What is the purpose of the small voltage regulator which supplies electrical power to the petrol and water temperature gauges on the instrument panel?
As far as I can tell, it just reduces battery voltage of 12 volts to 10 volts for the two instruments.
If so, this seems frivolous. It must have some other function but a mystery to me.
It caters to low battery voltage keeping the gages more consistent making those gages more accurate the oil pressure not considered as important .The car originally had a generator
Cheers
The IVR “regulates” the input (battery) voltage, which can vary between, say 10V and 15V depending on load. The regulated output is a constant 10V (pulse width modulated). Thus the fuel and temp gauges, set up to expect a 10V power source, are accurate independent of the battery voltage.
It’s not that the oil pressure gauge isn’t considered as important, but rather that the OP system contains its own voltage regulator. Operation is quite similar to the IVR in fact, except that the regulator it’s contained within the OP sender (which is why it’s so large).
Another style of instrument, used since the 50s I believe in some cars and finally by Jaguar beginning in the 1980s, uses a different style of internal regulation. Each gauge compares the battery (ignition) voltage, supplied via an extra terminal, to the voltage supplied by a resistive sender. Thus the needle position is proportional to the fraction of battery voltage that comes from the sender. This eliminates the need for a regulated voltage source per se. IMHO.
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Robert , what is " pulse width modulation". ( or do you.mean pulse with modulation?)
I measure voltage at input and output of my vr and find constant 12+ volts at input. But output measured at output pole cycles up and down from.zero to.12+ volts
Is that a measurement problem or a fault in my voltage regulator?
Perfect! That’s pulse width modulation. It’s the latest (digital electronics) thing but Jaguar had a mechanical version in the 1950s!
Say the input is 12V. To get a (time-averaged) output of 10V, the output stays on (12V) for, say, 500 ms and off for 100 ms, repeating every 600 ms. Thus the voltage is on for 5/6 of the time, which gives you an average of 5/6 of 12V, or 10V. If the input voltage goes up, the regulator output is on less than 5/6 of the time, if it goes down, more than 5/6 of the time.
The archives describe how it works in detail–a bimetallic strip bends in response to a bit of heat developed by resistance wire wrapped around it to cycle a switch contact. It’s ingenious! To work at mechanical speeds, the instruments themselves have to be slow-responding. And they are–also using coils wrapped around bimetallic strips, but to move the needle, not to switch the battery voltage on and off. IMHO.
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Thanks for tertific, clear explanation
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Now to steal the thread a bit, @Ray_Livingston, we haven’t heard a mouse whisper on the electronic instruments you started working on before Covid. It seemed you were very close to finishing them. Or did I miss the final result?
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All cars with electrical gauges have some sort of voltage regulation. American cars of the period used IVR’s identical in design and function to Lucas, but regulated to 5V. This dates back to the days when cars had 6V electrical systems.
If it’s playing up you can get a solid state voltage regulator or if handy look in your favorite electronics store for the regulator and put into original case if your very fussy.
I think @Michael_Frank has them at coolcat
That he does and I put one in my 69. Just one less old thing in the old thing LOL 
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I will also mention that there are instructions on JL that tell you how to make your own. Very simple, if I can do it…
Cheers,
LLynn
I couldn’t find it on JL but here’s a You Tube video: Build a DIY Gauge Voltage Stabilizer For Your Classic Car - CHEAP - YouTube
Cheers,
LLynn