A relay is an electrical switch which allows a low-current circuit to control a high-current circuit. For example you want a pair of spot lamps on the car (high current) to be turned on and off by a dashboard switch (the low current circuit). The relay allows a thin cable to be used on the control side, and a (hopefully!) shorter but thicker cable on the load side. This helps to preserve your on/off switch from arcing, and reduces voltage drop in thin, tired old cables.
A basic relay for this purpose will cost between £2 and £4 each, be rated at 12 volts and either 30 amps or 40 amps. You probably want the kind with a mounting bracket to allow a self tapping screw to mount the relay on the inner wing or wherever.
Most 12 volt relays have four terminals, each with a number. The numbers are normally 30, 85, 86 and 87; an indication of what should be connected where. (The numbers originated from a German specification).
The coil side (the circuit with the on/off switch) is connected as follows:-
- 86 is one connector on the control or coil side .
- 85 is the other connector of the control or coil side. It is normally connected to earth/ground. It does not matter which way "round" the relay is connected. You could connect 86 to earth and the relay will work quite happily. Convention says we use 85 connected to earth (in our case, the vehicle bodywork). The electrical symbol for a coil is a 4-sided box with a diagonal line. So the following sketch is not "correct" but you should get the idea!
The load which could be spot-lamps, etc are connected as follows:-
- 30 is the power input.
- 87 is the power output to the load.
When you supply power to the coil (terminals 85 and 86) it becomes an electromagnet. The coil (electromagnet) then pulls on and that closes the connection between the contacts 30 and 87 which are NORMALLY OPEN). Now power can flow through the relay to your lamps etc.
85 and 86 in more detail
Connections 85 and 86 are to the magnetic COIL inside the relay. Think of it this way, the coil pulls the switch contacts 30 and 87 together. Remove the power from the coil and the coil releases the switch contacts and the load circuit is broken.
One contact is probably going to be connected to an on/off switch. The other side of the switch will be connected to power (that is the battery). The other relay contact will be earthed. When the switch is operated, current flows from the battery, through the switch, through the coil in the relay and then to earth, which is in effect the other side of the battery. The circuit is complete and the relay will "click" and operate. You can create this simple circuit with a few bits of spare wire and try it out for yourself! It is a very satisfying click!
30 and 87 in more detail
Contacts 30 and 87 are used to switch the load, such as spot lamps. One contact goes to the lamps, the other to the power source (e.g. the battery).
When the relay operates, the circuit is from the battery, through the relay contacts, 30 and 87, to the lamps and hence to earth. The circuit is complete and the lamps will light. This will continue until the switch controlling the relay is opened. Current will stop flowing and the coil will release the contacts in the relay.
Use a maximum of 2 lamps per relay (so as not to exceed the capacity of the relay – a normal relay would be rated at 30 amps.)
You should always mount relays with the contacts pointing down, to prevent water collecting, entering and damaging the relay contacts.
Some relays have fuses, which are used to protect the circuit being switched (in our case the spot lamps). If a short-circuit develops in the circuit, for example the "live" main cable may short onto the chassis, the fuse in the relay will blow. The fuse size should be close to, but more than, the current drawn by the load. (Watts divided by Volts = Amps) So, for example 130Watt Lamps, 130/12 = 10.8, so use a 12.5A or 15A fuse. (Note:- Always use "12" volts to calculate your amperage. The voltage of your battery should be nearer to 13.2 volts, but it can be lower so this gives you a safety margin when calculating wire sizes and fuse capacities).
A fused relay is going to cost around £8 but gives you peace of mind and saves you having to install extra fusing in the lamp circuit.
For loads up to about 17.5 amps you will need 28/030mm, 2 sq mm cable which will cost around 35-40 pence per metre. This translates as 28 strands of copper, each strand being 0.30mm diameter. The cross-sectional area of the cable is 2 sq mm, which guides you towards fitting the correct terminal. The load quoted for this cable, 17.5 amps, is a continuous load of 17.5 amps. If you think you need a higher capacity cable, you’ll have to move up to a thicker cable, the next one being 44/0.30 mm 3sq mm and 27.5 amps.