The relay at rest. Heavy power can flow from 30 to 87a. The relay is not using any power, so it may as well be in a box on the shelf. A spring (not shown here) keeps the contacts from moving.
Horn and starter relays are usually rated for intermittent duty. The electric magnet coils will overheat and burn out unless allowed to cool.
In contrast, electric fans and fuel pumps use continuous duty relays which have magnet coils designed to run all the time.
A relay is a remote-control heavy duty switch. A very small switch in your vehicle can control a massive switch somewhere else.
The relay at work. A tiny current from a small switch circuit powers the coil, making it an electric magnet. The magnet makes the switch contacts move with a SNAP! Now heavy power can flow from terrminal 30 to 87.
This is why a small switch on a one amp circuit - like a horn button - can sound a 20 amp AH-OOGA horn on heavy wires.
Heavy wire is hard to run and expensive, plus it loses power over distance. Relays keep heavy wires shorter and provide maximum power.
Drawn by Jon Pardue 8-15-2006