Protecting components

Resistors can be used to protect other components (such as an LED) from damage by too much current.

Potential dividers

In a potential divider, a fixed resistor is used to split voltage between different parts of the circuit. Potential dividers (or voltage dividers) are used, for example, with LDRs in circuits which detect changes in light.

Timing applications

A fixed resistor can be used in series with a capacitor to control a time delay.

Variable resistors or potentiometers

There are two types of variable resistor:

· Variable resistors, which are altered continually as they work: eg for volume control in a radio.

· Pre-set potentiometers, which have a resistance control that is adjusted and then fixed. These resistors would normally be adjusted once only.

The main difference between the two types of variable resistor is their size. Pre-set potentiometers tend to be smaller and are usually adjusted with a screwdriver. A variable resistor has a long spindle with an operating knob attached.

Special resistors

Thermistors change resistance as temperatures change. Most thermistors have a negative temperature coefficient, meaning their resistance falls as temperature increases. Thermistors are used in temperature-sensing circuits.

Light-dependent resistors (LDRs) change resistance as light levels change. The light levels are detected by a photo-sensitive plate on the resistor. Most LDRs have a negative light coefficient, meaning that their resistance falls as the amount of light falling on them increases. LDRs are used in light-detection circuits.

Ohms and resistance values

Ohm

The ohm is the unit of resistance. Larger values are measured in kilo-ohms (1000 ohms) and mega-ohms (1,000,000 ohms). Resistors are marked, using a code specified in British Standard 1852, as follows:

· The letter R means ohm. Numbers coming before the R indicate a value more than one. So 1R (or 1R0) = 1 ohm; 47R = 47 ohms; and 4R7 = 4.7 ohms. Numbers coming after the R indicate a value less than one: so R56 = 0.56 ohms.

· The letter K means kilo-ohm. Numbers coming before the K indicate a value more than one, while numbers coming after the K indicate a value less than one. So 1K8 = 1.8 kilo-ohms and 5K6 = 5.6 kilo-ohms.

· The letter M means mega-ohm. Numbers coming before the M indicate a value of more than one, while numbers after the M indicate a value less than one. So 2M = 2 mega-ohms, and 2M2 = 2.2 mega-ohms.

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