Current Voltage Regulators

David

Why don't you take the opportunity to change your car to negative earth? Is there a positive earth radio?

Hi Graham I?m not rely sure what is involved to change from positive to negative earth, is this easily done? I have a positive earth radio but this does not work so needs replacing anyway. I just assumed it would be easier to change polarity on the regulator, but I?m open to all suggestions. Regards David

Bit of an FAQ that one. If I can just re-post Dave Plowman's words:

And that's it. It *is* easy to do; the battery terminals took longest when I did my A40. It's a sensible move in any case, there's no real justification for retaining positive earth.

Just curious, as I never did understand electrics - what's the advantage of changing to negative earth?

Geoff MacK

Good point.

Originally, negative earth was IIRC the only way used. Then someone discovered that plugs etc lasted longer with positive earth. The change was made - by some - in the '30s and '40s. Ohers - like Rolls - stayed with negative earth as did most European makers. Only really the US and UK went for positive earth in a big way. Some say that the polarity of the ground makes a difference to body corrosion too, but IMHO the jury's out on that one.

The advent of cheap silicone transistors (NPN) made negative earth attractive again - these in basic circuits are negative earth, although the first germanium transistors were PNP and positive ground.

The advantage of converting an older car is that you can use modern cheap and better electronics - say a larger alternator or a decent radio. You might have a problem if your car is positive earth and has an electronic revcounter etc, but these can usually either be converted or an identical looking negative earth one bought.

AFAIK there is no significant advantage. Up to the early 70's cars were still being made with positive earths, but since then negative earths seems to have become the std. I think the decision is based on the idea that electrolytic corrosion on a cars body is less with a negative rather than a positive earth, but as I say, I don't think the difference is that significant. It certainly makes life easier for the electrical accessory manufacturers. Mike.

Back to your original question, David.

I didn't think that regulators were current direction sensitive since they only operate on coils/relays. So I got out my 1956 Odhams Motor Manual which describes Voltage and Current-Voltage regulators, and there is no mention of positive and negative versions. I purchased a 1968/69 Daimler 250-V8 saloon recently (-ve earth with an alternator) but have checked the Part Catalogue and see that the Control Unit for your car is given as 37342/D-RB.340.

As a fellow owner make contact directly if you wish. I can check at our local friendly auto electrician and make doubly sure. Peter C.

Hi All Just a quick message to say thanks to all the readers who replied to my earlier posting. It?s always encouraging to know that there are people out there who will take the time and trouble to pass on their knowledge to others. I have been tied up with work this last week so have not done any more to Daimler. The general consensus appears to be that the regulator should work OK, so I think I will just fit it and see what happens, Thanks again David.