[R-390] RE: Parts Selection

[Drew Papanek]

Tim and Mark:

Thanks for your observations on carbon composition resistor burnout.  Like 
you guys, in my younger days I used to intentionally burn up resistors (and 
other components) usually by applying  110 VAC just to see what would 
happen.  Seldom was the CUIDT (Component Under Intentional Destructive Test) 
metered to read current/voltage.  I burned up a few back in the high school 
lab and watched the meters on the power supply, but I don't recall any 
behavior indicating reduction of resistance just before smoke release.  Then 
again, I don't recall much of anything from that time; maybe too much "smoke 
release"... :-)

Another complication is the cabon comp lookalikes made a while back by IRC.  
Those were actually metal films in plastic cases.  Overload supposedly 
carbonizes the case material, reducing resistance.

The red-hot overload mode of metal film resistors brings to mind an idea:  
cut a MF resistor's leads to stub length, overlap and solder on each a wire 
extension and (no mechanical connection other than solder).  When the 
resistor overheats, solder melts and the resistor body (hopefully) falls 
away, opening the circuit.  Add a spring for more positive action and you 
have basically borrowed from the arrangement used in slo-blo 3AG style 
fuses.

Drew

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