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'[EE] Desk-Magnifier - Failed Resistor'
2006\10\04@122240 by Rolf

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Hi all.

Looking for some insight as to the purpose of a resistor in a desk
lamp... specifically, an illuminated magnifier, 120V AC lamp that has a
circular Flourescent tube. The lamp was not working, and I tracked it
down to a failed-open resistor, 0.1 Ohm 5% 1W in series with the live AC
current.

120V live wire>--------------/\/\/\/\/\/\-------->to rest of circuit
                             0.1 Ohm

With the resistor failed open, the circuit of course does not work. The
resistor is not a current-sense resistor (there is no sensing circuit).
I have bypassed/shorted the failed resistor, and (briefly) tested the
circuit, and it all works (the light goes on... ;-).

I intend to replace the resistor with an equivalent (but higher power
rating), but my curiosity demands that I understand why there would be a
0.1Ohm resistor in series with Live. The best I can figure would be a
current/surge limiting resistor, but I would have thought it would be a
higher value. I was hoping for some other suggestions/confirmation.

Thanks

Rolf


2006\10\04@123909 by Marcel duchamp

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Sometimes manufacturers will do this with flame-proof resistors that are
there simply as a fuse in case of a short circuit else where drawing
excessive current.  Perhaps they are a few cents cheaper than a fuse.

Rolf wrote:
{Quote hidden}

2006\10\05@023823 by Russell McMahon

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> Looking for some insight as to the purpose of a resistor in a desk
> lamp... specifically, an illuminated magnifier, 120V AC lamp that
> has a
> circular Flourescent tube. The lamp was not working, and I tracked
> it
> down to a failed-open resistor, 0.1 Ohm 5% 1W in series with the
> live AC
> current.

*IF* it is what you say it is then it has to be intended to act as a
fuse - and as a very gross one at that. It serves no useful purpose
otherwise due to its low value.

BUT I = sqrt(Power/R) = sqrt(1/0.1) ~= 3 Amp at rated wattage.
That would only fail under heavy short circuit or a load of 1 kW plus.

POSSIBLY it is a PTC which is meant to have a higher value when hot.
This also seems unlikely.  The fact that it has failed suggests either
that you have been trying to run heaters from your lamp :-) or that it
is not what it appears to be (or that it was faulty).



       Russell


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