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 solenoids 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
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
- 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