A relay is simply a remote-control switch, which uses a small amount of current to control a large amount of current. A typical relay has a control circuit and a power circuit. The control circuit is fed current by the power source, and the current flows through a switch and an electromagnetic coil to ground. The power circuit is also fed current from the power source, and the current flows to an armature which can be attracted by the magnetic force on the coil.

In operation, when the control circuit switch is open, no current flows to the relay. The coil is not energized, the contacts are open, and no power goes to the load. When the control circuit switch is closed, however, current flows to the relay and energizes the coil. The resulting magnetic field pulls the armature down, closing the contacts and allowing power to the load.

Many relays are used on Toyotas for controlling high current in one circuit with low current in another circuit. The relay control circuit can be switched from the power supply side or, more common in Toyotas, from the ground side.

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