Relays do exactly the same thing a regular
switch. Instead of being moved by a finger, they are moved by an
electromagnet that attracts a metal switch. When the "switch" is
inactive (no power through electromagnet) the common and normally closed
contacts are tied together.
When the relay is activated (12 volts across
electromagnet), the switch is pulled, disconnecting normally closed contact,
and connecting the normally open contact with common terminal. As soon
a power is removed from the coil, the contacts go back to their original
Do you need a relay?
Most head units have an output to turn on
amplifiers, etc. These outputs are designed to turn on a small number
of devices, so they provide very little current.
On high-end systems, when many devices
(amplifiers, crossovers, equalizers, processors, fans, etc) have to be
connected to this turn-on wire the current output might not be enough.
If the circuit is overloaded, it can blow a fuse or even damage the head
There is an easy way around this
problem: Add a relay.
How many devices are too many? Depends
on how much current each device draws and how much current the head unit
provides. Check the specifications section of the manuals to
see. Typically, pieces of equipment such as amplifiers, crossovers and
equalizers draw very little current, since their turn-on switches are either
solid state or small relays. If you are hooking other devices that
draw more current such as neon lights, fans, actuators, motors, etc, then
you definitely need to add a relay. An easy way to tell how much
current devices are drawing is to check with a current
diagram to the left shows the connections required to get the turn-on
output. The relay can be located either behind the radio, trunk, or
elsewhere in the car. Usually, it is easier behind the radio because
wires going to the relay are shorter.
Terminal 87 goes to constant power
(+12v). It can be obtained from the same wire where the radio's memory
backup is connected.
Terminal 86 goes to ground (negative wire
going to the head unit or to a metal part that is connected to the chassis
of the vehicle).
Terminal 85 is connected to the remote
turn-on wire output at the head unit. Lastly, terminal 30 is run to all the
components that need to be turned on.