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Resistors



Jump to: Colour Coding Power Rating Voltage Rating Types of Resistors Effect of Age Effect of High Frequencies

Resistors, as their name implies, are designed to provide some desirable, or necessary, amount of resistance to current flow in a circuit. They can also be used to 'drop' voltage. As such they are the main elements used in circuit design to arrive at the desired current flows and voltages which work the circuit. Resistors do not generate electrical energy, but merely absorb it, which the resistor dissipates in the form of heat. The performance of a resistor is not affected by frequency, so it behaves the same way in both dc and ac circuits.

Resistors are specified by 1. resistance value in ohms; 2. tolerance as a percentage of the nominal value; and 3. power rating in watts. They are also categorized by the type of construction.

Resistance value and tolerance is normally indicated by a colour code consisting of four coloured rings, starting at or close to one end.
These are read as follows:
1st ring gives first digit.
2nd ring gives second digit.
3rd ring gives number of noughts(zeros) to put after first two didits.

The Universally adopted colour coding is:


The fourth ring gives the tolerance, viz: Absence of a fourth ring implies a tolerance of 20%.

Certain types of modern resistors of larger physical size may have letters and numbers indicate the numerical value and the following letter the multiplier, where:
E = *1 (multiplied by one)
K = *1000 (multiplied by thousand)
M = *1000000

A second letter then gives the tolerance:

M = 20% tolerance either side of the nominal value.
K = 10% tolerance either side of the nominal value.
J = 5% tolerance either side of the nominal value.
H = 2.5% tolerance either side of the nominal value.
G = 2% tolerance either side of the nominal value.
F = 1% tolerance either side of the nominal value.

The actual range of (nominal) resistance values to which resisistors are made is based on steps which give an approximately constant percentage change in resistance from one value to the next - not simple arithmetical steps like 1,2,3 etc. These are based on the preferred numbers:
1, 1.2, 1.5, 1.8, 2.2, 2.7, 3.3, 4.7, 5.6, 6.8, 8.2, 10, 12, 15, 18 etc

Thus, for example, a typical range of resistor values would be:
10, 12, 15, 18, 22, 27, 33, 39, 47, 56, 68, 82, and 100 ohms.
120, 150, 180, 220, 270, 330, 390, 470, 560, 680 and 820 ohms.
1, 1.2, 1.5, 1.8, 2.2, 2.7, 3.3, 3.9, 4.7, 5.6, 6.8, and 8.2 Kiloohms.
10, 12 etc Kilo ohms
1, 1.2 etc Kilo ohms

As regards tolerances, as a general rule resistors with a 10 percent tolerance are suitable for average circuit use. The actual resistance value of, say, a 1 kohm resistor would then be anything between 900 and 1100 ohms. For more critical work, such as radio circuits, resistors with a 5 percent tolerance would be preferred. Closer tolerances are not normally required, except for very critical circuits.

Power Rating:

The physical size (or shape) of a resistor is no clue to its resistance value, but can be a rough guide to its power rating. Physical sizes range from about 4mm long by 1mm diameter up to about 50mm long and 6mm or more diameter. The former would probably have a power rating of 1/20th watt and the latter possibly 10 watts. More specifiacally, however, the power rating is related to type as well as size. A general rule which does not apply to power rating, however is that whilst this figure nominally represents a safe maximum which the resistor can tolerate without damage, it is usually best to operate a resistor well below its power rating - say at 50 percent rating - particularly if components are crowded on a circuit, or the circuit is enclosed in a case with little or no ventilation.

Voltage Rating:

Maximum operating voltage may also be specified, but since this is usually of the order of 250 volts or more, this parameter is not important when choosing resistors for battery circuits. Resistors used on mains circuits must, however, have a suitable voltage rating.

Types of Construction:

Resistor types classified by construction are:

1. Carbon Resistors


(also called carbon-composition, moulded-carbon and carbon rod). These are in the form of a small rod moulded from carbon and a binder, with wire connections at each end. The rod is usually protected with a paper or ceramic sleeve, or a lacquer coating. These are the most common (and cheapest) type of resistor, generally available in 1/8, 1/4, 1/2, 1 and 2 watt ratings.
It is a general characteristic of carbon resistors that their value remains stable at normal temperatures, but above 60 degree centigrade their resistance increases rapidly with increasing temperature.

2. Carbon-film resistors


(also known as high-stability carbon resistors). These are manufactured by depositing a thin film of carbon on a small ceramic rod. The rod is fitted with metal end caps, to which wire leads are attached. The body of the resistor is usually protected by a varnish, paint or silicone resin coating, but some types may be encased in a ceramic, plastic or glass outer coating.
Carbon-film resistors are little affected by temperature changes (their stability is usually better than 1 percent) and are also characterized by low 'noise'. They are available in sub-miniature sizes (1/20 and 1/10 watt power rating); and in larger sizes up to 1 watt power rating. They are a preferred type for radio circuits, Particularly as they have frequency characteristics.

3. Metal-film resistors


These are made by depositing a metallic film (usually nickel-chromium) on a glass or ceramic rod. A helical track is then cut in the film to produce the required resistance value. Metallic end caps are then fitted, carrying the wire leads, and the body protected by a lacquer, paint or plastic coating. Stability characteristics are similar to those of carbon-film resistors, but they are more expensive. They are generally produced in miniature sizes with power ratings from 1/10 watt upwards.

4. Metal-oxide film resistors


Constuction is similar to that of a metal-film resistor except that the coating used is a metallic oxide (usually tin oxide), subsequently covered with a heat-resistant coating. This type of resistor is virtually proof against accidental overheating (e.g. when making soldered connections) and is also not affected by damp. Stability is very high (better than 1 percent), and the power ratings are high for their physical size.

5. Metal-Glaze resistors


In this type the resistive deposited on the rod is a cermet (metal-ceramic); otherwise construction is similar to metal-film resistors.
film-resistors may also be classified as thick-film or thin-film. As a general rule, individual resistors of this type are thick-film. Thick-film resistors are also made in groups on a small substrate and encapsulated in integrated circuit 'chips'. Thin-film resistors are made in a similar way, but on a considerably smaller scale for use in the manufacture of integrated circuits.

Effect of Age:

All resistors can be expected to undergo a change in resistance with age. This is most marked in the case of carbon-composition resistors where the change may be as much as 20 percent in a year or so's use. In the case of carbon-film and metallic film resistors, the change is seldom likely to be more than a few percent.

Effect of High Frequencies:

The genral effect of increasing frequency in ac circuits is to decrease the apparent value of the resistor, and the higher the resistor value the greater this change is likely to be. This effect is most marked with carbon-composition and wire-wound resistors (see below). Carbon-film and metal-film resitors all have stable high frequency characteristics.

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