Girault,
You are correct that the bi-metal strip has two modes, open (zero volts)
and closed (12 volts) the ratio of on to off time averages the voltage
to about the same average voltage as a constant 6 volts. The instruments
also have bi-metal elements in them which respond to the heating and not
heating of the current through them. The on off cycling is in the
neighborhood of 2 per second. There is enough thermal load in the
instruments to not rise and fall significantly at this rate. If you turn
the key on with a full fuel tank, watch the fuel gauge closely. You will
see the needle move up a bit, then stop, then move some more, and repeat
this until it reaches its final value. This is caused by the pulsing
voltage heating up the bi-metal strip some, then shutting off and
repeating. This type of instrument is very rugged in comparison to a
moving coil instrument that requires almost frictionless bearings
supporting the needle.
As John Sandhoff correctly pointed out in his reply, using a bi-metal
strip has the additional feature of adjusting the on/off ratio depending
on ambient temperature. All in all this is a quite inexpensive, rugged
and elegant method of driving instrument in a vehicle!
Peace,
Pat
girault@bellsouth.net wrote:
>Thanks to Tom, John, Gordon, Jones, Dan, and Bob for their responses to my
>electrical question. You know, I really enjoy the list every day. When I
>don't have time to work on the car, at least I can talk to somebody about it.
>But my curiousity is aroused over this devilish little voltage regulator. It
>has a bi-metallic spring with a contact inside. It's either open or closed.
>How can it regulate the current to the water temp and gas gauges in a
>constant manner? It seems like it would have two modes - either open or
>closed. Wouldn't that make the gauges jump when it activated?
>Thanks, Girault Jones
>
>
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