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MG Midget and Sprite Technical - Relay connections

I am adding relays for the headlights into the Frogeye loom. Mainly to protect the contacts in the ignition and dip switches. I am not going for laser lights, just standard.
I have two, 4 bladed 30A rated relays to be fed from the dip switch, Blue/red to one relay and Blue/white to the other. The relays have terminals 85 and 86 for the switching coil and 87 and 30 for the output pair.

The question is, does it matter which I connect to the dip switch, 85 or 86 ? ( the other will go to earth)

Similarly, is there a convention for which of 30 and 87 is the supply and which is the output to the lights? which
GuyW

Terminal numbers are as follow 30 is 12v supply
87 is load (lamps)
85 is earth
86 is relay switch wire.
That was the way I was always shown but 85 and 86 will work either way round.
Andy Tilney

Thanks Andy.
I thought there would be a standard convention for this.
GuyW

I've just looked it up Guy and DIN standard 72552 applies to car electrics. Needless to say there's a Wikipedia article which lists terminal numbers and the convention used. So I better use it too!
Bill Bretherton

And was Andy right then, Bill? ;-)
GuyW

Yes! The standard assumes negative earth presumably.
Bill Bretherton

Ah, so as my car is a positive earth Frog, are you suggesting that earth should be 86 and switched feed, 85? I thought that as far as the switch is concerned earth would be earth, whether that be positive or negative?

I am sure it would work either way round as obviously it is fully insulated, and it is just an enclosed switch.
GuyW

Some relays have internal diodes, in which case it is important to get your positive and negative the right way round.
Dave O'Neill 2

Guy, an interesting question! I don't think basic relays will have diodes so, that being the case, I'd still connect as Andy suggests so that 30 is battery feed (-ve in your case) and 85 is still earth.
Bill Bretherton

As Dave has rightly said relays with integrated diodes are polarity conscious, which I would assume yours are not that type, so it does not matter,
Andy Tilney

They are basic, cheap 4 terminal ones. 30 seems logical as the 12v feed, which happens to be -ve in my case. There are no diode symbols on the mini-circuit diagram printed on the case so I will stick with Andy's first instructions. ;-)
GuyW

Guy 12v relays with integrated diodes in are not very common, whenever I have wanted one I have has to order it in special, if you bought them in Kendal then they wont have diodes in,
Andy Tilney

Guy,

Terminal 30 is almost always the feed, as in a 4-pin relay the output is terminal 87, but with a 5-pin it is either 87 and 87a (changeover type), or 87 and 87b (double output type). The terminal connections are logic dependent, not battery polarity.

I have come across a 5-pin changeover relay application that switches the source of supply (87) or (87a), to feed a common output (30), but the norm is as above.

The convention for terminals 85 and 86 is as in the previous replies - 86 is the +12v feed from dip or other switch, and 85 goes to the body (earth). If you have a diode relay, the +12v must go to terminal 86 and terminal 85 to -12v.

For a standard 4-pin relay without a diode, terminals 85 and 86 perfectly well if reversed.

N.B. The are two types of 4-pin relay, type A and type B. The connection numbers are the same for both, but the pin layout is different, so are not interchangeable if plugged into a pre-wired base. Type B tend to be the most commonly used.

Richard
Richard Wale

Richard I was interested to read in your post the part.
I have come across a 5-pin changeover relay application that switches the source of supply (87) or (87a), to feed a common output (30), but the norm is as above. was this in an automotive application, if so could you explain this application further please,
Andy Tilney

Thanks Richard. You confirm Andy's original reply. The only thing is you say 86 is the +ve feed from the dipswitch. Only mine being an early car has a -ve feed from the dipswitch.
So conventionally, which fact takes precedent? The polarity or the fact that it is the switched feed?

Pretty certain it is not a diode relay.
GuyW

Guy,

The 'convention' is designed around negative earth vehicles, but works equally well with positive earth, UNLESS you have a diode relay, when terminal 86 MUST be connected to +12v.

It is probably easier to connect the 'supply', which for you is -12v, to terminal 86 and connect terminal 85 to earth (+12v), as that is what the mini diagram on the relay usually shows, i.e. ignore the polarity.

Richard
Richard Wale

Whilst on about electrics, is there any benefit in adding a voltage stabilizer like the later cars. I am keeping with a dynamo, and prefer to keep it simple unless there is a clear advantage.
GuyW

The voltage stabiliser was only for the fuel gauge, which was of a different design. If you are keeping the original gauge, the voltage stabiliser is not needed.
Dave O'Neill 2

Thanks Dave. Yes, l have the original gauge and tank sender units. So no voltage stabilizer then . That's good!
GuyW

Not quite to do with relays, but not wanting to start a new thread!

My dash guages were all missing their bulb holders but I have managed to collect together a selection. It now occurs to me that the ignition warning light one probably needs to be a bit different in that it needs to be in series with the regulator and ignition switch, rather than earthed through the body of the speedometer, as with the other lamps. I don't think I have one that is isolated from the guage like that, but presume this one from Autosparks (photo) is what I need? Am I right?



GuyW

Guy

I don't know about the Frog, but later cars had a bulbholder without the 'claws' which fitted into a black plastic tube in the back of the gauge.

The tube not only insulated the bulbholder from the gauge, but directed the light to the jewel in the face of the gauge.

Items 38 or 39, depending on bulb type.


Dave O'Neill 2

Here's a photo from Paul Hunt's MGB site, showing the black plastic tube on the back of the tacho.


Dave O'Neill 2

Guy

The frog has an insulated holder as you indicate. The one you illustrated would be fine.I may have a spare original one.
Bob Beaumont

Thanks for the offer Bob, but l have already ordered it. 😊
GuyW

Andy,

The application was on a Golf GTi that my wife had many years ago, and after fitting aftermarket Cibie fog lights, I wired them in so that the standard 2-position front/rear fog light switch worked as standard, i.e. position 1 is front fogs only, and position 2 is front and rear fogs.

In addition, via the attached diagram, the front fogs also came on with the main beam to give additional 'driving lights'. It worked very well indeed.

The 5-pin relay was wired so that in its un-energised state the front fogs were switched on by the standard VW fog light switch, but with headlights on main beam, the supply to the front fogs was switched from the default of the 2-position fog light switch, to the alternative of the main beam relay output.

The combination of 100/80W H4 outer headlight bulbs, 130W H3 inner headlight bulbs, plus the 55W H1 fog light bulbs made for real 'daylight' night time viewing!

All the circuits had relays added to 'unload' the standard VW switches.

Richard




Richard Wale

This thread was discussed between 22/04/2018 and 30/04/2018

MG Midget and Sprite Technical index

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