What is a relay circuit?

A relay circuit is typically a smaller switch or device which drives (opens/closes) an electric switch that is capable of carrying much larger current amounts. Or a circuit which operates the coil or electronic actuator from one source and uses a separate power source to drive an isolated device.

Answer

Generally speaking, a relay circuit is a circuit that uses a small mechanical switch or a semiconductor device (with associated circuitry) to energize a relay, which will then close a contact set to complete another circuit. This system is used by most people on a daily basis, and it is used to start a motor vehicle. The key switch (ignition switch) is turned to "start" and 12 volts (approximately) is applied to the starter solenoid (which is a big relay). The coil is energized, it shuts contacts, and the battery voltage is delivered through the heavy contact set (for high current capacity) to the starter motor. There are variations on this theme to which the term relay circuit can be applied, but the idea remains the same: a small switch of some kind controls switching in another (usually higher voltage and/or current) circuit. It could be argued that the telegraph is a relay circuit. Remember those old westerns? When a telegraph key is pushed down (thus completing the circuit), a remote (relay) coil is energized. The magnetic field created by energizing that coil pulls down an armature with the objective being to make a "click" instead of it being to close some electrical contacts. An early and dramatic application of the simple relay circuit, the telegraph, yes?

First answer by Quirkyquantummechanic. Last edit by Quirkyquantummechanic. Contributor trust: 840 [recommend contributor]. Question popularity: 43 [recommend question]