All about resistors


What is a Resistor?

A resistor is an electrical component that works on a passive two terminal system. The role of a resistor is to limit or restrict the flow of an electrical current into a circuit. Resistors can also be used to adjust signal levels, terminate transmission lines and divide voltages.

There are three main types of resistors; fixed, special and variable resistors. Fixed resistors are the most commonly used type of resistor and have three vital uses. Their first use is protecting other components from high currents that could damage them. Another use is to control time delays by working with a capacitator to control this. Finally, fixed resistors can also divide voltage into different parts of a circuit.

A special resistor is a component with a particular use. For example, light-dependent resistors use a photo-sensitive plates to detect light changes and therefore change the resistance levels dependent on this. Another type of special resistor is a thermistor, which are used in temperature sensing circuits. Thermistors contain negative temperature coefficients that increase the temperature as resistance falls.

A variable resistor comes in two forms; a resistor and a potentiometer. A variable resistor alters continually as it works whereas potentiometers control and fix resistance once adjusted. A way to distinguish the two of these is by their size. Potentiometers are usually smaller than resistors which tend to have a long spindle attached to an operating knob.

What are Ohms?

To measure the ability of a components resistance of an electrical flow we use ohms. Ohms are the unit of resistance. We use the letter ‘R’ to symbolise an ohm. The way we set this out is placing the ’R’ after a number which symbolises that the number is more than one i.e. 2R is equal to 2 Ohms. To symbolise a number is less than one we place it after the ‘R’ i.e. 2R5 is equal to 2.5 Ohms and R25 is equal to 0.25 Ohms.

What are Resistance Values?

All resistance values are symbolised in a series of colours. Each colour stands for a different unit which aids in distinguishing what units the resister holds when marked on the component.

There are four bands in total. The first band indicates tens and the second band is the units. The band colours represent difference units from 0 to 9, for example orange is 3 and blue is 6. So, a sequence of orange and blue would represent 38.

The third band is what you multiply the result of the first two bands by. The colour here also represents units e.g. silver equals 0.01 and red is 100. So here if the third band was red then the resistance would be 3.8 kilo-ohms.

Finally, the forth band shows the tolerance of the resistor. Resistor manufacturers can not totally guarantee that the first 3 bands on the component are the current resistance level. Therefore, the tolerance band gives a percentage of which the resistance can be higher or lower. A red band indicates a 2 % tolerance, a gold is 5 % and a silver is a 10 % tolerance. For example, a band with a resistance of 100 ohms and a tolerance of 5 % would have a resistance level between 95 and 105 ohms.