Home > Circuit Bricks > Solenoid
UPDATED:.21:13 21 October 2013


A solenoid is basically a coil of wire (electro-magnet) with a plunger in the middle that moves quickly inwards when a voltage is applied to the coil.

A solenoid is a coil with a plunger

This is a miniature PCB
mount type

They can be used to activate all kinds of things - like locks and water valves

Since a solenoid is an electromagnet, it produces EMF which can damage any electronic devices driving it. You must use a silicon diode D1 (1N4001 recommended) to absorb the EMF, connected in reverse bias. (in reverse).

Operating voltages range from 6 to 24 volts. Generally the higher the voltage, the more powerful the solenoid is.


A solenoid generally draws quite a large amount of current, from about 0.5 Amps up to 5Amps or more
Below are 2 recommended methods of driving them.

Method 1
Method 2

  • A Darlington transistor Q1 is used as a driver.
  • Must have diode protection across the solenoid.
  • It is advisable to use a heatsink for the transistor as it could get hot.
  • A transistor driving a relay - again a relay produces EMF as it has an electromagnet so use a protection diode.
  • Suitable for general on/off switching
Written by Phil Townshend - 2008
www.edutek.ltd.uk - Working Electronics For Students & Teachers