Subject: Re: [Shop-talk] Need a variable 85-285 ohm resistor

From: Darrell Walker <[email-masked]>
Date: Tue, 12 Mar 2013 19:48:38 -0700

Hi Randall,

It definitely isn't a bimetallic strip type.  There appears to be two windings
overlapping in a kind of inverted "V", and the needle is free to float between
them.  My guess is the windings create a magnetic field that moves the needle,
but how that magic happens is beyond me.

I ended up ordering a 500 ohm, 1/4 watt pot. When it arrives,  I'll set it to
500 ohms and report back!

-Darrell

On Mar 12, 2013, at 6:54 PM, Randall <[email-masked]> wrote:

> Good point about power dissipation.  Wish I had a good answer.
>
> There are several different designs of electric automotive gauges; and I
don't know which type Darrell has.  I believe the Jaguar oil pressure gauge
that I have uses a "balanced" magnetic movement somewhat similar to the design
used for the fuel gauge in my TR3 but with somewhat higher resistances.  Here
is a circuit diagram:
> http://mgaguru.com/mgtech/electric/fg_05.htm#fg_diag
>
> But I haven't had it hooked up yet, so I'm not certain.
>
> Another common movement type uses a bimetal strip and a heater.  As the
strip is heated, the differential expansion of the two dissimilar metals
forces the strip to bend.  That motion is transmitted to the needle through a
lever that greatly amplifies the movement.  This type of movement was used for
the fuel and temp gauges for TR4-6, as well as my 1970 Audi.
>
> None of which really answers the question.  Best suggestion I have is to
start with the pot at highest resistance, then measure the voltage across it
and calculate an equivalent resistance for the gauge.  Then you can calculate
the current and power dissipation at 85 ohms and see if you are in trouble.  A
higher power control will quickly get you into territory where it makes better
financial sense to just buy the sender.
>
> Unless maybe you can poke around and find one on the surplus market for
cheap.  There are a couple here:
> http://goo.gl/trCYq
> that would do.
> Randall
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From: Darrell Walker <[email-masked]>
Date: Wed, 13 Mar 2013 06:57:18 -0700

I did a little more inspection of the gauge, and checked the resistance
between the power connection and the sensor, and power and ground.  I see 328
ohms between power and ground on the gauge, and 178 between power and the
sensor connection.  Looking at the link that Randall provided, I believe the
operation of my gauge is the similar, though the construction is different.
One coil is wired from power directly to ground, and the other is wired
through the sensor, so the resistance in that path would vary between 263 and
463 ohms (178 plus the variable resistance from the sensor).  I believe this
causes two magnetic fields (one fixed, one variable) that act on the needle
and register the oil pressure (as presented by the variable resistance in the
sensor).

At least, that's my guess!
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