SOLENOIDS

A solenoid is the term used for a heavy duty relay and like a relay acts as a switch saver. Solenoids are used to switch circuits that require a very large current to operate. An example of this type of circuit is the starter circuit. Consider the diagram below.

Since the starter circuit requires a lot of current to operate, the diagram above would be impractical. This is because the contacts of the ignition switch would need to be very large to carry the large current. Also the large cables required to carry the current would need to be very long.

By using a solenoid, small ignition switch contacts can be used and the length of cable can often be reduced. This will reduce the voltage drop in the circuit.

Voltage drop is the term used to describe a reduction of voltage between two points. e.g. between the ignition switch and the starter solenoid. Voltage drop is proportional to resistance. Long cables, small diameter cables and corroded connections all cause an increase in resistance, which creates voltage drop. To avoid voltage drop, manufacturers design circuits so that, the shortest route, is used by the largest diameter cable with the minimum number of connections. Consider the diagram below.

Like a relay a solenoid has a winding and contacts. When the ignition switch is turned to the cranking position, current flows through the winding and around the soft iron core. Unlike a relay the soft iron core is free to move. The magnetic effect of the winding pulls the iron core into the winding. This brings the moving and fixed contacts together which allows current to flow through the contacts and turn the starter motor.

When the ignition switch is moved away from the cranking position, current flow to the winding ceases. The return spring then pulls the moving contacts and iron core away from the winding. This breaks the current path to the starter so stopping the motor from turning.

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