On Mar 2, 6:28*pm, (Don Klipstein) wrote:
In ,
George Herold wrote in part:

SNIP previously quoted material

Thanks John, *I must admit it's hard to understand how a 3 watt 100k
ohm resistor across the 240 V lines would blow even if everything down
stream of it was shorted. (As your calculation shows.) * A new circuit
board is in the mail, but this won't solve the mystery. *The board has
been redesigned and is no longer analog but digital. *Once I put the
new board in I'll look at the busted one and see if I can figure
anything out.

* I have seen resistors fail in ways that cause their resistance to
decrease until they get toasted and then go open circuit.

* My experience has been that this happens more with carbon composition
resistors. *Those tend to be cylinders with untapered ends. *Many times,
the composition is exposed. *The composition may absorb moisture and
experience a decrease in resistance, especially if it is operated only
intermittently. *This is more likely to be a problem where there is
exposure to moisture or high humidity or salty coastal air.

* And carbon's resistivity has a negative temperature coefficient.

* Preferably the resistor degrades rapidly and then goes open circuit
quickly by the time it allows anything else to get overloaded to a
damaging extent.

* A possible other explanation is that a severe line voltage surge caused
something in the resistor to carbonize. *The resistor becomes more
conductive and hotter. *The carbonization may then go into a runaway
situation, leading to sizzle-sizzle and then to *pow*, *poof* or *phut*.

*- Don Klipstein )

Thanks Don, This certianly wasn't a carbon composition. (Though I
was digging through a pile of 1-2 Watt 100-300k ohm carbon C's at work
today.) It doesn't seem like putting 100k in there can hurt anything,
though I don't expect it to work either... I figure something else
might have happened also.

George H.