<VV> Thermister Swap? two coil gauges

[corvairduval at cox.net]

The same two coil design was used for years on GM fuel gauges also. This is
one reason GM 6 volt fuel gauge will usually work if the car is converted
to 12 volts. 

Frank DuVal

Original email:
-----------------
From: Jim Simpson via VirtualVairs virtualvairs at corvair.org
Date: Mon, 8 Sep 2014 12:36:52 -0400
To: BobHelt at aol.com, virtualvairs at corvair.org
Subject: Re: <VV> Thermister Swap?


Bob, excellent question regarding the supply voltage.  I was using an HP
variable supply, set at 13.8 volts approximately.

I did wonder about the sensitivity of the gauge to supply voltage and found
that the readings didn't change over a very wide range.  I went as low as
about 8 volts and a bit over 14 with no observable change in the needle
position.

That said, I didn't check over the full range of the gauge, nor did I check
more than two gauges.

But this appears to be an area that GM got right.  They used bi-coil (two
coil) meter movements.  These are specifically designed to be insensitive
to supply voltage.  From the limited information I can find on this design,
they use two coils that essentially balance one another.  As long as the
current changes in both coils in the same ratio, the needle doesn't move.
One characteristic of this meter design is that when you turn off power,
the needle doesn't go to zero; it just sits there.

Jim Simpson

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