Automotive Relays

Common Relay Pin Configuration (Bosch-type)

Overview
Relays are widely used in electrical applications where one circuit is to be energized or turned "on" by the presence of a voltage, provided by another circuit. An example of this is when an automotive radio sends out a triggering voltage to turn on an external amplifier or activate a motorized antenna. Anywhere a switch can go in a circuit, a relay can replace it, (as long as there is a triggering voltage available to activate it).

The "switch" in a relay is more often called a solenoid. A solenoid is like a piston that pushes outward when energized with electricity. This push mechanically trips the switch in the relay, completing circuit and allowing the switched voltage output.

A relay can be triggered with an electrical pulse as small as 150 milliamps. The switched output can be as high as 30 or 40 amps.

Relay at rest

Relay energized

Connections
The terminals of a relay are defined as follows:

• 30 is the common or input voltage (or ground) to be switched.
• 87a is the normally closed connection (can be used as a switched voltage output when the relay is at rest). (This terminal offers no voltage when the relay is energized.)
• 87 is the normally open connection (switched voltage output when the relay is energized).
• 85 is connected to the ground of the triggering voltage.
• 86 is connected to the positive 12V of the triggering voltage.

Note: in many cases, the connection of pins 85 and 86 can be interchangeable, but NOT if there is a diode wired across the coil.