On Mon, 17 Jul 1995, Roger Garnett wrote:
>
> From: pwv@tc.fluke.com (Pat Vilbrandt)
> Subject: Re: Volage stabilizer, HELP!
> Cc: british-cars@autox.team.net
>
> >From the TR3-TR4A Bently Manual:
>
> "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 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.
>
> --- End of forwarded message from pwv@tc.fluke.COM (Pat Vilbrandt)
>
> Keep in mind, tho, that the mechanical voltage stabilizer/gauge combination
> may have inherent temperature compensation built in as a _system_. I seem to
> recall someone mentioning that after they had built a solid-state equivalent,
> their gauges showed some variability as the ambient temperature changed.
>
> Pat Vilbrandt Fluke Corporation Everett, Washington USA
> pwv@tc.fluke.COM or: { uunet, uw-beaver, sun, microsoft }!fluke!pwv
>
> ----
On my '79 B, the voltage reads about 10.25v at the fuel gauge w/a full
tank - Maybe Lucas changed the specs somewhere along the line without
telling anyone.
The current requirements would be higher with a full tank also - I
computed .67 amps @ 10v - Maybe a heat sink would be a good idea.
Jim Mellander
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