Resistor for neon indicator lamp

Jon

In , George Herold wrote in part:

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 ( snipped-for-privacy@misty.com)

es,

ed

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.

Wow. Thanks John

Not sure if my email got to you, George. So just verifying by the usual post-a-message-on-the-public-cork-board method, sometimes called a newsgroup.

Let me know.

Jon

Excellent, lots of reading there!

George H.