OK, this is from a thread that is a week old, but I'm just catching up.
I remembered this thread coming up 5 years ago, and sure enough, I saved
the winner. Here 'tis:
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Date: Tue, 24 Mar 92 21:39:22 PST
From: pwv@tc.fluke.COM (Pat Vilbrandt)
Subject: Re: Voltage Stabilizers & Gauges
> From Jerry Kaidor:
> This weekend I built an electronic gizmo that produces a steady,
> regulated voltage for my gauge. The only trouble is, I still don't know
> what voltage to adjust it for ( Pat, where are you? ).
I'm here. It just so happens that I was out in the garage this evening
getting the information I had promised you! And I typed this in to my PC
*before* I read your message! No, really! Have I ever lied to your before?!?
(I figured I'd send this to the whole BCars list, since there have been
requests in the past for information on the Lucas "Voltage Stabilizers"
used in our venerable LBCs.)
Well, it took me longer to get around to it than I had planned, but I
finally dug out the 'scope and took a look at the output of the voltage
stabilizer in my TR4. (The voltage stabilizer is a heater/bi-metallic
strip gizmo, not unlike a turn-signal flasher, that is used in this car
to crudely regulate the battery voltage for the fuel and temperature gauges.)
Before I tell you what I saw, here is what the Bently Manual has to say
about it:
"The voltage stabilizer is a small sealed unit, located under
the facia on the right hand side of the car, and is used to
provide a constant current [sic] of 10 volts for the operation
of the fuel contents gauge and the Temperature Indicator."
What I saw, looking at the output of the stabilizer with an oscilloscope,
with the battery fully charged (12.6V at 50 degrees F) and the engine off,
was a square wave swinging from 0 volts to -12 volts (positive "earth"
you know), with a frequency varying widely from about 3 Hz to less than 1
Hz. The duty cycle was a bit more consistent at just under 50%. This would
correspond to, not surprisingly, an average voltage value of right around
6 volts.
To double check this, I built a low frequency "averaging" filter out of a
10,000 uF capacitor and a 1 kohm resistor (which gives a time constant
similar to that of the gauges), and measured the filtered output, which
was 5.9 volts, close to the value estimated above.
The fuel and temperature gauges in my TR4 are accurate and stable, so I assume
that the 10 volts mentioned in the Bently is incorrect, and that the value of
6 volts I measured is the correct value, at least for my car.
So, for those of you (like Jerry) that have considered making a "stabilizer"
out of more modern components, it appears that an LM7906 3-terminal regulator,
with a 10 uF or so capacitor at the output, and a 1N4001 diode in series with
the input from the battery to block opposite-polarity transients, would make a
suitable substitute. The current requirements of the gauges are rather modest
(around 100 mA with a cold engine and a 1/4 tank of gas), so it doesn't even
look like a heat sink would be necessary for the LM7906.
There you have it, Jerry. Good luck in putting together a voltage stabilizer
look-alike for the TR4 temperature gauge you're putting into your TR3
engined TR2! (I *think* I got all that right!)
Pat Vilbrandt John Fluke Mfg. Co., Inc. Everett, Washington USA
UUCP: pwv@tc.fluke.COM or: { uunet, uw-beaver, sun, microsoft }!fluke!pwv
ARPA: fluke!pwv@uw-beaver.ARPA
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Lee M. Daniels Laboratory for Molecular Structure and Bonding
daniels@cryogen.com Texas A&M University
'74 TR6 '77 MGB http://acs.tamu.edu/~lmd1191
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