Solenoids

What is solenoid

Solenoid is a package of insulated wire and a plunger inside the coil. A solenoid is working on electromagnetic effect of electricity. When power is provided to the solenoid coil, it attracts or repel the metal or magnet core inside the coil which is known as plunger. The plunger is normally spring loaded. When power is applied to the coil, the plunger is attracted inside the coil and it compresses the spring. The other end of the solenoid is attached to the mechanism to be attracted or actuated. Then when power is cut off from the solenoid, the electromagnet coil loses its magnetism and the energy stored in the spring pushes out the plunger. The principle of a solenoid is somewhat like a relay. But in relay there are contacts attached to the armature whereas in solenoid the plunger is attached to the mechanism to be actuated.

In automotive the solenoid is used in several circuits like in door locks, self-starter, trunk lock, egr system, evap control system, automatic transmission and many more. The solenoid is controlled through a relay or sometimes it is driven by a transistor somewhere in the body control unit or ecu. When a solenoid is powered up or driven by a transistor, its one end is connected to battery positive through a fuse and the other end of the solenoid is connected to the driver transistor collector or drain in case of a mosfet transistor. When signal is applied to the base or gate of the transistor, the transistor conducts which causes current to flow in the solenoid and makes it working.

All solenoids used in automotive are normally 12v. They have different current or power rating. The current consumption or the power rating of a solenoid depends upon the size of the solenoid and the thickness of the wire used in solenoid coil. The bigger is solenoid, the thicker the wire used in its coil. The smaller the solenoid, the thinner the wire used in its coil.

Sometimes a diode is connected across the solenoid terminals. This diode is functioning like a flywheel, free wheel or damper diode. It damps the back emf produced in the solenoid when the solenoid is switched off. This is normally used when the solenoid is driven through a transistor. If this diode is not used the back emf can easily destroy the driving transistor.