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How To Wire A Simple Relay

Products Explained How To Wire A Simple Relay

We frequently see the question asked, "Which wire goes where on a relay? How do I wire it up?" so perhaps this article will help you out a bit.

A relay is an electrical switch which allows a low current circuit to control a high current circuit. For example, you upgrade to halogen headlamps (the high current), using your original switch wiring (the low current). Using a relay allows you to use a lighter cable on the control side, and a heavier, shorter cable on the lamp side, thus reducing voltage drop in your original wiring, and also saving your switch from arcing.

A basic 12v relay for this purpose will have 4 terminals and will be either 30 or 40 amps, often supplied with a mounting bracket to fix to the bodywork. These relays terminals are usually numbered, 30, 85, 86 and 87.

relay terminals.jpg

The coil side of the relay (this is the circuit with the on/off switch) is connected as follows.

86 is one connection on the coil, or control, side.

85 is the other connector on the coil, or control, side. 85 is normally connected to ground/earth, usually via the vehicles bodywork, but if you look at the following illustration you'll see that it doesn't really matter which way round you connect these two terminals, it will work either way, but generally 85 is the ground.

Relay electrics.jpg

The circuit which carries the load to your lamps would be connected as follows....

30 is the power input (from the battery, via a fuse)

87 is the power output (to your lamps)

So, when you flick the switch on, the power from the switch goes to terminal 86, which energises the coil, which is an electromagnet. The magnet in turn makes the connection allowing current to flow through from terminal 30 (from the battery) to terminal 87 (to your lamps)

"Aha", I hear you say..."I have a relay that has FIVE terminals! So, what's that about then?"

These, 5 terminal, relays are often referred to as a 'change over' relay. The extra terminal is numbered 87a, and this is used where a changeover is required. For example, you have a 2 speed fan, terminal 87 would be to operate the higher speed. In this type of relay, terminals 30 and 87a are normally connected. When you operate the relay (selecting the higher fan speed) the relay disconnects terminal 87a from terminal 30, and connects terminal 87. NOTE.. The load on terminal 87a should never be greater than 20 amps.

5 blade relay changeover.jpg

A relay will set you back between £2 and £4, and a fused one about £7 or £8.

Some additional information

Always mount the relays the right way up! It won't make any difference to how it operates, but if you mount it with the terminals facing downwards, then water cannot get in and stop the relay from working.

Use a maximum of two lamps per relay. Relays are normally rated at 30 amps, so this ensures you won't exceed the relays capacity.

Always use a fuse in the circuit. You can buy fused relays, which will save you having to fit an extra fuse in the wiring. Fuse size should be close to, but more than the current which is drawn by the load. So, if you fitted a 100watt lamp, then to work out the fuse size, use Watts divided by Volts, which gives you the Amps (Watts/Volts= Amps), Thus.... 100/12=8.3, so a 10 amp fuse can be used. Incidentally, the battery will actually be showing nearer 13.5 volts if you were to check it, but by using 12volts we give ourselves a little extra safety margin.


It's important to use the right size of cable. No good using 17.5 amp cable in a heater plug circuit for example. (For reference, 17.5amp cable is usually sold described as "28/030mm, 2 sq mm" which means it has 28 strands of copper, each of which is 0.3mm in diameter. The cable has a cross section of 2 sq mm.) The cable you would use for the heater plugs would be more like 27.5 amp, or even 35amp, and for this circuit you would most probably use a 40amp relay.
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