note: Bob H- here's another one for the tech page. rg
>From jurrasm@genesis.torrington.comFri Jul 14 18:22:19 1995
> >I'm am wondering if anyone has replaced the voltage stabilizer with a
> >three terminal regulator. This seems a more straight forward approch
> >since no dropping resister needs to be added to protect the zener.
This was thrashed out pretty completly here a few years ago. At the time Pat
Vilbrandt, our resident electrical engineer did a pretty good analysis, and
developed a working replacement. Of course, the voltage stabilizers do usually
do a pretty decent job, but if you want to update, here's what you do:
From: pwv@tc.fluke.com (Pat Vilbrandt)
Subject: Re: Volage stabilizer, HELP!
Cc: british-cars@autox.team.net
> I think Pat Vilbrandt posted a schematic a while back for making a solid
> state version of this device...
More of a verbal description, actually. Here is what I sent out:
>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
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