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 unit.
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 meter.
The 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.
