Resistors are passive components that reduce, resist or moderate the amount of electrical current running through a circuit. This is because some components are unable to function at certain levels of voltage. The higher the value of the resistor, the less current there is available for a given voltage.

Resistors also act to limit temperature coefficient, noise and inductance. They work according to Ohm's Law - the physical theory which states that the current through a conductor between two points is inversely proportional to the resistance across the two points. As a formula it is written as V=IR (where V is Voltage, I is Current and R resistance).

Resistance is measured in Ohms (O). Since 1 ohm is very small, most resistor values are written as 'K' (1000O) or 'M' (1000000O). Carbon film and metal film are the most popular resistor materials, and resistors are available in fixed or variable values. The kind of resistor you need will not only depend on the difference between the voltage supplied and the voltage that powers the circuit. Tolerance and the power rating of the resistor are other factors which need to be taken into account.

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