Automotive Relays

Find out more about automotive relays and their use in renewable applications

Relays (1) are a vital component of many renewable energy applications, particularly where the output from a low voltage low current electronic controller is used to switch a high current device such as a pump (2), motor, or heater.

For applications where a load current of under 10 Amps is to be switched there are many relays available which can be PCB mounted (e.g this 10A relay (3) available in the REUK Shop (4)) and are not very expensive. When higher levels of current are to be switched, then PCB mounted relays are no longer suitable and here is where automotive relays come into their own.

Automotive Relays

As the name indicates, automotive relays are designed for use with cars. Cars operate on a 12 Volt system, and so (as Ohm's Law (5) tells us that current = power/voltage) high power devices such as amplifiers and air-conditioning units draw very high currents. A 300W amplifier for the stereo for example will draw a current of 300/12 = 25 Amps, and so a high current switching relay is essential.

As there are so many cars manufactured with so many relays, automotive relays are typically not too expensive while also being very well made, robust, and relatively indestructable. This makes them the obvious choice for anyone who needs to switch a high current device.

Types of Automotive Relay

The commonest type of automotive relay is rated at 40A, with 30A and 20A coming close behind. Where a PCB mounted 12V relay will typically need a current of 30-50mA to energise, large automotive relays require from 100mA - 200mA with higher rated relays typically requiring a higher energising current.

Automotive relays typically have either 4 pins or 5 pins. In both cases two of the pins are for the coil across which 12V are put to move the internal switch in the relay. This is called energising the coil.

In 5-pin relays (pictured above), the three (non coil) pins are labelled COMMON, NO (normally open), and NC (normally closed). When the coil is energised COMMON is connected to NO, and when the coil is not energised COMMON is connected to NC.

Connections for a 12V automotive 4-pin relay

In 4-pin relays (pictured above), the non-coil pins are connected either side of the internal switch. This switch is open when the coil is not energised, and closed when it is energised. (This is equivalent to the contacts being COMMON and NO (normally open).

Using Automotive Relays

With PCB mounted relays it is very easy to either solder leads onto the pins or solder the relay onto a circuit board. With automotive relays things are a little more complicated since the pins need to be so much larger and the cable must be much thicker in order to carry large currents.

Therefore the pins of automotive relays are typically shaped to match standard female spade connectors. These can be crimped or soldered onto the ends of cables, and then simply pushed onto the relay pins making a very secure connection.

Automotive relay holders (pictured above) are also available. These can be clipped together to enable multiple relays to be fitted securely and tidily.

Buy Automotive Relays

There are always a great selection of automotive relays (and automotive relay accessories) listed for sale on eBay (6) at affordable prices. Click here to search now for Automotive Relay (7) to view the latest listings.

Web Link References

(1) http://www.reuk.co.uk/Relays-and-Renewable-Energy.htm
(2) http://www.reuk.co.uk/Pumps-for-Solar-Water-Heating.htm
(3) http://www.reuk.co.uk/buy-12V-10A-RELAY.htm
(4) http://www.reuk.co.uk/REUK-Products-For-Sale.htm
(5) http://www.reuk.co.uk/Ohms-Law.htm
(6) http://rover.ebay.com/rover/1/710-53481-19255-0/1?type=1&campid=5335956244&toolid=10001&customid=
(7) http://rover.ebay.com/rover/1/710-53481-19255-0/1?type=3&campid=5335956244&toolid=10001&customid=&ext=automotive+relay&satitle=automotive+relay