basicelectronics

Basic Electronics

This page contains some basic electronics information

Electricity is a very interesting subject. Flow of electrons through conductors, creation of magnetic field, accumulation of charges over non-conducting substances (static electricity) and the more recently know properties shown by silicon chips are all fascinating subjects unto themselves. In this web page I intend to give a brief information of what I know about electricity, electric and electronic components.

Why do some substances conduct electricity while others do not?

A substance can conduct electricity if it has free electrons that will flow through the substance, effectively conducting electricity. To be truthful, every substance conducts electricity to some degree. However, substances like metals and solutions of ionic compounds (such as salts of sodium and potassium) conduct electricity to a much higher degree than other substances like plastics and porcelain. We do not have substances that span the whole range of possible conductivity. This means that we have a very large gap in the conductivity shown by "conductors" and that shown by "insulators". On one hand, we have perfect conductors known as superconductors (some elements at sub cryogenic temperatures (zero resistance)), to conductors (like metals) to semiconductors (like carbon, silicon and germanium) to insulators (glass, teflon etc.)

How do we measure the degree of conductivity?

By a unit called conductance. Well, usually we define its inverse, called resistance. Resistance is defined as the ratio of voltage over current that flows through a substance. Wikipedia defines electrical resistivity as follows. Electrical resistivity (also known as specific electrical resistance) is a measure of how strongly a material opposes the flow of electric current. A low resistivity indicates a material that readily allows the movement of electrical charge. The SI unit of electrical resistivity is the ohm metre. We have to use the unit ohm-metre since resistance will change depending on the length of the "wire".

What is a resistor?

A resistor is an electronic component that has a fixed electrical resistivity. It is very convenient to use a tiny component than to incorporate a very long wire to create the required resistance. Remember, however, that it is possible to create resistance of about any magnitude using a (thin) long wire. In fact that is what a potentiometer is all about. It is not possible to create a custom required resistance using (a single) resistor. Potentiometers can give the exact resistance that you require (but they may drift quite a lot over time). To identify the magnitude of a resistance (and also its tolerance), all resistors have a color code. To remember the colors, we have an acronym BBROYGBVGW (B. B. Roy, Great Britain, Very Good Wife) (Black Brown Red Orange Yellow Green Blue Voilet Gray White) assigned values 0 though 9. I would like to refer you to the Wikipedia article for color codes to get a complete understanding of how these color codes are used. You may also want to enjoy some other (inappropriate for classroom) mnemonics for BBROYGBVGW on the same site!

As a side note, let me add that resistance behaves in the same way for both AC and DC currents.

So, are there components that behave differently for AC and DC currents?

Yes. Inductors and Capacitors behave differently for AC and DC currents. When I was in school, I wanted to create a unified theory of physics (and become a Nobel laureate). My theory was that the effect opposes the cause (as simple as that, the unified theory has to be simple, by rule!) I thought of many examples. Inductance, is one of them. Wikipedia defines it as follows. Inductance (L) (measured in henrys) is an effect which results from the magnetic field that forms around a current-carrying conductor. Electric current through the conductor creates a magnetic flux proportional to the current. A change in this current creates a change in magnetic flux that, in turn, generates an electromotive force (EMF) that acts to oppose this change in current. Inductance is a measure of the amount of EMF generated for a unit change in current.

If current flowing through the inductor does not change, there is no EMF. For a constant DC current, an inductor behaves like a short circuit (or a very small resistor), while for a AC current, it behaves well, like an inductor. Usually we use inductors of magnitudes in Milli-henry.

Capacitance is a measure of the amount of electric charge stored (or separated) for a given electric potential. The most common form of charge storage device is a two-plate capacitor. The unit of capacitance is Farad. We use capacitances of several magnitudes, but usually, much lower than the Farad. Typically, we come across values like pico-farad, nano-farad and micro-farad. Touching an unshorted capacitor can be dangerous. Capacitors must be stored so that they are and remain uncharged by shorting their two legs using metal connectors. A capacitor is like open circuit (very high resistance) to a DC current. For an AC current, it is like a capacitor.

So most of the electronic items are made of only these three components?

Quite to the contrary. Though resistance, capacitance and inductance forms the basic components in electrical circuitry, modern electronics contains many other basic components. Most of these components are silicon based. Fundamental to these silicon devices are diodes and transistors.

But I though Silicon is only a semiconductor?

Correct. And we are fortunate to have it. The amazing physical properties that Silicon shows are due to its 4 electrons in the outermost orbit. It can be fabricated to conduct electricity in a direction in which we want it to flow.

And what do I do with a unidirectional flow of electricity?

We can create a diode using this property. It works like a non-return valve (my Father loves this analogy). When AC current is passed through it, only the positive wave will appear on the other side. The negative part of the wave is cut out. This is the basic component of rectifiers, or AC-DC converters (or the (heavy) "adapters" connected to printers, computer speakers, and other equipment like synthesizers). A diode behaves in this fashion because of a P-N junction fabricated in Silicon. Two sides of a silicon wafer are doped with opposite dopant to create a P-side and a N-side. Both these "sides" are conductive, but when merged together, the junction is non-conducting. It creates a boundary called the depletion zone which cause diodes to behave in their characteristic way.

While most diodes are used in forward bias mode, a particular type of diode is used, almost exclusively in the reverse bias mode. I like to reproduce what Wikipedia writes about the Zener diode.

A conventional solid-state diode will not allow significant current if it is reverse-biased below its reverse breakdown voltage. [If very high voltage is applied in the reverse direction,] the diode will be permanently damaged. In case of large forward bias (current in the direction of the arrow), the diode exhibits a voltage drop due to its junction built-in voltage and internal resistance. The amount of the voltage drop depends on the semiconductor material and the doping concentrations.

A Zener diode exhibits almost the same properties, except the device is specially designed so as to have a greatly reduced breakdown voltage, the so-called Zener voltage. A Zener diode contains a heavily doped p-n junction allowing electrons to tunnel from the valence band of the p-type material to the conduction band of the n-type material. In the atomic model, this tunneling corresponds to the ionization of covalent bonds. A reverse-biased Zener diode will exhibit a controlled breakdown and allow the current, to keep the voltage across the Zener diode at the Zener voltage. For example, a diode with a Zener breakdown voltage of 3.2 V will exhibit a voltage drop of 3.2 V if reverse bias voltage applied across it is more than its Zener voltage. ... the Zener diode is typically used to generate a reference voltage for an amplifier stage, or as a voltage stabilizer for low-current applications.

It was a large copy-paste and I am sorry about it, but I could have not written any better.

And what were you saying about transistors?

Transistors are three-legged electronic components that have changed the face of the world. A transistor uses two P-N junctions and provides some very interesting properties. One of them is signal amplification. It allows very small currents to control circuits with much higher currents. Transistors can act as an electronic switch or an amplifier. The three legs of a transistor are called Emitter, Base and Collector. Transistors are of many types called as BJT, FET, MOSFET etc. BJTs are the simplest to understand. The Wikipedia articles about Transistors and BJT are really very good.

Transistors are the building block of all electronic components including registers, logic circuits and memories. An integrated circuit (IC) is mostly made of hundreds of tiny transistors. A computer's microprocessor, graphics processor and a lot of chips these days, contain millions of transistors packed together in one package. The technology that made this possible is called LSI and VLSI (Very Large Scale Integration).