Relays are used in various applications including for home automation, automobiles and in the control of AC mains appliances. Essentially they are remote switches which activate an electro-magnet, which in turn pulls contacts to switch circuits on and off.
The coil of the relay is energised by a small amount of current, which then closes contacts that feed power to an accessory that needs a larger current to operate (typically over 20 amps). Wiring a 12v relay can be tricky and requires a high level of electrical knowledge and skill.
If you use the wrong relay or wire the relay incorrectly, it could cause damage to your vehicle’s electrical system or even harm the vehicle’s computer systems. If you have a high current circuit, it’s important to use a fuse and switch to reduce the risk of damage to the coil or other parts.
Relays normally have a coil rating of between 25A and 40A with 30A typically being the highest. The current drawn from the coil is shown on the circuit schematic and should be measured using a multi-meter with a suitable current sensor. A very high resistance reading or open circuit is a sign of a damaged coil so should be checked before use.
A relay’s terminals are typically 6.3mm wide however some more specialist relays can have wider terminals, especially those with inbuilt electronics. This increase in width can be beneficial for signalling purposes or to allow for additional connections within the relay itself.
To connect a relay to an accessory, first wire the common terminal of the relay to your battery supply and the accessory’s power lead to one of the normally closed pins (87 and 87a). Then, run a wire from the coil of the relay to the accessories’ circuit so that the accessory is activated by the relay when it receives the power.
If you have a timer, you can set it to control the activation of the relay by choosing times for its “ON” and “OFF” status. Using the timer to activate the relay will save you money, and can also help you avoid overheating your appliance by regulating its current usage.
Some relays have a *diode across the coil to suppress high voltage spikes when the coil is de-energised, which can hurt sensitive electronic equipment upstream of the +12V coil supply. The diode absorbs the spikes, forcing them back into the electro-magnet where they dissipate as heat.
Another common application is in automotives, where there are several high current and power components including headlights, heater/cooling systems, blower motors and audio systems. These components are often triggered by momentary switches so it is helpful to have a relay to handle the load from these devices.
When wiring a 12v relay, it’s best to make at least four connections, two from the coil of the relay and two from the relay’s contacts. It’s always a good idea to test all of the connections before they are made so that any faults can be resolved quickly and easily.