A relay is used when the load you want to switch draws more current than the switch available for the application. For instance, you want to install auxilliary/spot lights on your off road vehicle. The ignition switch may not be able to cope with the additional 15 or 20 Amps required. Enter the relay. It uses a small current to supply a large amount of power from the battery to where you need it.
On the bottom of the relay are numbers stamped next to the connecting tabs, they are wired as follows:
- 30 - Connected to positive of the power source(battery). Put a fuse in line with this as close to the power source as possible.
- 87 - Connected to load. This would be whatever you want to power ie fan, lights, electrical motor etc. The other side of the load is connected to earth(negative of the power source)
- 87a - Anything connected to pin 30 is routed inside the relay to thispin when the relay is off. When the relay is switched on, this pin isdisconnected internally from 30.
- 85 and 86 These pins are used to switch the relay. 85 isusually connected to earth. 86 is then connected to the activation switch and the otherside of the switch is then connected to your power source. (It does not matter which way round you connect these pins polarity wise, since between these 2 pins is a coil used to pull in the contact points and as long as they have power across them, the coil will activate.)
Below is a typical application. In this case to power a radiator fan.When wired as below the fan will work only with the ignition switchedon and the radiator hot. (Thermo switch - v - activated) This is not only suitable to Jeep but Dodge, Volkswagen, Ford, Chrysler, any vehicle as long as you keep to the basic configuration.
Let's get some air over that cooling system. Referring to the diagram:
- i is the battery
- ii is a suitable fuse(see below)
- iii is a suitable switch or in this case the vehicle's ignition switch.
- iv is the relay (Beware, there are different relay pinout configurations but the numbers stay the same)
- v is the load (In this case the radiator fan)
- vi is a thermostatic switch mounted on the radiator
When purchasing make sure firstly that the wire used is of the correct guage(thickness/current rating) for the given application. You dont want the wire melting because it is too thin and cannot handle the current. Find out how many amps the load will draw and add at least another 20% for reserve. The fuse rating should then be matched to the wire's capability since it prevents the wire and power supply (battery) from overload and fire.
If you wanted to wire your headlamps by this circuit, iii would be the high beam switch. The headlamp "on" switch should be wired between fuse ii and relay pin 30. Low beam is connected to pin 87A and high beam to pin 87 instead of the fan. The thermo switch would of course not be needed and can be replaced with a wire straight from pin 85 to earth.