The resistor is a component commonly used in electrical engineering and electronics. It has two terminals, and may be connected either way round within a circuit. The purpose of a resistor is simply to resist the flow of electrical current. Resistors have many applications, and can be found in virtually all electrical devices and electronic circuits. Most of the resistors found in circuits have a fixed resistance value and behave in a manner that is consistent with Ohm's Law. This means that the current flowing through the resistor will vary in direct proportion to the voltage applied across it. There are a number of different types of resistor, made from many different materials. Some of the different types of resistor available are described below. Resistors can vary in value from the very small (a fraction of an ohm) up to very large values (several million ohms). A typical resistor, together with the symbols commonly used to represent resistors in circuit diagrams, are illustrated below.
A typical resistor and its circuit symbols
Carbon composition resistors
This type of resistor was widely used during the 1960s and consists of a solid cylindrical core made up from a mixture of powdered carbon and an insulating (usually ceramic) material, within which are embedded connecting leads. The core materials are bonded with resin, and the ratio of the two materials within the core determines its resistance value. The core is typically surrounded by an insulating layer of plastic or enamel. Carbon composition resistors, although fairly reliable, had quite broad tolerances and their resistance could change if they were subjected to higher than expected voltages. They have now largely been replaced by other types of resistor that offer smaller tolerances and maintain their resistance value when subjected to voltages that are higher than those indicated by their design specification.
Carbon film resistors
In this type of resistor, a carbon film is deposited on an insulating material (such as a ceramic rod) and a spiral is cut into it in such a way as to create a long and narrow (and hence resistive) conducting pathway. Resistance values for carbon film resistors vary from 1 ohm to 10 million ohms, and maximum working voltages can range from 200 to 600 volts. Similar techniques can be used to produce resistors (metal film resistors, for example) that employ other resistive materials and can be manufactured to exhibit very small tolerances and excellent performance characteristics. Carbon film resistors are relatively cheap to manufacture, however, and can be produced with tolerances of either 10% or 5%. This makes them both affordable and generally suitable for most common applications.
Wire-wound resistors are usually constructed using thin metal wire wound around a non-conducting core (usually ceramic of plastic). Each end of the wire is soldered to a metal cap on each end of the core, and the assembly is coated with a layer of plastic or enamel. Wire-wound resistors generally can carry more current than other types of resistor, and those designed for high-power applications may have an additional outer layer made from aluminium or some kind of ceramic material. The method used for winding the wire around the core material are often designed to reduce the electromagnetic effects produced by the coil when carrying current, since these can be detrimental for some applications. A wire-wound resistor sometimes has an adjustable slider to enable the resistance value to be changed. Such a resistor is known (for obvious reasons) as a variable resistor.
Resistor power rating
Electrical power (P) is measured in Watts, and is the product of voltage and current (P = VI). Electrical power is often dissipated by an electrical circuit element (such as a resistor) in the form of heat. As current flows through a component, it will get warm because some of the electrical energy is converted into heat energy. As long as the heat can be lost to the surroundings at the same rate, the resistor should not overheat and suffer damage. The power rating of a resistor is an indication of how much power (in the form of heat) the resistor can safely dissipate. Generally speaking, the larger the surface area of the resistor, the more heat it can dissipate. Since power is directly proportional to current, a resistor with a high power rating will be able to carry more current than one with a lower power rating without suffering any adverse effects. The resistors most commonly used in electronic applications have power rating of either 0.25 W or 0.5 W. Higher power ratings are generally required only when the resistor values are relatively low (i.e. less than 300 Ω) or the applied voltage is relatively high (i.e. greater than 15 V).
Resistor colour coding
The resistance value of most common types of resistor is identified by coloured bands that are painted onto the body of the resistor. The colour of the first and second bands will yield a two-digit number, while a third band identifies a power of ten by which the two-digit number must be multiplied to obtain its resistance value in ohms. A fourth band, if present, identifies the tolerance within which the resistor's nominal value is deemed to be accurate. Gold indicates a tolerance of plus or minus 5%, while silver indicates a tolerance of plus or minus 10%. The absence of a band normally indicates a tolerance of 20%.The tables below illustrates the values represented by each colour.
In the resistor illustrated below, the first band is orange, indicating that the first digit is 3. The second band is black, indicating that the second digit is 0. A red band in the third position tells us we have to multiply the number obtained (30) by 100, giving a nominal resistance value of 3,000 Ω (or 3 kΩ). The gold band in the fourth position indicates a tolerance of 5%, which means that the actual value of the resistor can be anywhere from 2.85 kΩ up to 3.15 kΩ.
This colour coding indicates a nominal resistor value of 3 kΩ
Note that gold and silver are used in the third band when resistor values of less than 10 Ω are to be shown, and indicate multiplier values of 0.1 and 0.01 respectively. A resistor with red, violet and gold as the first three colours, for example, would therefore indicate a nominal resistance value of 27 × 0.1 = 2.7 Ω.
Standard resistor values
Standard resistors are not manufactured at every possible resistance value. This would be impractical, not to mention uneconomical, since the range of resistances used in electrical and electronic circuits is so broad. It would also be somewhat pointless, since the tolerances allowed for standard resistor types mean that they can vary across a known range of values around their nominal value. For this reason, several series of standard values have evolved. Each series has a fixed number of standard values, expressed as a two digit number multiplied by a power of ten. Each series is based on a specific tolerance, and the number of two-digit values used increases as the tolerances get smaller, with values carefully chosen so that there is a (very small) overlap between the range of possible values for one nominal value and the next in the series.
For example, the E6 series is used for resistors with a tolerance of 20%. The two-digit values used in this series are 10, 15, 22, 33, 47, and 68. Notice that the interval between each successive value becomes successively larger, because of the increasingly large potential variation in actual values. Consider that the next value in the series will be 100 (10 × 10). At the lower end of its possible range of values, a 100 Ω resistor with a 20% tolerance could have an actual value as low as 80 Ω. The 68 Ω resistor, on the other hand, could have an actual value as high as 81.6 Ω (68 Ω + 20%). You can perhaps see, therefore, why no intermediate values are manufactured for this series, since there is already a small overlap between the two possible value ranges. If it is necessary to restrict the range of possible values in a particular application, a resistor with a smaller tolerance should be chosen. The standard values available for the E6, E12 and E24 series of resistors are given in the tables below.