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MG MGB Technical - voltage stabiliser

Can anyone advise whether the polarity of the voltage stabiliser matters to its proper function? I have a 64 Roadster later converted to negative earth. In trying to get more accurate fuel guage readings should a new unit be negative earth since I assume the existing unit will be an early positive earth version?
Thank you

Ron Stevenson, Ayrshire, Scotland
R Stevenson

Don't bother with one of those horrible bi-metal things. Get an electronic one like this, ebay item number 130512139446. Reliable and accurate.
Mike Howlett

The fuel gauge works off 10 volts (you might ask why......don't know!) I bought a voltage regulator from Maplins. Gives out 10 volts with an input anywhere between 8 and 30 volts. It's a tiny, solid state thing costing a couple of quid, AND it works AND is small enough to fit in the old unit.
Allan Reeling

Hi Ron,

To answer your question, the original regulator is not voltage sensitive, can be connected either positive or negative. Electronic ones though are polarity sensitive.
If you are purchasing a unit and there is an option of positive or negative earth, then you will need a negative earth one, to use with you negative earth car.

Herb
Herb Adler

Herb is correct. The OEM bi-metal voltage regulator is not polarity sensitive. It will regulate a positive or negative input voltage.

As to the the statement that the solid state electronic regulators are better than the OEM is not always true.

If the solid state regulator does not have a low dropout regulator installed then I would say the OEM is better.

I say this because some three terminal 10v electronic regulators will not start to regulate until it sees around thirteen volts at its input. This means that the gauge will not work until you start the engine and the regulator in the alternator starts regulating.

The low dropout regulators will regulate down to around 10.5 volts so the gauge will work with the engine off.

The OEM regulator is very forgiving down to low voltages.

And when you think about it, how much accuracy can you get from a simple wire wound resistor in the fuel level sender. Not much.

You should go to Paul Hunt's web site for information on how to calibrate your fuel gauge. (If you install an electronic regulator you will have to re-calibrate your gauge anyway.)
Steven 67GT

The original positive ground voltage stabilizers were not polarity sensitive, as has already been mentioned. They are extremely reliable and often outlast the car itself. My '67 has the original stabilizer and I converted over to negative ground almost 30 years ago with no ill effects. I've replaced the fuel sender one time in all those years. RAY
rjm RAY

Hi there,
Alan Reeling can you tell me which voltage regulator you bought from maplins please. I'm having trouble with mine. Also does it work with the engine off at all. Hope to hear from you soon. Many thanks Aaron
A Taylor

"The fuel gauge works off 10 volts (you might ask why......don't know!)"

The old gauges use a dual resistance coil set up with a shunt so one side is always reading the sender, and the other is reading the base voltage (minus the shunt). A pointer with a large base is attracted to the highest value coil and the pointer swings. If the voltage varied, the gauge won't read accurately across its scale. So they used a voltage that would be always lower than the battery/charging circuit voltage which could vary from 12 volts to 16 volts depending on the car's system, and get that voltage from the voltage stabilizer. I hope this makes sense, I may have simplified it too far.

warmly,
dave
Dave Braun

"The original positive ground voltage stabilizers were not polarity sensitive" which is why they weren't 'positive ground' anyway. The stabiliser did change at around the same time as the change in polarity, but it wasn't connected, and it is isn't an issue when changing a positive earth car to a negative.

Up to Oct 64 the gauge was of the undamped type. This didn't need a voltage stabiliser but was still independent of voltage changes as it used a Wheatstone Bridge arrangement of four resistors/windings - two in the sender and two in the gauge. Three were of known value, the fourth operated by the slider varied according to fuel level and its effective resistance can be calculated, or displayed on a meter or gauge, by referencing it to the three known values, and it is independent of the supply voltage. The later damped type uses the heating effect of a current on a bimetal strip, which is totally dependant on supply voltage to give a consistent reading, hence the need for a stabilised voltage supply. As Dave implies any voltage lower than the lowest expected voltage likely to be encountered in the cars electrics will do, and it is nominally 10v. the irony is that far from being a stabilised voltage, the output of the 'stabiliser' is actually constantly switching between 0v and the cars system voltage, which is varying of course. The voltage is used to heat up another bi-metal strip in the stabiliser, which operates a contact switching the supply voltage on and off from the output (and the stabilisers heating coil). Exactly the same as in the gauge the heating effect in the stabiliser is dependent on current which is dependent on voltage. When voltage is low the bimetal strip takes longer to heat up and open the contact, so the voltage is connected to the gauge for longer, and vice-versa. The net result is that one cancels out the other, and the gauge reads consistently with a varying supply voltage.

Hope I haven't complicated it too far ...
PaulH Solihull

This thread was discussed between 24/04/2011 and 15/05/2011

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