Electromagnets, Solenoids and Relays

Electromagnets, Solenoids and Relays


In the field of physics any coil of wire used for its electromagnetic properties is called a solenoid. However, in electronics and mechanics, the term "solenoid" is used for an electromagnetic actuator.

A Solenoid

An electromagnetic solenoid. When current flows through the coil the magnetic field pulls the armature into the coil.

A solenoid is used in a car to engage the starter motor with the engine. Solenoids may also be used to operate valves, coin returns in vending machines or any other mechanical device that needs to be operated electrically. The armature of a solenoid may be based on a permanent magnet. If so it can either be pulled or pushed depending on the polarity of the voltage connected to the coil.


A solenoid-operated switch is called a relay. A typical relay has a hinged armature connected to switch contacts.

A Relay
A relay. The coil to the right is an electromagnet that operates the switch to the left.
Relay Symbol
Schematic symbol for an SPDT relay. This symbol indicates the off position. When the coil is energized the switch will move to the bottom contact.

Relays are usually used to allow a small current to switch a much larger current. For example, the “solenoid” of a car is actually a relay.1 It takes far more current to operate a starter motor than the key switch can handle. Therefore, the key switch energizes the electromagnet of the solenoid/relay, which can easily handle the 500 amperes or more that it takes to crank an engine. Relays are also used for remote operation of many switches at one time. One small switch or other driver circuit operates the relay coil where the relay has several switches that work in unison. There are nearly as many types of relays as there are hand-operated switches. The simplest is a single-pole-single-throw. The schematic symbol for a relay is simply an inductor near a switch. Sometimes, as in the example above, the switch contacts are drawn in a style that resembles the mechanical operation of the relay.

Electromagnets, Solenoids and Relays