Automotive Relays
Automotive Relays
Find out more about automotive relays and their use in renewable applications
electric circuit | generalRelays 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, 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 available in the
REUK Shop) 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 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.
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 at affordable prices. Click here to search now for
Automotive Relay to view the latest listings.
Article Last Modified: 10:50, 24th Oct 2008Comment on this Article
If you have any comments on this article, please email them to
neil@reuk.co.uk.
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