Electrical Polarity

[JAG]

Hi I have just purchased my first Lotus, a 1968 elan plus 2. There is not a battery fitted and I am unsure whether it is a positive or negative earth vehicle. The chassis no is 0229 so I suspect it is a positive earth. Is there any way to check it before I install the battery the wrong way?

Regards

John

[MarkDa]

The battery leads may be labelled, the rev counter should save on the face.

[Concrete-crusher]

As far as I recall it's only the clock if you have one that's polarity sensative so if you connect up and the clock works you should be ok

The tacho will say posative or negative earth but that's no help if its been changed, the tacho needs an internal mod for this (easy to do)

Another clue would be the presence of an alternator rather than dynamo that would indicate negative earth

Steve

[Craven]

On older style battery the connection posts are of different size !!

[billwill]

The rev Counter is polarity sensitive. It has a zener diode inside to stabalise the voltage for the actual meter, but I am not at all sure that the arrangement will protect the electronics if you supply the voltage the wrong way around.

[billwill]

If you have a dynamo (not an alternator) and you connect the battery wrong way around it will probably polarise the dynamo so that it will generate electricity wrong way around. If you subsequently connect an uncharged battery the right way around that will be rather bad for the battery, I think.


Has it got a radio? That might have a red lead for positive and a black lead for negative which may help you work out what polarity your car is now. You cannot rely on what it was when it left the factory (or what the Rev Counter says on the dial) as a previous owner might have reversed the polarity.

[h20hamelan]

viewtopic.php?f=38&t=28654&start=

Might help

[ericbushby]

Hi Bill,
That worried me as well.
When I bought my car it had been changed to negative earth and fitted with an alternator.
The rev counter, although connected, did not work (along with many other things).
When I realised it had two transistors in, I assumed they would have failed.
I opened it up to change the polarity and replace transistors and capacitors.
But on testing it worked correctly and is still running on the original GET 113 germanium transistors, now 52 years old.
I still do not know how they survived.
Eric in Burnley
1967 S3SE DHC

[billwill]

There are two basic circuits for the Smiths tachometer. This is the one most people show, unfortunately it was drawn by a non-electronic engineer, so is not drawn in the normal way for such a circuit.


When the polarity is correct, the zener diode conducts (through R7 and R8) such that the voltage across it remains steady at 6 volts (i think) and this is the stabalised supply voltage for the transistors. If the polarity is reversed the zener will be forward biased and will conduct more heavily bringing the voltage across it down to about 1 to 1,5 volts. I guess that the transistors are able to withstand this amount of wrong-way supply.

The other circuit is like this:


As you see there is no zener, and so I am not at all sure that this circuit can stand the wrong polarity of supply.

[ericbushby]

Thanks Bill,
Properly explained as usual,
Eric