Some spare time... I just *have* to wander into this one...
On July 17, 2010 04:46:01 pm fons@email-addr-hidden wrote:
> Measuring the power button switch with a multimeter showed
> an unstable resistance value of between 1 and 3k while it
> was not pressed. So I removed the thing, which turned out
> to be a cheap miniature switch, a little cube of around 8
> mm size. I opened and disassembled it, and noticed that the
> contacts had some black dirt on them. Cleaned with aceton
> and reassembled, and things worked perfectly again.
You say the switch had conductance when not pressed?
Then that black dirt was not just dirt, it was something conductive.
It couldn't be oxide. Oxide increases the switch resistance.
Was the switch that simple membrane style I like to call
"juice bottle safety pop-up cap"? Or did it have an actuator
arm which travels when you push it?
Either way a switch with undue conductance is extremely rare.
I'd say it's a total fluke. Some conductive solder may have fallen inside.
Also, are you sure it was not an unwanted stray thin conducting solder bridge
or wire somewhere on or near the switch, or on the board it connects to?
You'd be surprised the odd things I've seen.
Cleaning 'black dirt' from the contacts should *not* have solved it
unless the 'dirt' itself was conductive.
>
> What I don't understand is how the contacts got so dirty.
> If a resistance of a few kOhm is enough to make it look
> as a closed contact then it can't be handling large currents,
> so there should not be any arcing.
It's a low current soft power switch. The PSU does the heavy switching.
I would like to put in a plug for Caig Laboratories' DeoxIT chemical
products. Some of the best contact cleaner I've ever used.
And when *that* fails on stubborn oxidation and you're *still*
too lazy to disassemble the switch and clean the contacts by hand
with a *pencil* *eraser* (which is the ultimate solution), then try
spraying some powerful PC board flux remover in there.
That got me out of a few jams but watch out, it eats cosmetic plastics...
Cheers. Tim.
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Received on Mon Jul 19 12:15:01 2010
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