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:
- Fixed resistors
- Variable resistors
- Special 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
- In timing applications. In this role a fixed resistor is used with a capacitor in series.
- 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.
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.