What Is a Solenoid Switch?

by Pauline Gill

Solenoid switches are used to switch high power circuits on and off using a much smaller electrical control signal to actuate the switching. This allows extensive logic and decision making circuitry to be performed on inexpensive microchips and small electronic parts, with the actual switching of the high power signals being limited to the very last step. The result is cooler running of less sophisticated equipment. It also enables the high power switching equipment to be limited to a remote location. Solenoid switches are typically used on automotive engine starting systems.

Solenoid Switch Operation

Solenoids are wound wire magnetic coils with an open core to receive a sliding cylindrical plunger. When the coil is energized with an electrical current, a magnetic field is created in the hollow opening which pulls the plunger into it or pushes it out, based on the orientation of the solenoid and the poles of the plunger. The plunger is mechanically connected to a set of switch contacts, which perform the high power switching. The there are a minimum of four connector terminals on a solenoid switch. The coil requires two and is most often isolated from all the other terminals, which keeps the coil wires completely independent of the current being switched. The switched current terminals are usually substantially heavier than the coil terminals.

Switch Types

Most solenoid switches have only one switched pole due to the amount of current being passed through them. Some are only momentarily operated, such as is the case with starter solenoids on automobiles. Once the automotive engine is started, the starter motor and solenoid are completely isolated from the operation of the rest of the electrical system. Some non-inertial automotive starting systems also use the moving plunger to slide the starter pinion along the starter shaft to engage the flywheel, in addition to actually powering the starter motor.

Solenoid Switches Versus Relays

Whereas solenoids impart substantial motion to their plungers, either for switches or valves, the coils of electrical relays are wound around a ferrous magnetic post which becomes magnetized and attracts a steel plate across a small gap to close a set of electrical contacts. These contacts may switch more than one pole and are commonly used to switch AC line currents at 120, 240 or 480 volts, although relays are available for both AC and DC switching and any voltage range. Solenoid switches perform both electrical switching and coordinated mechanical motion.

Solenoid Switch Applications

Besides wide scale engine starting applications, solenoid switches are used to switch on many other types of motors while mechanically engaging or disengaging their shafts. This allows latching and opening mechanisms for windows, doors and hatches to derive two functions from the same piece of coordinated equipment.

Engage Then Turn On

Since solenoid switches typically place the switch contacts at the very end of the plunger stroke, they act as self contained logic interlocks, not allowing a motor to start or a valve to open until the plunger has moved its entire distance first.

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