Resistors are passive two-terminal electrical components that affect electrical resistance in a circuit element. In electronic circuits, resistors are used to reduce current flow, adjust signal levels, divide voltages, bias active elements, and terminate transmission lines, but they also have other applications.
There are numerous types of Resistor used in electronic circuits. Resistors either control the flow of current or produce a voltage drop. In order to do this, a resistor needs to have some form of ‘resistive’ or ‘resistance’ value. Therefore, resistors are available in a large range of resistance values from fractions of an Ohm (Ω) to millions of Ohms.
When the body of the resistor is big enough to read the print, such as power resistors, Resistance value, tolerance, and wattage rating are generally printed onto the body of the resistor in the form of numbers or letters. But most resistors are small, these specifications are shown as coloured bands that indicate their resistive and tolerance values along with the physical size of the resistor indicating its wattage rating.
Calculating Resistor Values
The ‘left-hand’, most important coloured band is the band that is nearest to a connecting lead with the colour coded bands being read from left-to-right as follows:
Digit, Digit, Multiplier = Colour, Colour x 10 colour in Ohm’s (Ω)
For example, a resistor has the following-coloured markings.
The fourth and fifth bands are used to determine the percentage tolerance of the resistor. Resistor tolerance is a measure of the resistor’s variation from the specified resistive value and is a result of the manufacturing process and conveyed as a percentage of its ‘nominal’ or preferred value.
Typical resistor tolerances for film resistors range from 1-10% whereas carbon resistors have tolerances up to 20%. Resistors with tolerances lower than 2% are called precision resistors with lower tolerance resistors, thereby being more expensive.
The majority of five band resistors are precision resistors with tolerances of either 1-2% while most of the four band resistors have tolerances of 5%, 10% or 20%. The colour code used to denote the tolerance rating of a resistor is given as one of the following:
If the resistor has no fourth tolerance band then the default tolerance would be 20%.