Resistors are components which restrict or resist the flow of current. The ability of a material or component to resist current flow is measured in ohms. There are three main types of resistor:

  1. Fixed resistors
  2. Variable resistors
  3. Special resistors

Fixed resistors

These are the most common type of resistor. They are found in nearly every electronic circuit. Their three most important uses are:

      • Protecting other components (such as an LED) from damage by too much current.
      • A light-emitting diode (LED) protected by a fixed resistor
      • As potential dividers (or voltage dividers). A fixed resistor is used to split voltage between different parts of the circuit. Potential dividers are used, for example, with LDRs in circuits which detect changes in light.
A circuit diagram for a temperature detector
      • A circuit diagram for a temperature detector
      • In timing applications. In this role a fixed resistor is used with a capacitor in series.
A circuit diagram for a timing application
      • A circuit diagram for a timing application

Variable resistors or potentiometers

There are two types of variable resistor:

  • The first type of variable resistor can be altered continually as they work. For example the volume control in a radio.
  • The second type is called a pre-set potentiometer. It has a resistance control that is adjusted and then fixed. These resistors would normally be adjusted once only. You may have had to calibrate a pH meter, in this you were adjusting the value of the pot.

The main difference between the two types of Potentiometers is their size. The pre-set potentiometers tend to be smaller and are usually adjusted with a screwdriver. A variable resistor is generally provided with a long spindle onto which an operating knob is 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) have a resistance which changes in response to changes in light levels, as 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.