﻿ Introduction

Introduction to Electronics

When I first started teaching computer architecture in the late 1970s, I could reasonably expect that many of my students would be familiar with the basic principles of electronics; for example, the meaning of volts and amps, an understanding of Ohm’s law, and an ability to design and construct circuits.

Over the years that changed as the educational background of my students changed. Fewer students were coming to computer science via an interest in physics or electronics.

In general it’s not vital that students have an in depth understanding of electronics. They should, at least, know the basic units of electricity and have an understanding of some of the areas of computer architecture and design where electronics has a direct impact (power dissipation, electromagnetic radiation, bus termination, propagation delays on buses).

These notes provide a short introduction to electronics.

What is Electricity?

Electricity is like gravity. Everyone knows something about its effects, but few have any understanding of what it is.  This is because both electricity and gravity have their origins in the fundamental structure of matter and the subatomic particles that make up matter. For our purposes, two of the particles that make up matter are the electron and the proton. An important characteristic of the electron is that it carries a negative charge and the proton has a positive charge. These two charges are equal and opposite. Protons form part of an atom’s nucleus and are relatively massive compared to electrons. Atoms with an equal numbers of protons and electrons have no overall charge, because the positive and negative charges cancel out. The electrons are said to orbit the nucleus, although that is rather a fiction because an electron does not exist at a particular point in space.

In some materials called conductors, electrons are able to move through the bulk material, hopping from one atom to another; that is, some of the electrons are shared throughout the material. The movement of electrons through a conductor is called an electrical current. Note that electrons move incredibly slowly through a conductor. They drift at around a few millimeters per hour. However, the effect of the electric current moves through the conductor at close to the speed of light, 300,000 km/s (3.0 x 108 m/s).

Insulators may loose their electrical neutrality; for example, by rubbing them. When this happens a surfeit or deficit of electrons builds up and the insulator is said to be charged. This charge is popularly called static electricity. A static charge is sufficient to destroy transistors in electronic circuits which is why we are supposed to discharge static electricity by ensuring that we are earthed (grounded) before handling ICs and circuit boards.

A moving electric current has four effects.

• It heats up conductors through which it moves (heaters and incandescent lights).
• It can cause chemical changes in conductors through which it moves (electrolysis and batteries).
• It generates a magnetic field (loudspeakers, motors, radio waves).
• It can release photons in materials (X-rays, light-emitting diodes and displays).

Units of Electricity

The basic units associated with electricity are:

Charge: The charge on an electron is 1.602176487×10−19 C, where C is the symbol for the Coulomb, the basic unit of electrical charge.

Electric potential:  Water flows downhill because of gravity. Electrons flow through a conductor because of the potential difference across the conductor. The greater the potential difference, the greater the movement of charge.  The unit of potential difference is the volt and the symbol for electric potential in equations is normally E.

Semiconductor electronics generally operate using potential differences of about 1 to 5 volts. The potential difference generated by a lithium ion cell is approximately 3.6 V and the potential difference generated by a nickel-metal hydride cell (NiMH) is typically 1.2 V. The domestic power supply is 110 V in the USA and 240 V in Europe. The domestic power supply uses alternating current in which the voltage varies in a sinusoidal way between a positive maximum value and a negative maximum value. The quoted voltage is actually the equivalent dc voltage that would produce the same heating power.

Current: Current is a flow of electric charge and is measured in Coulombs per second. The common unit of current is the ampere. Incidentally, the symbol for current is I which is derived from the French expression intensité de courant. The ampere is defined as a charge of one Coulomb passing a point in one second; that is, 6.241 x 1018 elections.

Power: Power is defined as the rate at which work is done. The symbol for power is usually P. One way of measuring power is to compare it to the rate at which a horse can work; hence the term horse power. The unit of electrical power is the Watt and is defined as the work done when a charge of 1 Coulomb moves through a potential difference of 1 Volt in 1 second. Domestic and industrial electric power is usually measured in terms of the kilowatt hour, which is 1,000 W for one hour. In physics the Watt is also expressed at the joule per second, J/s). Note that a horse power is defined as equivalent to 746 W and is the power necessary to lift 550 pound by one foot in one second. The calorie is a unit of energy and is equivalent to 4.184 J. So, if your CPU dissipates 70 W, it is dissipating 70 J/s or  292 calories/s (note that the domestic Calorie is actually the kilo calorie and equivalent to 1,000 calories so that the CPU actually dissipates 0.292 C/s).

Resistance:  Matter has a property called resistance that impedes the flow of electricity. In fact, the range of resistance values between a near-perfect insulator and a good conductor is probably the widest range of physical property. The unit of resistance is the Ohm, Ω, and a conductor is said to have a resistance of 1 Ω. The relationship between voltage current and resistance is expressed by Ohm’s law as I = E/R (current = voltage/resistance). The resistance of the human body can be as high as 100,000 Ω (dry skin) or 1,000 Ω (wet skin).

Capacitance:  Capacitance is the ability to store a charge. A capacitor can be thought of as two flat conducting plates separated by an insulator. The charge is stored on the surface of the insulator. The unit of capacitance is the Farad and is equivalent to one Coulomb of charge when there is a 1 V potential difference between the places.

Relationships

Q = C.V                Charge = capacity x voltage

W = E.I                 Power = voltage x current

I = E/R                  Current = voltage/resistance

P = I2.R                 Power = current2 x resistance

The following references provide links to pages related to electronics.

History of Electronics

Our world depends on electronics and electricity. This article briefly describes highlights in the development of electronics, or to be more precise, the discovery of electricity and its properties. This happened in a remarkably brief historical span.

Semiconductors

The key to the modern computer is the integrated circuit which consists of many (up to billions) of transistors on a single chip.

Batteries

Today’s world is a mobile world. Portable electronics requires power. Here three trends are at play: the ever increasing complexity of mobile devices, the continuing improvements in low-power technology, and the development of improved batteries with greater efficiency. This article looks at battery technology.

Electronic Circuits

Here we provide a very simple description of electronic circuits.