Electricity > Variable Resistor

Variable resistors consist of a track which is made from a coil of wire, carbon or a ceramic and metal mixture and a wiper which touches the track at one point. The wiper can be moved along the track to create more or less resistance. Imagine the track is a metal spring connected into a circuit. Because the spring consists of a lot of wire, it acts as a resistor. If you disconnect one end of the spring and touch the middle of the spring with the lead, it halves the amount of resistance the spring is applying.

Variable resistors, usually in a straight format with two connectors are called Rheostats. Many schools have large rheostats that can be used with labpacks to vary the current. This is the simplest type of variable resistor as it has only two terminals, one at each end of the track and a moving wiper which can travel along its length. The standard symbol for a rheostat is below:

Fig 1: Rheostat symbol

Electronics suppliers usually sell variable resistors as 'potentiometers' or 'pots' for short. They are rated like resistors but their listed value in ohms is their maximum resistance. They are available in a linear or dial type and usually have three terminals see diagram:

Fig 2: Potentiometer

The standard symbol for a potentiometer is below:

Fig 3: Potentiometer symbol

Presets are small potentiometers which are 'preset' to a particular resistance. They are used to initially set up an alarm level, light level or similar. They can be found on MFA boards where they are used to adjust light sensitivity of LDRs. They are normally soldered directly onto circuit boards. The symbol for a preset:

Fig 4: Preset symbol