You're playing on my turf here, so if I may, I'd like to add a comment to
"chrysler1978" 's statement:
Heat is generated in conformance with the expression P=I(squared)R, or in
English, the power disippated is equal to the square of the current times
the resistance, for a purely resistive circuit. So for a given current flow
through any path, the heat generated will be directly proportional to the
resistance of the circuit. If a switch contact develops significant
resistance due to crud, loose contact or corruption, the current will be
reduced somewhat, but the increased resistance will cause some localized
heating at the point of resistance, and at a particular range of values for
the resistance, there will be a very high heat generated at the point of
resistance. The maximum heating is when the value of the introduced
resistance of the switch contact equals the resistance of the design load,
but anything within a factor of 10 will cause enough heat to do damage to
the insulator that holds the contacts. Anyone who has had to replace a
fried blower switch on our beloved 81-83 coupes will be very familiar with
this phenomenon.
Neil's problem with his dash rheostat, however, is more likely to be a
situation where the nichrome wire of the rheostat has become coated with an
insulating layer of oxide or some other crud, which will go away if the knob
is rotated 187 times with the lights on. Do this at night where you can
notice the dash lights flicker when there is momentary contact through the
offending layer, and work with that spot, exercising the rheostat knob back
and forth; you'll see the good spot expand slowly, until finally the dash
lights will work like new again. This is true of any dash light control
for any year, if it is the type that controls the gauge illumination by a
rotary knob on the headlight switch.
Dick Benjamin (who has a 46 year old sheepskin somewhere which says I'm
qualified to speak on this subject.)
(I'm sorry to be thick, but "fruit bat impression"? OH! Hanging upside
down - Ding! I get it!)
----- Original Message -----
From: <chrysler1978@xxxxxxxxxxx>
To: <mailing-list@xxxxxxxxxxxxxxxx>
Sent: Monday, September 16, 2002 4:19 PM
Subject: IML: What causes heat in conductors?
> I must point out. Resistance does NOT cause heat. Current flow causes
> heat! If there is a heat problem in a conductor, that means there is more
> current flowing in that conductor than the conductor can carry without
> heating up. The misconception that resistance causes heat is a sure way
to
> religate one's self to never being able to properly diagnose electrical
> problems.
>
> Why then does a corroded conductor often over heat? Different reasons.
> 1. If the conductor has corroded and is now smaller than it originally
was,
> it will heat up.
> 2. If there is a build up of corrosion in an electrical connection, that
> corroded area will heat up when current flows through it.
> ...and others.
>
> What has to be remembered is, CURRENT FLOW CAUSES HEAT! That is a fact of
> physics and will never be proven wrong.
>
> ----- Original Message -----
> From: "Neil Thorpe" <nt014b6628@xxxxxxxxxxxxxxxx>
> To: <mailing-list@xxxxxxxxxxxxxxxx>
> Sent: Monday, September 16, 2002 1:29 PM
> Subject: Re: IML: 81-83 headlamp switch and disapearing dash display
>
>
> I had tried to clean up the old light switch before
> ordering a replacement with electrical switch
> cleaner which didn't make any difference.
>
> The rheostat was corroded probably due to
> my car spending more time parked than driving
> 47000 miles in 20 years (ave 2350 miles per year)
>
> The corrosion had probably acted as an insulator
> instead of creating a high resistance therefore
> i have hopefully avoided the burnt wire
> problems you have encountered.
>
> One bonus of playing with the light switch so
> much is my fruit bat impression is getting
> much better.
>
> Neil Thorpe black 82 imp
> London, England.
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