Automotive relay pinout?

[Steve_D]

Can't think of a better place for this question.

A simple automotive relay has the following pin numbers 30, 85, 86, 87.
Pins 85 and 86 are for the coil that activates the relay.

After extensive research (Google) I have failed to find anything conclusive to say which of these pins should be positive or negative. For the most part it makes no difference.

Some relays have a protection diode across these pins which should have given the answer but I have found examples of the diode being either way round.

Can anyone provide proof positive of the ‘correct’ way to connect these relays?

Many thanks
Steve

[jhfozzy]

For 85 & 86, either way is good, it's a coil of wire, it doesn't care which way round pos and neg are. Diodes and resistors in the coil circuit are only used when the circuit demands is to stop backfeeding up the line etc.

30 is the supply.

87 & 87a are to the load.

[OlegI]

If it has a diode and you connect it the wrong way round, won't you short circuit the coil?

If you've got a multimeter handy, measure the resistance across the coil both ways round. If it has a diode, you'll see a low resistance one way and a high resistance the other. The high resistance means you've got + on the cathode & - on the anode, which is the way it should be connected in circuit.

Edited by OlegI on Thursday 25th October 09:21

[PaulKemp]

Very informative diagram
The plague of frogs concerns me, I leapfrog down the road sometimes
Do I have a relay wired wrong????

[OlegI]

Steve,

There is a problem...I normally work on 380V 3 phase and I'm not that familiar with automotive components. So I got hold of a car relay & typical rectifier diode to check. What I wrote is technically correct, the problem is that the resistance of the diode is so high in the forward direction (circa 1.2Mohm) that when in parallel with the resistance of the coil (circa 75ohm), it makes very little difference to the total resistance (73.9ohm vs 74.0ohm). When you add in possible resistance from dirty contacts, the difference may not be large enough to be significant on a standard multimeter.

So ignore what I wrote about using resistance to check. For the relays you have that include an integrated diode, is it marked on the case which way round the diode is connected?

Serves me right for being a smartarse

[Steve_D]

Thanks for the update (and the PM).

Where a diode is not fitted then, functionally, it makes no difference which way round the coil is wired.
Where a diode is fitted the marking on the case tells you which way it is polarised.

I do quite a lot of wiring on kitcars and it bugs me that I can't find something that positively defines an industry standard.

The weight of evidence is leaning towards pin85 to negative.

Steve

[OlegI]

I guess maybe there isn't a standard.

As long as the diode polarity is marked on the case for each relay, you're ok.

[OlegI]

PaulKemp said:

Very informative diagram
The plague of frogs concerns me, I leapfrog down the road sometimes
Do I have a relay wired wrong????

Getting a diode wrong on 380V is an error of near biblical proportions. I guess at 12V you may be at risk of 1 or 2 small tadpoles. Have you looked thouroughly?