There are two main reasons for using resistors:
1) A resistor in a circuit effectively turns a change in current into a change of voltage. For example, in a voltage amplifier, the transistor (or valve) changes its collector current (or anode current) in response to a change to its input, (base or grid respectively). This change in current is converted into a change of voltage by the output load resistor.
2) A resistor controls the current or voltage supplied to another device. For example, the base current of a transistor sets the operating point of the input/output characteristic of the transistor. In turn the base current is set to its optimum by using the correct value(s) of resistor(s) in the base circuit.
There is also a host of subsidiary reasons for using them, such as safely dissipating RF power which it is not required to radiate, as in a “Matched” or “dummy” load. They are also used for providing a matched load for infinitesimally small amounts of power on the unused output ports of some mixers, (where they would otherwise result in unwanted mixer product frequencies), and in directional coupler “isolation arms”, etc. Unfortunately, there is no such thing as a pure resistor, i.e. one which does not also have a small amount of unwanted or “parasitic” inductance and capacitance. In many cases this does not matter but in others it does. An example of where it does matter is in a high power matched load where one is adjusting the circuit for an exact match, i.e. a VSWR of 1:1 into a specified load resistance, (usually 50 Ohms). In this and some other respects, a resistor of precisely specified value must be used in addition to its having sufficient power dissipation.
Resistors used in circuits are usually either wire ended or designed for “surface mounting”. As far as power dissipation is concerned, size matters. Wire ended resistors ¼ inch long are usually rated at ⅛Watt; those about ½ inch long are usually rated at ¼ or ½ W, while those ¾ inch long or more will dissipate 1 to 2W. The resistive material used is discussed in more detail later. There is usually a maximum voltage rating of 150V for small resistors, and 250V for medium-sized ones, regardless of power. Resistors using metal wire as the resistive material usually have higher power ratings, (often printed on the resistor), and they can get very hot at full power. They can have sizes up to several inches in length. Their resistance tends to increase with temperature. Resistors using a carbon composite as the resistive material show a decrease in resistance as they get hotter.
Apart from precision resistors designed mainly for instrumentation purposes, resistors, are made to three main value specifications, E6, E12 and E24. The “E” number determines the number of “steps” or “intervals” there are between each value and the next. In this, “Exponent” system, the size of adjacent steps is a constant ratio within each series, rounded to two significant figures It also determines the precision with which the resistor’s nominal value is realised. For example; the “E6 series” resistors have a ratio of 1:1.5 between adjacent values, so values between 10 and 100 Ohms are: 10, 15, 22, 33, 47, and 68 Ohms. and similarly for other decades. These resistors have a “tolerance” of ±20% so an “E6” resistor with a nominal value of, say, 10Ω may have an actual value between 8Ω and 12Ω. Similarly, an E6 resistor of nominal value 15Ω may have a value of between 12Ω and 18Ω etc.
The E12 series has a ratio of 1:1.2 between steps and a tolerance of ±10%, so values within the decade are: 10, 12, 15, 18, 22, 27, 33, 39, 47, 56, 68, and 82. The E24 series has a tolerance of ±5%, etc.
Surface mount resistors are frequently too small to be marked, and it is best either to measure each one before inserting it into a circuit, or to rely on what is printed on the packet.
Value at a glance
The nominal value of a wire ended resistor is usually marked with a series of coloured rings to be read from one end. In the “colour code” the first two rings indicate digits and the third is a multiplier. The colours in the sequence 0 to 9 are:- black, brown, red, orange, yellow, green, blue, violet, grey, white. A fourth ring of gold or silver indicates a tolerance of ±5 per cent or ±10 per cent respectively. Where no fourth ring exists the tolerance is ± 20 per cent. Other codes are used by some manufacturers, but codes not conforming to the above colour code are unreliable and it is best not to assume a value from a non-standard code, but to measure the resistance before inserting the resistor into a circuit.
Types of construction
In principle, any conductive material which is made long enough and/or thin enough can be made into any value of resistance. However, to pack a certain value of resistance into a small space with other desirable characteristics requires carefully selected materials. At one time nearly all resistors were composed of a mixture of carbon and a clay binder which was fired into a hard brick-like substance in the form of a rod with wrapped wire connections at its ends. Carbon based resistors are still made but they have a sizable negative temperature coefficient. They were superseded, first by metal film resistors in which metal was deposited on a ceramic tube or rod, then by metal oxide resistors, and finally by a new metal film technique which has a lower temperature coefficient and is more stable with age than its predecessors. The metal film is sometimes cut with a spiral groove to increase its resistance. In the early days of radio, the graphite-clay mixture used in pencil lead was sometimes used by amateurs; whole lengths of pencil lead for low value resistors for dropping filament voltage to its correct value, and pencil scribble on paper for high value resistors as used for “grid leaks”. The author has used the former technique in his “teen-age” years.
Wire wound resistors use Nichrome wire, (because it has an almost zero temperature coefficient), wound into a coil onto a ceramic tube These are often coated with a vitreous enamel. They are suitable for powers up to several tens of Watts, but their main disadvantage is that they have significant inductance which may be relevant at radio frequencies.
There are many special types of resistor. A voltage dependent resistor has a resistance which falls rapidly as the voltage across it reaches some critical level It is used to absorb voltage spikes. A Thermistor has a resistance when hot which is at least an order of magnitude larger or smaller, (depending on the type), than its resistance when cold. Some thermistors are designed to be heated by a component whose temperature is to be measured, while others are self heating to provide a “soft start” to equipment.
Variable resistors are usually constructed as Potentiometers (“pots”,) and low power ones, (½W power dissipation), are usually of carbon film construction. The resistance variation is accomplished by rotation of a spindle or by a “slider”. The log-law, (logarithmic), potentiometer has a graduated carbon film, so that on rotating the spindle or moving the slider, the resistance increases very slowly at first but rapidly towards the end of the movement. This type is often used as a volume control, because the ear responds logarithmically to sound power. Wire-wound potentiometers can be obtained for powers up to 50W. Miniature “pre-set” potentiometers are often mounted on circuit boards. Some types of potentiometer are capable of 3600 rotation and are used for rotation sensing in servo systems such as beam aerial rotators, and remote tuning units, while others are capable of many turns for high precision of setting resistance.
Resistors are usually very reliable components, usually lasting for many years or tens of years, but, as they are used in the largest numbers compared to other components in many circuits their failure is one of the commonest causes of equipment failure. When replacing them when repairing circuits, care should be taken as they can be damaged and have their resistance changed by too long an exposure to the heat of a soldering iron. A future article will take a brief look at inductors and capacitors.