Relay - Step by step to wire up a relay to operate a 12v fridge from either mains or leisure battery.

The point of this write up is to give you an insight into how relays work and also how to to auto switch the electricity supply to your 12v coolbox or fridge between Leisure Battery (LB) or 240v mains hook up without physically having to connect or disconnect anything. Its kind of rewarding doing it yourself. You will need a test meter to complete this, some wire crimpers and terminals plus a relay (detailed below) all of which will be a lot cheaper than buying this setup off the shelf.

This is useful if you want to run your 12v fridge/coolbox etc while on the go and then when you get to the campsite and connect your van to hookup then the fridge is run by the site hookup electric, not your Leisure Battery (which you could charge up separately overnight.)

This made sense when I was doing it although in some ways it was lucky I did not understand relays fully at the time or I might never have attempted it as someone later told me I'd in effect wired up a relay backwards but its perfectly safe and has achieved a perfectly good solution. Even if you know little about electrics or relays you should be able to do this. Re read it a few times and it will make sense, it looks complicated but its not once you actually do it.

You need a relay, a 5 pin, will cost you about 3. 10amp rated will be enough. You can use heavy duty (higher amps) relays but bit of a waste unless its all you have. Just ensure its got more amps than your appliance you want to power.
Amps are the key, if you run 3 stereos at 5amps you must have more than 15 amp wire and a fuse of 15amps or you'll melt and catch on fire. 10 amps should be ample for a fridge but it will say somewhere, the biggest draw in electric current is normally when they start up, mine apparently goes as high as 8 amp draw then settles down to 3 ish max.

Shopping list
5 pin relay, sometimes called an accessory relay. capable of 10 amp
some 16 amp cable and blue connectors to go with it, get 10 male and 10 female (search for pearly wot nots).

A 240v to 12v adaptor costs as little as 4 on ebay (prices vary). This comes with a plug on one end, which you plug into your hookup normal house plug socket and an output on the other end which is converted to 12v supply.

You cut the end off 12v supply end giving you 2 strands of cable, one for live and one for earth/ground. How do you know which one is which? well just before you cut it make a note of which wire is which so you could, in theory, join them back together. The easy to touch bit in your hand is earth (outer casing), the bit down the middle is live. Put your test meter to continuity mode to work our what's what. There's only 2.

Crimpers (for crimping the terminals and a fuse and fuse holder to match your fridge. The above is all available in halfords etc and even if you bought everything from new incl crimpers it shouldn't cost more than 25.

5 pin relay, look at the pins, there's a number next to each one:

85, 86, 87, 87a and 30

85 is always negative/ground.
30 is live - output in this instance
87 and 87a are the two live inputs, one from LB and one from a 240 to 12v power supply that becomes alive when you plug it into the campsite hookup.

86 is the switcher between the 87 and 87a so we physically join (with wire) 86 and 87. What this does is make it so that when external campsite power becomes available/on the trigger, 86, switches the supply source to the fridge across from LB supply to campsite hookup supply.

A relay has states, in its normal state 87a and 30 are connected inside the relay. When it is switched you are rerouting the power (which in our case will become the 87 pin to 30) once its switched across. If you disconnect the new power source then 86 cannot hold the gate open and the relay will switch back to its normal state so when you drive off.

Note, even though 85 relay terminal is ground/earthed you also need to earth you appliance, in this case the fridge. You possibly could join them up but the thing about earths is it needs to match the positive side coming in so never have a smaller cable on the earth side, if its too small then the circuit breaks down so better extra separate earths.

So wire 30 to the fridge. In that run of wire to the fridge cut it in half and put the fuse inline, this is the 12v supply out to the fridge (or anything you want powered)

87a to your LB

a 240 > 12v adaptor supply runs to 86 which you bridge to 87

85 to earth

And this is what will happen: Remember the gate/switch flicks between 87 and 87a.

So initially 87a has the battery supply running into it and the output (#30) goes to the fridge, the fridge and relay are earthed (85).

As soon as external hookup power is connected the relay switch inside has power and 86 triggers the gate to move across. Now power runs in from 87 up to 30.

One hookup power is removed the 86 (switch) cannot maintain the link open so the gate switches back to 87a (normal state) and 87a leisure battery resumes service to 30.

Normal State

87a & 30 are joined/connected

87 is the alternative supply

30 is the accessory so the fridge

85 & 86 left

85 is earth for the switch inside and

86 is the trigger

So when power goes to 86 it switches from 87a to 87 into 30

That's two +ve feeds, earth all goes normal.