What is a relay and why do I care?

A relay is Simply a switch that opens and closes under the control of another electrical circuit. There are many types and sizes available but for the examples found here we will be concerned with the 12v automotive style or bosch relays. In this form the switch is operated by an electromagnet to open or close one or more sets of contacts.

Relays are rated with contacts capable of switching greater loads than a standard toggle switch, thus we can use them to power lights, fans, motors & pumps that would normally have too great a current draw to run off of a simple toggle switch. Relays can also be used to change polarity of current and a multitude of other tasks.

Pictured here would be the terminals one would find on a standard 5 pole automotive style relay. 4 Pole relays are available and do not have terminal #87a.

Terminal #85 & #86 are each ends of a coil, when current is applied the coil creates a magnetic field that moves the switch located inside the relay.

Terminal #30 is the common terminal of the relay, when no current is being applied to the relay the connection between terminal #87a and #30 is closed.

When voltage is supplied across the coil this connection is broken and the connection between terminal #87 and #30 is closed.