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MG MGB Technical - Voltage stabiliser

Has anybody fitted the LM2940T solid state stabiliser recommended by Rick Anstley to an eary MK2 (1968)?

I have blown two up they are OK (9.8V) until I start the car, then they fail and give the same voltage out as in.

An electronics type has suggested a capacitor on the input as a surge supressor
T J Malloch

Connect a voltmeter from the power lead (brown wire or green with the key on) to ground. with the engine running at 2000 RPM the voltage should not exceed 14.5. If is higher your voltage regulator is bad and the stabiliser is being ruined by too high an input voltage.
John H

Are they failing during cranking? As the engine starts? Or a little while later.

Max input voltage on those is 26v, can't see how you are exceeding that. Output current is only 1 amp though, and a US 68 has all three gauges served by the regulator. The fuel and temp senders can go as low as 30 ohms each, and the oil sender possibly the same. The resistance of the gauge is lower than that, I think. So three gauges could well get up to the limit of the regulator under some circumstances. Or there is simply a wiring fault.
Paul Hunt 2

Thanks for the replies
John, At fast idle voltage is 13.8 and doesn't go much higher with more revs
Paul, In Australia (at least on my model) the stabiliser only serves the fuel gauge which at prsent level shows 138ohm, so overcurrent shouldn't be a problem.
I have the older tacho that is in line with the coil feed wire woould this cause spikes

Terry
T J Malloch

A capacitor may well help. They are probably being destroyed by an inductive spike as the starter motor stops cranking. A 10U tantulum in parallel with a 1000 nF mica or similar high self resonant type is usually recomended.
Stan Best

If they are going to work at all I'd expect them to be protected against spikes, there are spikes of around 250v in the ignition circuit every time the points open, and HT current actually flows through the coil, ignition circuit and battery to the body to complete the circuit. All cars up to 73 had the current pulse type tach, if this type of regulator didn't work with these I'm sure Rick Astley would have said. He *does* specifically say that a capacitor shouldn't be needed. However something seems to be destroying these regulators, so maybe it will help even though the cause is actually something else. In which case it may work for a while, and then fail. This is another piece of electronic stuff I won't be touching, to go along with electronic ignition and electronic pumps.
Paul Hunt 2

Hi

I have put a 1ohm 1W resistor between the battery and the + input of LM2940T. Have a 20V 1W zener diod between the + input and ground of the LM. Note the "ring" of the zerer shall be to the + input of LM. I also have 0,47uF capacitor at the input to ground and a 22uF at the output of LM.
The zener will protect from high voltage spikes and the capacitors will eliminate oscillations. Note the the oscillations can break the LM2940T regulator.
The regulator need to be tighten to some metal for cooling. It has been working for some years.
Tjellvar

Thanks for your comments everybody.
Local advice was very close to Tjellvar's. I installed a simalar unit today and it is working well.

Great Ocean Road on Monday???????

Terry
T J Malloch

This thread was discussed between 11/02/2008 and 16/02/2008

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