Vic writes:
>This brings up an interesting topic. Can anyone explain the operation of
>the voltage stabilizer? In my mind, I always thought it operated similar to
>a voltage regulator and could easily be replaced with an electronic
>version.
Well...you're not alone in this thinking. But you know those Brits
...they're tricky. Leave it to them to come up with a slightly
different word usage to throw us Yanks off the trail. :-)
The voltage stabiliser as used in Spits and other LBCs is not a
voltage regulator. It's not designed to deliver constant voltage.
Rather, it's designed to deliver a variable averaging voltage with
regard to current draw and ambient temperature. Geesh...what a
horrible sentence. :-)
The stabiliser is very specifically designed to operate with heater
type gauges. For the Spitfire this means only the fuel and
temperature gauges...nothing else.
Heater type gauges can be good things. They're cheap and rugged.
They're slow to react, so you won't see your fuel gauge dancing around
on bumps in sync with the float sensor. The operation of the gauge is
simple. Current passes through a bimetal heater causing it to bend as
it heats up. The gauge needle is mechanically attached so it moves as
the heater deflects. The more current that passes through the heater,
the more it will bend and deflect the needle...BUT, for a given amount
of current it will read lower on a cold day than a warm one. It will
also tend to "drift" upwards as the gauge itself gets warmer. This is
where the stabiliser comes into play.
The stabiliser also has a bimetal heater. That heater is opening and
closing *its own* contacts that supply the gauges. The length of time
that the contacts stay closed is determined by the current draw from
the gauges AND ambient temperature. If ambient temperature is low
then the stabiliser contacts will tend to stay closed longer and
deliver more current to the gauges. More current to the gauges on a
chilly day means that *they* will read more accurately. Thermal drift
in the gauges due to resistance changes in the gauge heaters will tend
to be offset by thermal drift in the stabiliser. Neat huh? :-)
Solid state regulators aren't temperature compensated the way the
stabiliser is. They're also can't run in a non-linear mode that
would compenate for changes in heater resistance. Hmmm....just
thought of something. What if we used a three terminal regulator with
a thermistor on the controlling leg...then we'll add a varis...
Gotta chat with Dan on this one...we could be rich! :-)
>Same issue with the fuel sensor. The one that I know of is just a
>wire-wound variable resistor. The amount of current isn't large and I doubt
>that it heats up and I don't think that you would want a heat source in the
>fuel tank. The voltage source that drives the circuit (thru the resistor
>and gauge) is the from the stabilized voltage. I don't understand how one
>will offset the other.
Vic...the sensors, in this case the fuel and temp sender, aren't
really a factor a far as compensation goes. They themselves don't
heat up and change the circuit operation to any appreciable degree.
They're pretty much just dumb variable resistors. The temperature
compensation loop is just between the gauges and the stabiliser.
BTW...I'd love to take credit for this brilliant observation, but alas
it's not mine. :-( Long ago list members Gregory Meldon, Jerry Kaidor
and a few other EEs were experimenting with solid state regulators and
found them wanting.
They went BACK to that old, rotten, thermo-electric "piece of sh*t"
that came with their cars. Not always easy to improve on good, solid
LUCAS engineering. :-)
Cheers!
Tom O'Malley
'74, '77 Spits
|