed in units called volts, short for voltage. A volt can be defined as the pressure required to force a current of one ampere through a resistance of one ohm.
To make this easier to visualise, consider the water pressure (voltage) required to pass a litre of water (current) through a copper pipe of a certain small diameter (resistance).
Also try and visualise water going through other pipes of varying diameters (smaller to larger in size). Either the water pressure required would vary or the volume delivered would vary, or both.
You have just grasped the basics of ohms law, where E = voltage; I = current in amperes and R = reistance in ohms:

This voltage can be generated in many different ways

Some examples:
Chemical (batteries) e.g. dry cell 1.5V, wet cell storage about 2.1V
Electromagnetic (generators)
Thermal (heating junctions of dis-similar metals)
Piezoelectric (mechanical vibration of certain crystals)
Photoelectric (light sensitive cells)

CURRENT

A flow of electrons forced into motion by voltage is known as current. The atoms in good conductors such as copper wire have one or more free electrons of the outer ring constantly flying off. Electrons from other nearby atoms fill in the holes. There are billions of electrons moving aimlessly in all directions, all the time in conductors.
When an emf (voltage) is impressed across a conductor it drives these free electrons away from the negative force toward the positive. This action takes place at near the speed of light, 300,000,000 metres per second although individual electrons do not move far they have a shunting effect. This is similar to a number of cars pulled up at traffic lights when the last vehicle fails to stop and hits the second last vehicle which in turn hits the third last vehicle...............
The amount of current in a circuit is measured in amperes (amps). Smaller units used in electronics are milli-amps mA (1 / 1,000th of an ampere) and micro-amps uA (1 / 1,000,000th of an ampere). An ampere is the number of electrons going past a certain point in one second.
The quantity of electrons used in determining an ampere is called "coulomb" which one ampere is one coulomb per second. A coulomb is 6,280,000,000,000,000,000 or 6.28 X 10 ¹⁸ electrons.
This (a coulomb) is the unit of measuring electrical quantity or charge.

Resistance

What is resistance?

In the topic current we learnt that certain materials such as copper have many free electrons. Other materials have fewer free electrons and substances such as glass, rubber, mica have practically no free electron movement therefore making good insulators. Between the extremes of good conductors such as silver, copper and good insulators such as glass and rubber lay other conductors of reduced conducting ability, they "resist" the flow of electrons hence the term resistance.
The specific resistance of a conductor is the number of ohms in a 1' (305mm) long 0.001" dia round wire of that material.
Some examples on that basis are Silver = 9.75 ohms, Copper = 10.55 ohms, Nickel = 53.0 ohms and Nichrome = 660 ohms
From this information we can deduce that for a voltage applied to a piece of Nichrome wire , only around 10.55 / 660 = 0.016 of the amount of current will flow as opposed to the the current flowing in the same size copper wire.
The unit of resistance is the ohm and 1 ohm is considered the resistance of round copper wire, 0.001" diameter, 0.88" (22.35 mm) long at 32 deg F (0 deg C).

Resistance in series and parallel

It follows if two such pieces of wire were connected end to end (in series) then the resistance would be doubled, on the other hand if they were placed side by side (in parallel) then the resistance would be halved!
This is a most important lesson about resistance. Resistors in series add together as R1 + R2 + R3 + ..... While resistors in parallel reduce by 1 / (1 / R1 + 1 / R2 + 1 / R3 + .....)
Consider three resistors of 10, 22, and 47 ohms respectively. Added in series we get 10 + 22 + 47 = 79 ohms. While in parallel we would get 1 / (1 / 10 + 1 / 22 + 1 / 47) = 5.997 ohms.

Resistance and Power

Next we need to consider the power handling capability of our resistors. Resistors which are deliberately designed to handle and radiate large amounts of power are electric cooktops, ovens, radiators, electric jugs and toasters. These are all made to take advantage of power handling capabilities of certain materials.
From our topic on ohms law we learnt that P = I * I * R that is, power equals the current squared times the resistance. Consider our example above of the three resistors in series providing a total resistance of 79 ohms. If these resistors were placed across a 24 volt power supply then the amount of current flowing, from ohms law, is I = E / R = 24 / 79 = 0.304 amperes.
Using any of our power formulas we determine that 0.304 amperes flowing through our 79 ohm resistance dissipates a combined 7.3 watts of power! Worse, because our resistors are of unequal value the power distribution will be unequal with the greater dissipation in the largest resistor.
It follows as a fundamental rule in using resistors in electronic circuits that the resistor must be able to comfortably handle the power it will dissipate. A rule of thumb is to use a wattage rating of at least twice the expected dissipation.
Common resistors in use in electronics today come in power ratings of 0.25W, 0.5W, 1W and 5W. Other special types are available to order. Because of precision manufacturing processes it is possible to obtain resistors in the lower wattage ratings which are quite close in tolerance of their designated values. Typical of this type are the .25W range which exhibit a tolerance of plus / minus 2% of the value.
Resistors come in a range of values but the two most common are the E12 and E24 series. The E12 series comes in twelve values for every decade. The E24 series comes in twenty four values per decade.
E12 series - 10, 12, 15, 18, 22, 27, 33, 39, 47, 56, 68, 82
E24 series - 10, 11, 12, 13, 15, 16, 18, 20, 22, 24, 27, 30, 33, 36, 39, 43, 47, 51, 56, 62, 68, 75, 82, 91
You will notice with the E12 values that each succeeding value falls within the plus / minus 10% of the previous values. This stems from the real old days when resistances were stated as within 20% tolerance (accuracy). Later values of plus / minus 5% tolerance led to the E24 range of resistance. Quite common today are 2% tolerance metal films types but for general purpose use we tend to stick to E12 values of resistance in either 1%, 2% or 5% tolerance.
Cost is the determining factor and many retailers now stock the 2% range of resistance as a standard to minimise stocking levels and also at reasonably low cost.
As examples of say the "22" types (red - red) from the E12 series we get 0.22, 2.2, 22, 220, 2,200, 22,000, 220,000 and 2,200,000 or eight decades of resistors.
In my opinion these ought to be referred to respectively as R22, 2R2, 22R, 220R, 2K2, 22K, 220K and 2M2. Here the R, K and M hold places where no decimal points are used to cause confusion.
Consider if I meant to write (in the old fashioned way) 2.2K in for a circuit value but forgot to type in the "K" so you just had 2.2, would the circuit work? No! How easy is it for you to read decimal points above.
Isn't 2K2 easier to see as meaning 2,200 ohms as against 2.2K? What if you didn't see the decimal point in 2.2K, couldn't it be taken to mean 22K or 22,000 ohms? Now you know why I prefer to use 2K2 or 22K or 22R - no confusion.

Resistance colour chart codes

Here in this large colour chart is the resistance colour code - learn the sequence forever -
BLACK, BROWN, RED, ORANGE, YELLOW, GREEN, BLUE, PURPLE, SILVER, WHITE
I have accommodated two current colour banding of resistances - four band and five band resistance colour code. It should be pretty self explanatory I hope.
The five band code is more likely to be associated with the more precision 1% and 2% types. Your "garden variety" 5% general purpose types will be four band resistance codes.

Atoms and Electrons

Everybody knows about atoms and electrons don't they? Well we could skip this part but of course we won't because you will likely learn something new.
Electron theory states all matter is comprised of molecules, which in turn are comprised of atoms, which are again comprised of protons, neutrons and electrons. A molecule is the smallest part of matter which can exist by itself and contains one or more atoms.
If you turn on a light switch for example you will see the light bulb (globe) glow and emit light into the room. So what caused this to happen? How does energy travel through copper wires to light the bulb? How does energy travel through space? What makes a motor turn, a radio play?
To understand these processes requires an understanding of the basic principles. For the light to glow requires energy to find a path through the light switch, through the copper wire and this movement is called electron flow. It is also called current flow in electronics. This is the first important principle to understand.
The word matter includes almost everything. It includes copper, wood, water, air....virtually everything. If we were able to take a piece of matter such as a drop of water, divided it by two and kept dividing by two until it couldn't be divided any further whileit was still water we would eventually have a molecule of water.
A molecule, the smallest particle which can exist, of water comprises two atoms of Hydrogen and one atom of Oxygen - H₂O.
An atom is also divisible - into protons and electrons. Both are electrical particles and neither is divisible. Electrons are the smallest and lightest and are said to be negatively charged. Protons on the other hand are about 1800 times the mass of electrons and are positively charged. Each are thought to have lines of forces (electric fields) surrounding them. In theory, negative lines of force will not join other negative lines of force. In fact they tend to repel each other. Similarly positive lines of force act in the same way.
The fact that electrons repel electrons and protons repel protons, but electrons and protons attract one another follows the basic law of physics:

Like forces repel and unlike forces attract.

Sounds a bit like a teenage romance - opposites attract.
When an electron and proton are brought in close proximity to one another it is the electron which moves because the proton is 1800 times heavier. It is the electron which moves in electricity. Even though the electron is much smaller, its field is quite strong negatively and is equal to the positive field of the proton.
If the field strength around an electron at a distance of 1,000,000th of a centimetre was a certain amount, then the field strength around an electron at a distance of 2,000,000th of a centimetre will be 1/4 as much. This is because the field decreases inversely with the distance squared. If an increase in one thing causes an increase in something else, these two things are said to vary directly. 2,000,000 electrons on an object produce twice the negative charge than 1,000,000 electrons would.
Since the electric-field strength of an electron varies inversely with the distance squared, the field strength a centimetre away would be quite weak. The fields surrounding protons and electrons are known as electrostatic fields. "Static" means stationary or not moving.
When electrons are made to move, the result is dynamic electricity. "Dynamic" means movement. To produce a movement of an electron it is necessary to either have a negatively charged field "push it", a positively charged field "pull it", or, as normally occurs in an electric circuit, a negative and positive charge (a pushing and pulling of forces).
There are more than one hundred different atoms or elements. The simplest and lightest is Hydrogen. An atom of Hydrogen consists of one electron whirling around one proton much like the moon revolving around the earth. The next atom in terms of weight is Helium (He) consisting two protons and two electrons. The third atom is Lithium (Li) with three protons and three electrons and so it goes on.
Some of the elements and their atomic weights are:
Hydrogen (1); Helium (2); Lithium (3); Carbon (6); Oxygen (8); Aluminium (13); Silicon (14); Iron (26); Nickel (28); Copper (29); Germanium (32); Gold (79); Lead (82).
Most atoms have a nucleus consisting of all the protons of the atom and also one or more neutrons. The remainder of the electrons (always equal in number to the nuclear protons) are whirling around the nucleus in different layers. The first layer of electrons outside the nucleus can only accomodate two electrons. If the atom has three electrons then two will be in the first layer and the third will be in the next layer. The second layer is completely filled when eight electrons are whirling around it. The third is filled when eighteen electrons are whirling around.
Don't think these electrons whirl around in some haphazard manner, they don't. The electrons in an element of a large atomic number are grouped into rings having a definite number of electrons. The only atoms in which these rings are completely filled are those of inert gaseous elements such as Helium, Neon, Argon, Krypton, Xenon and Radon.
All the other elements have one or more uncompleted rings of electrons.
Some of the electrons in the outer orbit of atoms such as copper or silver can be easily dislodged. These electrons travel out into the wide open spaces between the atoms and molecules and may be termed free electrons. It is the ability of these electrons to drift from atom to atom which makes electric current possible. Other electrons will resist dislodgement and are called bound electrons.

Electron Theory and Metals

It would be impossible for electronics to exist without metals and they are crucial to modern technology. Here are some of the properties of a few metals commonly used in electronics.

Figure 1. - properties of selected metals

Note:  Iron is the only metal significantly affected by a magnet.

1.  Density at 20^° C is Kg per M³

2.  Ohms ^-1

3.  These properties can be altered dramatically by the presence of relatively small amounts of impurities.
 
Another property of metals is malleability. This is because rows of positive ions can easiy slide over one another and still maintain a regular pattern. This is the reason why metals can be stretched without breaking.

Alloys

Most metals in use today are in fact alloys. Common examples are stainless steel, high speed steel from which our drill bits are made and in common use in electronics - Solder (60% Sn, 40% Pb - that's tin and lead) and; Nichrome for resistance wire and electrical heating elements (80% Ni, 20% Cr - that's nickel and chrome).

Electronics

Define Basic Electronics ?
The goal of this chapter is to provide some basic information about electronic circuits. We make the assumption that you have no prior knowledge of electronics, electricity, or circuits, and start from the basics. This is an unconventional approach, so it may be interesting, or at least amusing, even if you do have some experience. So, the first question is ``What is an electronic circuit?'' A circuit is a structure that directs and controls electric currents, presumably to perform some useful function. The very name "circuit" implies that the structure is closed, something like a loop. That is all very well, but this answer immediately raises a new question: "What is an electric current?" Again, the name "current" indicates that it refers to some type of flow, and in this case we mean a flow of electric charge, which is usually just called charge because electric charge is really the only kind there is. Finally we come to the basic question:

What is Charge?

 
No one knows what charge really is anymore than anyone knows what gravity is. Both are models, constructions, fabrications if you like, to describe and represent something that can be measured in the real world, specifically a force. Gravity is the name for a force between masses that we can feel and measure. Early workers observed that bodies in "certain electrical condition" also exerted forces on one another that they could measure, and they invented charge to explain their observations. Amazingly, only three simple postulates or assumptions, plus some experimental observations, are necessary to explain all electrical phenomena. Everything: currents, electronics, radio waves, and light. Not many things are so simple, so it is worth stating the three postulates clearly.

Charge exists.   

We just invent the name to represent the source of the physical force that can be observed. The assumption is that the more charge something has, the more force will be exerted. Charge is measured in units of Coulombs, abbreviated C. The unit was named to honor Charles Augustin Coulomb (1736-1806) the French aristocrat and engineer who first measured the force between charged objects using a sensitive torsion balance he invented. Coulomb lived in a time of political unrest and new ideas, the age of Voltaire and Rousseau. Fortunately, Coulomb completed most of his work before the revolution and prudently left Paris with the storming of the Bastille.

Charge comes in two styles.

We call the two styles positive charge, + , and (you guessed it) negative charge, - . Charge also comes in lumps of 1.6 �10^-19C , which is about two ten-million-trillionths of a Coulomb. The  discrete nature of charge is not important for this discussion, but it does serve to indicate that a Coulomb is a LOT of charge.

Charge is conserved.

You cannot create it and you cannot annihilate it. You can, however, neutralize it. Early workers observed experimentally that if they took equal amounts of positive and negative charge and combined them on some object, then that object neither exerted nor responded to electrical forces; effectively it had zero net charge. This experiment suggests that it might be possible to take uncharged, or neutral, material and to separate somehow the latent positive and negative charges. If you have ever rubbed a balloon on wool to make it stick to the wall, you have separated charges using mechanical action.
Those are the three postulates. Now we will present some of the experimental findings that both led to them and amplify their significance.

Voltage   

First we return to the basic assumption that forces are the result of charges. Specifically, bodies with opposite charges attract, they exert a force on each other pulling them together. The magnitude of the force is proportional to the product of the charge on each mass. This is just like gravity, where we use the term "mass" to represent the quality of bodies that results in the attractive force that pulls them together (see Fig. 4.1).

  

Figure 4.1: Opposite charges exert an attractive force on each other, just like two masses attract. External force is required to hold them apart, and work is required to move them farther apart.

Electrical force, like gravity, also depends inversely on the distance squared between the two bodies; short separation means big forces. Thus it takes an opposing force to keep two charges of opposite sign apart, just like it takes force to keep an apple from falling to earth. It also takes work and the expenditure of energy to pull positive and negative charges apart, just like it takes work to raise a big mass against gravity, or to stretch a spring. This stored or potential energy can be recovered and put to work to do some useful task. A falling mass can raise a bucket of water; a retracting spring can pull a door shut or run a clock. It requires some imagination to devise ways one might hook on to charges of opposite sign to get some useful work done, but it should be possible.
The potential that separated opposite charges have for doing work if they are released to fly together is called voltage, measured in units of volts (V). (Sadly, the unit volt is not named for Voltaire, but rather for Volta, an Italian scientist.) The greater the amount of charge and the greater the physical separation, the greater the voltage or stored energy. The greater the voltage, the greater the force that is driving the charges together. Voltage is always measured between two points, in this case, the positive and negative charges. If you want to compare the voltage of several charged bodies, the relative force driving the various charges, it makes sense to keep one point constant for the measurements. Traditionally, that common point is called "ground."
Early workers, like Coulomb, also observed that two bodies with charges of the same type, either both positive or both negative, repelled each other (Fig. 4.2). They experience a force pushing

  

Figure 4.2: Like charges exert a repulsive force on each other. External force is required to hold them together, and work is required to push them closer.

them apart, and an opposing force is necessary to hold them together, like holding a compressed spring. Work can potentially be done by letting the charges fly apart, just like releasing the spring. Our analogy with gravity must end here: no one has observed negative mass, negative gravity, or uncharged bodies flying apart unaided. Too bad, it would be a great way to launch a space probe. The voltage between two separated like charges is negative; they have already done their work by running apart, and it will take external energy and work to force them back together.
So how do you tell if a particular bunch of charge is positive or negative? You can't in isolation. Even with two charges, you can only tell if they are the same (they repel) or opposite (they attract). The names are relative; someone has to define which one is "positive." Similarly, the voltage between two points A and B , V_AB , is relative. If V_AB is positive you know the two points are oppositely charged, but you cannot tell if point A has positive charge and point B negative, or visa versa. However, if you make a second measurement between A and another point C , you can at least tell if B and C have the same charge by the relative sign of the two voltages, V_AB and V_AC to your common point A . You can even determine the voltage between B and C without measuring it: V_BC = V_AC - V_AB . This is the advantage of defining a common point, like A , as ground and making all voltage measurements with respect to it. If one further defines the charge at point A to be negative charge, then a positive V_AB means point B is positively charged, by definition. The names and the signs are all relative, and sometimes confusing if one forgets what the reference or ground point is.

Current   

Charge is mobile and can flow freely in certain materials, called conductors. Metals and a few other elements and compounds are conductors. Materials that charge cannot flow through are called insulators. Air, glass, most plastics, and rubber are insulators, for example. And then there are some materials called semiconductors, that, historically, seemed to be good conductors sometimes but much less so other times. Silicon and germanium are two such materials. Today, we know that the difference in electrical behavior of different samples of these materials is due to extremely small amounts of impurities of different kinds, which could not be measured earlier. This recognition, and the ability to precisely control the "impurities" has led to the massive semiconductor electronics industry and the near-magical devices it produces, including those on your RoboBoard. We will discuss semiconductor devices later; now let us return to conductors and charges.
Imagine two oppositely charged bodies, say metal spheres, that are being held apart, as in Fig. 4.3.

  

Figure 4.3: Two spheres with opposite charges are connected by a conductor, allowing charge to flow.

There is a force between them, the potential for work, and thus a voltage. Now we connect a conductor between them, a metal wire. On the positively charged sphere, positive charges rush along the wire to the other sphere, repelled by the nearby similar charges and attracted to the distant opposite charges. The same thing occurs on the other sphere and negative charge flows out on the wire. Positive and negative charges combine to neutralize each other, and the flow continues until there are no charge differences between any points of the entire connected system. There may be a net residual charge if the amounts of original positive and negative charge were not equal, but that charge will be distributed evenly so all the forces are balanced. If they were not, more charge would flow. The charge flow is driven by voltage or potential differences. After things have quieted down, there is no voltage difference between any two points of the system and no potential for work. All the work has been done by the moving charges heating up the wire.
The flow of charge is called electrical current. Current is measured in amperes (a), amps for short (named after another French scientist who worked mostly with magnetic effects). An ampere is defined as a flow of one Coulomb of charge in one second past some point. While a Coulomb is a lot of charge to have in one place, an ampere is a common amount of current; about one ampere flows through a 100 watt incandescent light bulb, and a stove burner or a large motor would require ten or more amperes. On the other hand low power digital circuits use only a fraction of an ampere, and so we often use units of 1/1000 of an ampere, a milliamp, abbreviated as ma, and even 1/1000 of a milliamp, or a microamp, �a . The currents on the RoboBoard are generally in the milliamp range, except for the motors, which can require a full ampere under heavy load. Current has a direction, and we define a positive current from point A to B as the flow of positive charges in the same direction. Negative charges can flow as well, in fact, most current is actually the result of negative charges moving. Negative charges flowing from A to B would be a negative current, but, and here is the tricky part, negative charges flowing from B to A would represent a positive current from A to B . The net effect is the same: positive charges flowing to neutralize negative charge or negative charges flowing to neutralize positive charge; in both cases the voltage is reduced and by the same amount.

Batteries   

Charges can be separated by several means to produce a voltage. A battery uses a chemical reaction to produce energy and separate opposite sign charges onto its two terminals. As the charge is drawn off by an external circuit, doing work and finally returning to the opposite terminal, more chemicals in the battery react to restore the charge difference and the voltage. The particular type of chemical reaction used determines the voltage of the battery, but for most commercial batteries the voltage is about 1.5 V per chemical section or cell. Batteries with higher voltages really contain multiple cells inside connected together in series. Now you know why there are 3 V, 6 V, 9 V, and 12 V batteries, but no 4 or 7 V batteries. The current a battery can supply depends on the speed of the chemical reaction supplying charge, which in turn often depends on the physical size of the cell and the surface area of the electrodes. The size of a battery also limits the amount of chemical reactants stored. During use, the chemical reactants are depleted and eventually the voltage drops and the current stops. Even with no current flow, the chemical reaction proceeds at a very slow rate (and there is some internal current flow), so a battery has a finite storage or shelf life, about a year or two in most cases. In some types of batteries, like the ones we use for the robot, the chemical reaction is reversible: applying an external voltage and forcing a current through the battery, which requires work, reverses the chemical reaction and restores most, but not all, the chemical reactants. This cycle can be repeated many times. Batteries are specified in terms of their terminal voltage, the maximum current they can deliver, and the total current capacity in ampere-hours.
You should handle batteries carefully, especially the ones we use in this course. Chemicals are a very efficient and compact way of storing energy. Just consider the power of gasoline or explosives, or the fact that you can play soccer for several hours powered only by a slice of cold pizza for breakfast. Never connect the terminals of a battery together with a wire or other good conductor. The battery we use for the RoboBoard is similar to the battery in cars, which uses lead and sulphuric acid as reactants. Such batteries can deliver very large currents through a short circuit, hundreds of  amperes. The large current will heat the wire and possibly burn you; the resulting rapid internal chemical reactions also produce heat and the battery can explode, spreading nasty, reactive chemicals about. Charging these batteries with too large a current can have the same effect. Double check the circuit and instructions before connecting a battery to any circuit. More information on batteries can be found in Chapter 7.

Circuit Elements

Resistors   

  
We need some way to control the flow of current from a voltage source, like a battery, so we do not melt wires and blow up batteries. If you think of current, charge flow, in terms of water flow, a good electrical conductor is like big water pipe. Water mains and fire hoses have their uses, but you do not want to take a drink from one. Rather, we use small pipes, valves, and other devices to limit water flow to practical levels. Resistors do the same for current; they resist the flow of charge; they are poor conductors. The value of a resistor is measured in ohms and represented by the Greek letter capital omega. There are many different ways to make a resistor. Some are just a coil of wire made of a material that is a poor conductor. The most common and inexpensive type is made from powdered carbon and a glue-like binder. Such carbon composition resistors usually have a brown cylindrical body with a wire lead on each end, and colored bands that indicate the value of the resistor. The key to reading these values is given in Chapter 2.
There are other types of resistors in your robot kit. The potentiometer is a variable resistor. When the knob of a potentiometer is turned, a slider moves along the resistance element. Potentiometers generally have three terminals, a common slider terminal, and one that exhibits increasing resistance and one that has decreasing resistance relative to the slider as the  shaft is turned in one direction. The resistance between the two stationary contacts is, of course, fixed, and is the value specified for the potentiometer. The photoresistor or photocell is composed of a light sensitive material. When the photocell is exposed to more light, the resistance decreases. This type of resistor makes an excellent light sensor.

Ohm's Law

Ohm's law describes the relationship between voltage, V , which is trying to force charge to flow, resistance, R , which is resisting that flow, and the actual resulting current I . The relationship is simple and very basic: .   Thus large voltages and/or low resistances produce large currents. Large resistors limit current to low values. Almost every circuit is more complicated than just a battery and a resistor, so which voltage does the formula refer to? It refers to the voltage across the resistor, the voltage between the two terminal wires. Looked at another way, that voltage is actually produced by the resistor. The resistor is restricting the flow of charge, slowing it down, and this creates a traffic jam on one side, forming an excess of charge with respect to the other side. Any such charge difference or separation results in a voltage between the two points, as explained above. Ohm's law tells us how to calculate that voltage if we know the resistor value and the current flow. This voltage drop is analogous to the drop in water pressure through a small pipe or small nozzle.

Power   

Current flowing through a poor conductor produces heat by an effect similar to mechanical friction. That heat represents energy that comes from the charge traveling across the voltage difference. Remember that separated charges have the potential to do work and provide energy. The work involved in heating a resistor is not very useful, unless we are making a hotplate; rather it is a byproduct of restricting the current flow. Power is measured in units of watts (W), named after James Watt, the Englishman who invented the steam engine, a device for producing lots of useful power. The power that is released into the resistor as heat can be calculated as P=VI , where I is the current flowing through the resistor and V is the voltage across it. Ohm's law relates these two quantities, so we can also calculate the power as The power produced in a resistor raises its temperature and can change its value or destroy it. Most resistors are air-cooled and they are made with different power handling capacity. The most common values are 1/8, 1/4, 1, and 2 watt resistors, and the bigger the wattage rating, the bigger the resistor physically. Some high power applications use special water cooled resistors. Most of the resistors on the RoboBoard are 1/8 watt.

Combinations of Resistors   

 
Resistors are often connected together in a circuit, so it is necessary to know how to determine the resistance of a combination of two or more resistors. There are two basic ways in which resistors can be connected: in series and in parallel. A simple series resistance circuit is shown in Figure 4.4.

  

Figure 4.4: Two Resistors in Series

Determining the total resistance for two or more resistors in series is very simple. Total resistance equals the sum of the individual resistances. In this case, R_T=R₁+R2 . This makes common sense; if you think again in terms of water flow, a series of obstructions in a pipe add up to slow the flow more than any one. The resistance of a series combination is always greater than any of the individual resistors.
The other method of connecting resistors is shown in Figure 4.5, which shows a simple parallel resistance circuit.

  

Figure 4.5: Two Resistors in Parallel

Our water pipe analogy indicates that it should be easier for current to flow through this multiplicity of paths, even easier than it would be to flow through any single path. Thus, we expect a parallel combination of resistors to have less resistance than any one of the resistors. Some of the total current will flow through R1 and some will flow through R2, causing an equal voltage drop across each resistor. More current, however, will flow through the path of least resistance. The formula for total resistance in a parallel circuit is more complex than for a series circuit:

RT={1{1R1}+{1R2}...+{1Rn}}

(1)

Parallel and series circuits can be combined to make more complex structures, but the resulting complex resistor circuits can be broken down and analyzed in terms of simple series or parallel circuits. Why would you want to use such combinations? There are several reasons; you might use a combination to get a value of resistance that you needed but did not have in a single resistor. Resistors have a maximum voltage rating, so a series of resistors might be used across a high voltage. Also, several low power resistors can be combined to handle higher power. What type of connection would you use?

Capacitors   

  
Capacitors are another element used to control the flow of charge in a circuit. The name derives from their capacity to store charge, rather like a small battery. Capacitors consist of two conducting surfaces separated by an insulator; a wire lead is connected to each surface. You can imagine a capacitor as two large metal plates separated by air, although in reality they usually consist of thin metal foils or films separated by plastic film or another solid insulator, and rolled up in a compact package. Consider connecting a capacitor across a battery, as in Fig. 4.6.

  

Figure 4.6: A simple capacitor connected to a battery through a resistor.

As soon as the connection is made charge flows from the battery terminals, along the wire and onto the plates, positive charge on one plate, negative charge on the other. Why? The like-sign charges on each terminal want to get away from each other. In addition to that repulsion, there is an attraction to the opposite-sign charge on the other nearby plate. Initially the current is large, because in a sense the charges can not tell immediately that the wire does not really go anywhere, that there is no complete circuit of wire. The initial current is limited by the resistance of the wires, or perhaps by a real resistor, as we have shown in Fig. 4.6. But as charge builds up on the plates, charge repulsion resists the flow of more charge and the current is reduced. Eventually, the repulsive force from charge on the plate is strong enough to balance the force from charge on the battery terminal, and all current stops. Figure 4.7 shows how the current might vary with

  

Figure 4.7: The time dependence of the current in the circuit of Fig. 4.6 for two values of resistance.

time for two different values of resistors. For a large resistor, the whole process is slowed because the current is less, but in the end, the same amount of charge must exist on the capacitor plates in both cases. The magnitude of the charge on each plate is equal.
The existence of the separated charges on the plates means there must be a voltage between the plates, and this voltage be equal to the battery voltage when all current stops. After all, since the points are connected by conductors, they should have the same voltage; even if there is a resistor in the circuit, there is no voltage across the resistor if the current is zero, according to Ohm's law. The amount of charge that collects on the plates to produce the voltage is a measure of the value of the capacitor, its capacitance, measured in farads (f). The relationship is C = Q/V , where Q is the charge in Coulombs. Large capacitors have plates with a large area to hold lots of charge, separated by a small distance, which implies a small voltage. A one farad capacitor is extremely large, and generally we deal with microfarads ( �f ), one millionth of a farad, or picofarads (pf), one trillionth (10^-12) of a farad.
Consider the   circuit of Fig. 4.6 again. Suppose we cut the wires after all current has stopped flowing. The charge on the plates is now trapped, so there is still a voltage between the terminal wires. The charged capacitor looks somewhat like a battery now. If we connected a resistor across it, current would flow as the positive and negative charges raced to neutralize each other. Unlike a battery, there is no mechanism to replace the charge on the plates removed by the current, so the voltage drops, the current drops, and finally there is no net charge left and no voltage differences anywhere in the circuit. The behavior in time of the current, the charge on the plates, and the voltage looks just like the graph in Fig. 4.7. This curve is an exponential function: exp(-t/RC) . The voltage, current, and charge fall to about 37% of their starting values in a time of R �C seconds, which is called the characteristic time or the time constant of the circuit. The RC time constant is a measure of how fast the circuit can respond to changes in conditions, such as attaching the battery across the uncharged capacitor or attaching a resistor across the charged capacitor. The voltage across a capacitor cannot change immediately; it takes time for the charge to flow, especially if a large resistor is opposing that flow. Thus, capacitors are used in a circuit to damp out rapid changes of voltage.

Combinations of Capacitors   

Like resistors, capacitors can be joined together in two basic ways: parallel and series. It should be obvious from the physical construction of capacitors that connecting two together in parallel results in a bigger capacitance value. A parallel connection results in bigger capacitor plate area, which means they can hold more charge for the same voltage. Thus, the formula for total capacitance in a parallel circuit is:

CT=C1+C2...+Cn ,

(2)

the same form of equation for resistors in series, which can be confusing unless you think about the physics of what is happening.
The capacitance of a series connection is lower than any capacitor because for a given voltage across the entire group, there will be less charge on each plate. The total capacitance in a series circuit is

CT={1{1C1}+{1C2}...+{1Cn}}.

(3)

Again, this is easy to confuse with the formula for parallel resistors, but there is a nice symmetry here.

Inductors   

Inductors are the third and final type of basic circuit component. An inductor is a coil of wire with many windings, often wound around a core made of a magnetic material, like iron. The properties of inductors derive from a different type of force than the one we invented charge to explain: magnetic force rather than electric force. When current flows through a coil (or any wire) it produces a magnetic field in the space outside the wire, and the coil acts just like any natural, permanent magnet, attracting iron and other magnets. If you move a wire through a magnetic field, a current will be generated in the wire and will flow through the associated circuit. It takes energy to move the wire through the field, and that mechanical energy is transformed to electrical energy. This is how an electrical generator works. If the current through a coil is stopped, the magnetic field must also disappear, but it cannot do so immediately. The field represents stored energy and that energy must go somewhere. The field contracts toward the coil, and the effect of the field moving through the wire of the coil is the same as moving a wire through a stationary field: a current is generated in the coil. This induced current acts to keep the current flowing in the coil; the induced current opposes any change, an increase or a decrease, in the current through the inductor. Inductors are used in circuits to smooth the flow of current and prevent any rapid changes.
The current in an inductor is analogous to the voltage across a capacitor. It takes time to change the voltage across a capacitor, and if you try, a large current flows initially. Similarly, it takes time to change the current through an inductor, and if you insist, say by opening a switch, a large voltage will be produced across the inductor as it tries to force current to flow. Such induced voltages can be very large and can damage other circuit components, so it is common to connect some element, like a resistor or even a capacitor across the inductor to provide a current path and absorb the induced voltage. (Often, a diode, which we will discuss later, is used.)
Inductors are measured in henrys (h), another very big unit, so you are more likely to see millihenries, and microhenries. There are almost no inductors on the RoboBoard, but you will be using some indirectly: the motors act like inductors in many ways. In a sense an electric motor is the opposite of an electrical generator. If current flows through a wire that is in a magnetic field (produced either by a permanent magnet or current flowing through a coil), a mechanical force will be generated on the wire. That force can do work. In a motor, the wire that moves through the field and experiences the force is also in the form of a coil of wire, connected mechanically to the shaft of the motor. This coil looks like and acts like an inductor; if you turn off the current (to stop the motor), the coil will still be moving through the magnetic field, and the motor now looks like a generator and can produce a large voltage. The resulting inductive voltage spike can damage components, such as the circuit that controls the motor current. In the past this effect destroyed a lot of motor controller chips and other RoboBoard components. The present board design contains special diodes that will withstand and safely dissipate the induced voltages -- we hope.

Combinations of Inductors

You already know how inductors act in combination because they act just like resistors. Inductance adds in series. This makes physical sense because two coils of wire connected in series just looks like a longer coil. Parallel connection reduces inductance because the current is split between the several coils and the fields in each are thus weaker.

Semiconductor Devices

 

The Truth About Charge   

Our statements above about charge are not wrong, but they are simple and incomplete. In order to understand how semiconductor devices work one needs a more complete description of the nature of charge in the real world. Charge does not exist independently; it is carried by subatomic particles. For this discussion we will be concerned primarily with electrons, which carry a negative charge of 1.6 � 10^-19 C , the minimum amount of charge that can exist in isolation. At least, no one has found any smaller amount than this fundamental quantum of charge.
Electrons are one component of atoms and molecules. Atoms are the building blocks out of which all matter is constructed. Atoms bond with each other to form substances. Substances composed of just one type of atom are called elements. For example, copper, gold and silver are all elements; that is, each of them consists of only one type of atom. More complex substances are made up of more than one atom and are known as compounds. Water, which has both hydrogen and oxygen atoms, is such a compound. The smallest unit of a compound is a molecule. A water molecule, for example, contains two hydrogen atoms and one oxygen atom.
Atoms themselves are made up of even smaller components: protons, neutrons and electrons. Protons and neutrons form the nucleus of an atom, while the electrons orbit the nucleus. Protons carry positive charge and electrons carry negative charge; the magnitude of the charge for both particles is the same, one quantum charge, 1.6 �10^-19 C . Neutrons are not charged. Normally, atoms have the same number of protons and electrons and have no net electrical charge.

  

Figure 4.8: Structure of an Atom

Electrons that are far from the nucleus are relatively free to move around under the influence of external fields because the force of attraction from the positive charge in the nucleus is weak at large distances. In fact, it takes little force in many cases to completely remove an outer electron from an atom, leaving an ion with a net positive charge. Once free, electrons can move at speeds approaching the speed of light (roughly 670 million miles per hour) through metals, gases and vacuum. They can also become attached to another atom, forming an ion with net negative charge.
Electric current in metal conductors consists of a flow of free electrons. Because electrons have negative charge, the flow of electrons is in a direction opposite to the positive current. Free electrons traveling through a conductor drift until they hit other electrons attached to atoms. These electrons are then dislodged from their orbits and replaced by the formerly free electrons. The newly freed electrons then start the process anew. At the microscopic level, electron flow through a conductor is not a steady stream, like water flowing from a faucet, but rather a series of short bursts.

  

Figure 4.9: A Simple Model of Electron Flow

Silicon   

 
Semiconductor devices are made primarily of silicon (silicon's element symbol is "Si"). Pure silicon forms rigid crystals because of its four valence (outermost) electron structure -- one Si  atom bonds to four other Si atoms forming a very regularly shaped diamond pattern. Figure 4.10 shows part of a silicon crystal structure.

  

Figure 4.10: A Silicon Crystal Structure

Pure silicon is not a conductor because there are no free electrons; all the electrons are tightly bound to neighboring atoms. To make silicon conducting, producers combine or "dope" pure silicon with very small amounts of other elements like boron or phosphorus. Phosphorus has five outer valence electrons. When three silicon atoms and one phosphorus atom bind together in the basic silicon crystal cell of four atoms, there is an extra electron and a net negative charge. Figure4.11 shows the crystal structure of phosphorus doped silicon.

  

Figure 4.11: Silicon Doped with Phosphorus

This type of material is called n-type silicon. The extra electron in the crystal cell is not strongly attached and can be released by normal thermal energy to carry current; the conductivity depends on the amount of phosphorus added to the silicon.
Boron has only three valance electrons. When three silicon atoms and one boron atom bind with each other there is a "hole" where another electron would be if the boron atom were silicon; see Fig. 4.12. This gives the crystal cell a positive net charge (referred to as p-type silicon), and the ability to pick up an electron easily from a neighboring cell.

  

Figure 4.12: Silicon Doped with Boron

The resulting migration of electron vacancies or holes acts like a flow of positive charge through the crystal and can support a current. It is sometimes convenient to refer to this current as a flow of positive holes, but in fact the current is really the result of electrons moving in the opposite direction from vacancy to vacancy.

Diodes   

  
Both p-type and n-type silicon will conduct electricity just like any conductor; however, if a piece of silicon is doped p-type in one section and n-type in an adjacent section, current will flow in only one direction across the junction between the two regions. This device is called a diode and is one of the most basic semiconductor devices.
A diode is called forward biased if it has a positive voltage across it from from the p- to n-type material. In this condition, the diode acts rather like a good conductor, and current can flow, as in Fig. 4.13.

  

Figure 4.13: A Forward Biased Diode

There will be a small voltage across the diode, about 0.6 volts for Si, and this voltage will be largely independent of the current, very different from a resistor.
If the polarity of the applied voltage is reversed, then the diode will be reverse biased and will appear nonconducting (Fig. 4.14). Almost no current will flow and there will be a large voltage across the device.

  

Figure 4.14: A Reverse Biased Diode

The non-symmetric behavior is due to the detailed properties of the pn-junction. The diode acts like a one-way valve for current and this is a very useful characteristic. One application is to convert alternating current (AC), which changes polarity periodically, into direct current (DC), which always has the same polarity. Normal household power is AC while batteries provide DC, and converting from AC to DC is called rectification. Diodes are used so commonly for this purpose that they are sometimes called rectifiers, although there are other types of rectifying devices. Figure 4.15 shows the input and output current for a simple half-wave

  

Figure 4.15: A Half-Wave Rectifier

rectifier. The circuits gets its name from the fact that the output is just the positive half of the input waveform. A full-wave rectifier circuit (shown in Figure 4.16) uses four diodes arranged so that both polarities of the input waveform can be used at the output.

  

Figure 4.16: A Full-Wave Rectifier

The full-wave circuit is more efficient than the half-wave one.