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Subject: Re: dashboard rheostat etc...
Author: S1500@aol.com at smtp
Date: 9/8/95 6:20 PM
Bob Wrote:
>These rheostats are often salvageable. The 2 most likely failure modes
>are: resistance wire broken / burned out (burned out due to dash circuit
>shorting at some time), or corrosion of the "wiper" / resistance wire.
>This would be caused by moisture entering the device.....on a Triumph?
>nah, what am I thinking ? :-)
>
>Either way, if you are pretty good at dismantling things with small fiddly
>bits, you can open it up and take a look to see if it's fixable. What
>have you got to lose? As Jeremy pointed out, you can just bypass it.
>
>I'm not sure of the exact values, but I would guess that any "garden variety"
>rheostat from a junk car would work if a mechanical fit can bodged. Power
>dissipation should not be a problem. A Spit has very few dash lamps.
In my 1980 Spitfire circuit diagram, there are 7 lamps controlled by the
dimmer. Each lamp dissipates a nominal 2.2 watts. The rheostat must be able to
handle the 1.1 amps all these lamps draw at full brightness, but as the rheostat
is used to dim the lamps, this current falls so the actual power dissipated by
the rheostat is slightly less than 4 watts worst case (half power because of the
way series dimmers work). Any rheostat capable of handling about 5 watts with a
maximum resistance of twenty ohms or more should work for most LBCs. As a rule
of thumb, rated rheostat power of 2.5 to 3.0 times the total power of the lamps
and a maximum resistance 2.5 to 3.0 times the composite resistance of the dimmed
bulbs will give satisfactory service. I will measure the actual resistance of
my Spitfire's dimmer tonight and post back what I find.
regards, John Pratchios
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