Car Audio Electronic Components cont.

Bosch type relay Image courtesy of Parts Express

A relay is an electromechanical switch that is most widely used for car security systems. It is also commonly used as a way of protecting your head unit from damage if you have more than two devices (amplifiers, signal processors, etc.) being switched on by your head unit's remote turn-on lead. Basically a relay allows a small current to control a much larger current. The common Bosch type SPDT (Single Pole Double Throw) relay is show in the photo at right.

A relay consists of four main parts. A coil, denoted by terminals 85 and 86, a common terminal, denoted by terminal 30, a normally closed terminal, labeled terminal 87a and a normally open terminal labeled terminal 87. Depending on whether your controlling voltage is negative or positive, terminal 85 or 86 will be either grounded or connected to +12 volts. If you have a positive controlling voltage, such as a head unit remote turn-on lead, you will need to ground either terminal 85 or 86. It doesn't matter which one you choose, but the other terminal will be connected to the remote turn-on lead of your head unit. The common lead, terminal 30, would be connected to +12 volts because that is what we want to apply to terminal 87 when the relay coil is energized. Terminal 87 would be connected to the remote turn-on terminals of our various electronic components (amplifiers, crossovers, equalizers, etc.).

Here is an illustration of how we would wire up our example. When our remote turn-on lead applies +12 volts to the relay, the coil becomes energized and connects terminal 30 to terminal 87. We could have also connected terminal 30 to the electronic components and terminal 87 to +12 volts. Either way would give us the results we're looking for.