Resistors

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 [ohms: units of electrical resistance, usually shown by the symbol R. 1 volt will force a current of 1 amp through a resistance of 1 ohm. ]. There are three main types of resistor:

• Fixed resistors
• Variable resistors, and
• Special resistors, such as thermistors and light-dependant resistors (LDRs)

Fixed resistors

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

• 

  A light-emitting diode (LED) protected by a fixed resistor

  Protecting other components (such as an LED) from damage by too much current.

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  A circuit diagram for a temperature detector

  As potential dividers [potential dividers: components which split a circuit's voltage into two. Potential dividers consist of two resistors in series. ] (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 timing application

  In timing applications. In this role a fixed resistor is used with a capacitor in series.

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.

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.