"rijo1" bravely wrote to "All" (19 Nov 04 15:30:55)
--- on the heady topic of "Ohm's Question"

ri From: rijo1
ri I am working on a circuit board out of a mig welder. The problem is a
ri resister burnt and I can't read the some of the color bands. This
ri circuit board controls wire speed, arc stabilizer, etc. There is also
ri high voltage on this board . How would be the best way to determine
ri the proper resistance of the burnt resister ? Thanks for any help in
ri advance .

Just to add to what has been said that sometimes using a hand
microscrope with a strong light to view the underside of the resistor
body side which sustained the least burn can sometimes turn up flecks
of pigment which still survive.

Another trick with unknown resistors is to measure the resistance of
whatever is left and estimate the value from the physical dimensions.
This involves using needle probes, etc.

Another strategy is to try a ballpark value. For example a small
resistor value is 1 ohm and a large value is 1 Meg, so pick the
geometric mean or 1K ohms. Generally 1K ohms is a middle value for
many circuits. Consider that if 1K is too large it won't damage much
and if it is too small it also won't damage much either.

But you should try figuring out what the resistor does first!

A*s*i*m*o*v

.... Thomas Edison invented the "Light Emitting Resistor"