3. BASIC ELECTRONICS

3.1 FUNDAMENTAL UNITS: OHM'S LAW

The fundamental or units of electricity are:

Voltage - Electrical pressure or force (EMF) measured in volts, symbols V or E.

Current - Electrical flow, measured in amps, symbols A or I.

Resistance - Resistance to current flow, measured in ohms. Symbol is R.

Taking the often quoted water analogy of an open topped water tank in the roof space, with a vertical pipe from its bottom going down to the ground level, then

Voltage-being pressure , force, (electromotive force or EMF) or potential difference is equivalent to the head of water, or the height of the tank above the end of the pipe.

Current-is the electrical flow equivalent to the flow of water.

Resistance-is the equivalent to the diameter of the pipe - the larger the pipe, the lower the resistance.

It can be seen that the flow (of water or electricity) will increase with increase of pressure (increase in voltage or height of tank) and with decreases of resistance (increasing diameter of pipe).

That's all there is to it. The relationship between these three electrical functions is defined by a very simple formula known as Ohm's law, which is very simply


It can be seen that increasing the voltage or decreasing the resistance will increase the current flow.

If we have a 1 volt battery connected across a 1 Ohm resistance, then according to the formula, the current will be a current of 1 amp registered by the amp meter.


The voltmeter will measure the voltage across the resistor which equals the source voltage.
Likewise, if we increase the voltage to, say, 12 volts, then the formula becomes

Referencing the water analogy, increasing head of water (height of tank) increases the flow: increasing the diameter of the pipe (decreasing the resistance to flow) does the same.

3.2 CHANGING THE SUBJECT OF THE EQUATION

This sounds much more difficult than it really is; actually it is very simple. In any single equation, one can calculate any one unknown from a knowledge of all other functions or terms. If we know voltage and current, we can calculate resistance. If we know current and resistance, we can calculate voltage. If we know voltage and resistance, we can calculate current.

The fundamental rule is to put the unknown term on its own on one side of the equation (or equal sign) with all the other terms on the other side.
The two basic rules for changing the subject of an equation are:
  -a multiplied function on one side becomes a division function when transferred to the other (and vice versa).
   -a sum on one side (addition) becomes a difference (subtraction) when transferred to the other side (and vice versa).
Thus A = V/R    is OK for working out the amperage or
            current, if we know the voltage and
          resistance.

However, if we know the voltage and current flowing, we can calculate the resistance. For this we need R as the subject of the equation (i.e. , on its own on the left hand side).
From the first of the three basic rules above, R, which is a division on the right hand side, can be transferred to the left hand side where it becomes a multiplier:
A X R = V or AR = V
Now, to take A over to the right hand side, as it is a multiplier (A X R) on the left, it becomes a division on the right:
R = V : A or R = V/A

The third possibility, where V is the unknown and which we want on its own, we have already seen above:
AR = V which is the same as V = AR.
So far, we have only made use of the first of the three rules. An example of its use is as follows:
Given a 12 volt car battery and a 2 ohm resistor, calculate the current flow A.
A = V /R = 12/2 = 6 amp

If we had two 6 volt batteries in series to make up the 12 volts, the equation becomes

thus we have to do the addition (or subtraction) first before carrying out the multiplications or divisions.

3.3 POWER

In our water analogy, power is pressure times flow, in electrical terms it is Volts times Amps:
Power = Volts X Amps or W = VA
Thus in the example above
W = VA = 12 x 6 = 72 watts
Finally, we can define watts including resistance by substituting Ohm's law.
W = VA, but V = AR,
Thus W = A x A x R = A R
Similarly W = VA but A=V/R
Thus W = V x V /R = V/R
By way of example, and checking on our previous sum, we can see that
Thus, in our example, the resistance of a 72 watt auto headlight bulb can be calculated back as follows:
W = V /R , thus WR = V and R = V/W

Power is an instantaneous measure, measures in watts or horsepower.

Another unit is Work Done (W. D.) which is power times "time", or power applied for a given amount of time. Electrically speaking, watt seconds is a measure of work done. As this is a very small unit, this is usually multiplied up in the form of watt hours, or more commonly, kilowatt hours.

Be careful, "kilowatt hours" is technically not "power" consumed (kilowatts), but work done (kilowatt-hours).

3.4 MULTIPLIERS

As we have see just above, units may not be convenient to use in their basic form. They may be too large or too small. So we apply multipliers. The more common of these are"

109 = 1,000,000,000 or 1 Billion or Giga (as in gigahertz)
106 = 1,000,000 or 1 Million or Mega (as in Megohm)
103 = 1,000 or 1 Thousand or Kilo (as in kilometers)
101 = 10 or Deka
100 = 1
10-1 = 1/10 or 0.1 = deci (as in decibels)
10-2 = 1/100 or 0.01 = centi (as in centimeters
10-3 = 1/1000 or 0.001 = mili (as in miliampere)
10-6 = 1/ 1,000,000 or 0.000001 or 1/1m = micro, u, (as in microamps)
10-9 = 1/Billionth = nano (as in nanofarads)
10-12 = 1/Trillionth = pico (as in picofarad or micro micro farad)

Thus, kilowatts are "thousands of watts". Picofarads (the capacity of small capacitors are trillionths of a Farad. Milliamps are thousandths of an amp, thus 1000mA or milliamps is the same as 1 amp. Megacycles per second (the same as Megahertz) are millions of cycles per second; your favorite radio program may well be on 90.3 Megahertz VHF/FM.

3.5 DIRECT CURRENT (D.C.) AND ALTERNATING CURRENT (A.C.)

A battery, such as an automobile battery, produces direct current: the positive terminal is always positive and the negative terminal always negative. Either the positive or the negative may be connected to "earth" or "ground"; that is, a spike in the ground or the chassis of an automobile, or they may both be left floating. In the case of an automobile, they are now mostly "negative ground", but twenty or thirty years ago, they were mostly "positive ground". (i.e., battery positive connected to auto frame.)

A typical example of alternating current is that provided by the regular 117 VAC household electrical outlets. The current alternates (i.e., reverses) 60 times every second. (In the United states, that is 117 VAC, 60 CPS; other countries may vary from 90 to 240 VAC, 50 to 60 CPS.) If one examines the signal produced by a microphone, it is alternating at the frequency of the incident sound: voice, music or noise. This audio alternating voltage is very small, requiring amplification before being applied to a loudspeaker. The movement of the cone of the speaker follows the alternations of the applied voltage. Apply D.C. to a loudspeaker and the cone will move suddenly in one direction to take up a new static position until the voltage is removed. Apply A.C. and the cone will move in and out following the applied A.C. faithfully.

A.C. is measured in volts and amps in just the same way as D.C. However, and unlike D.C., we have to be careful. If we are looking at an A.C. voltage from the point of view of whether it will break down the insulation of a cable or a component, or jump a gap, then it is the peak voltage that counts. However, if we want to talk about power (volts times amps) such as would produce the same heating effect in an electric heater as the same D.C. volts and amps, then the actual volts and amps are less than the peak volts and amps, because A.C. voltages (at the A.C. receptacle or at the amplifier output) are sine waves. Only if the A.C. was composed of square waves would peak A.C. volts and amps produce the same power as D.C. volts and amps. The area of the sine wave equivalent to the area of the square wave (which corresponds to power) requires the peak voltage to be higher than that of the equivalent D.C. or square wave by a factor of 1.414 times. This happens to be the same as the square root of 2, thus what we measure is the RMS or root mean square value of the A.C. waveform. The regular 117 VAC domestic electricity supply actually has a peak value of 117 X root 2 (117 X 1.414) or 165 volts. 117 volts is actually the root mean square value.

3.6 CAPACITORS AND INDUCTORS

Capacitors and inductors have the ability (among other things) to store energy. If a D.C. voltage is applied across the plates of a capacitor, a current flows (the value of which depends on the value or capacity of the capacitor) diminishing exponentially until the plates are at the same voltage as the applied D.C. voltage. Disconnect the D.C. voltage and the plates stay charged for a time dependent on the quality of the capacitor; that is, its leakage characteristics. One example of a capacitor is the automobile which builds up a static electrical charge in very dry weather; the auto is acting as a capacitor (with respect to ground) which gives the occupant a kick in getting into or out of the auto. Nowadays, tires are deliberately made partially electrically conducting (by loading with carbon) in order to leak to ground the static voltage induced in the auto by movement, thunderstorms, etc.

Inductors act in much the same way, except that they do not hold their charge very long for mechanical/physical reasons. However, they can be very efficient in doing so for long enough to transfer energy from one winding to an adjacent winding in the case of transformers, car ignition coils, etc.

You will note that when applying a D.C. voltage across a capacitor, a current flowed until the plastic had assumed the voltage of the battery. If we reversed the polarity of the battery, a current would flow temporarily in the opposite direction. Thus, if we apply A.C. to a capacitor, a continuous but reversing (A.C.) current will flow across (through) the capacitor. But with D.C., no current flows once the plates have taken up the applied voltage. This is important in telephone work where the D.C. exchange (switching center) voltage may be blocked by a capacitor which, however, passes the A.C. (speech or ringer voltage) with relatively little hindrance.

The capacity of a capacitor is measured in microfarads or picofarads; the inductance of an inductor is measured in Henrys or microhenrys. The value of current which will pass across a capacitor or inductor is proportional to the speed of reversal (frequency) of the applied A.C. voltage, as well as to the value of the voltage itself. The apparent A.C. resistance is known as impedance, (measured as usual in ohms) and varies with frequency. Thus the impedance at 60 hertz (cycles per second) will be different from the impedance at, say 1000 hertz (approximate middle of the voice frequency). In the case of a capacitor, at 1000 hertz it will be much lower, in the case of an inductor, it will be much higher, than at 60 hertz.

3.7 DIODES AND RECTIFIERS

Diodes are one way valves or switches, like a non-return or flap valve in a water system. The valve on a car tire is a pneumatic example of a diode: one way only. And that's it.

With a car tire, a constant high pressure on the outside (such as from the garage compressor with a high pressure air tank) will cause a constant air exchange from the compressor tank into the tire (at least until the pressure inside the tire becomes the same as that within the compressor tank). If the tire valve was to be marked with a positive sign, it would be put on the inside of the tire - where the positive pressure is, and not on the outside air supply.

The same with diodes. The red (or ringed) end of the diode is where a positive voltage will remain once any driving voltage has been taken away. This marked end is sometimes called the cathode, from former vacuum tube days.

Back to car tires. Now try pumping it up with a foot pump. On the downstroke, the pressure outside the tire is higher and air flows into the tire. On the upstroke, the pressure in the pump reverses and a vacuum forms, but the valve holds firm until the pressure reverses back again on the next downstroke, and that is a fair representation of A.C. passing through a diode.

Now consider connecting four diodes in the form of a bridge, and applying A.C. as shown below.
When line A1 goes positive, electrons flow through D1 making B+ positive. Electrons also flow through D2 from A2 to B-. Now what happens on the "upstroke"? A1 goes negative and A2 goes positive. So electrons flow from DC= through D3 to A2, and return from A1 through D4 to B-. Thus, a bridge rectifier is an example of a full wave rectifier in which there is practically no time (except a short moment during reversal crossover) when electrons are not flowing in the circuit.

What is often not appreciated is that a bridge rectifier can be used equally well for DC. It doesn't matter which way round the battery is connected to terminals A1 and A2, B+ will always end up positive and B- negative. This is often used in telephone work so that line polarity (which reverses in many systems for signaling purposes) does not affect the operation of electronic circuits within the telephone itself.

Note that diodes are non-linear devices; that is, they also have a current-against-voltage characteristic which is also non-linear, especially at low voltages, but that is another matter.

3.8 AMPLIFIERS

To cover the field in any depth would fill a volume on its own. Therefore, the intention here is only to touch on the subject. Amplifiers are often used to amplify small AC signals (such as audio from microphones), the resulting large signal being capable of driving headphones or loudspeakers.
A basic circuit shown above will amplify the microphone input, which will appear at the "amplified output". A capacitor placed in the amplified output lead will block the DC voltage appearing there while allowing the amplified AC to pass through.

The above circuit is only one of many configurations possible.

A transistor is a relatively low impedance device. Like a water system, low head of water but very wide bore pipes. A special type of transistor is available which is generally a high impedance device - a higher head of water with narrower pipes to achieve a similar result. These are known as Field Effect Transistors or FET's. They are often found as microphone amplifiers, and as the first amplifier stages in radio receivers. Their operational effect is very similar to transistors, but their electrodes are normally renamed: Source, Drain, and Gate.

Transistors and FET's, and also integrated circuits (which are made up of hundreds of transistors, diodes and other components) are also non-linear devices. This should be appreciated when searching for any electronic device with a "Non-linear junction detector" which will indicate the presence of such devices even when they are switched off.

3.9 Further Information

These notes are intended only as an introduction to the knowledge of basic electronics needed to understand some of the technical implications encountered in Technical Surveillance Countermeasures.

Further information is contained in standard textbooks readily available from other sources.

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