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Resistors: Metal film and Carbon
Old 3rd April 2013 | Show parent
  #31
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Quote:
Originally Posted by cbc6403 ➡️
Except in one very dramatic way. I think the OP mentioned replacing the power supply rail decoupling resistors at one point. If something goes wrong with that circuit (like a bypass tantalum shorting) a carbon film resistor will open up like a fuse, while a metal film will sit there and burn (if the PSU can supply enough current), actually going lower in value as it cooks.

This isn't just theory. I've seen it happen in several Amek 9098 preamps. When the caps shorted, the current draw wasn't enough to blow the line fuse, but it eventually burned out the custom toroidal power transformer, making what would have been a fairly simple repair much more expensive and time consuming. If the designer had used 1/4W carbon film resistors instead of the same value in metal film, the fault would still have occurred, but the resistor would have opened up like a fuse, and it would not have had such dire consequences. The originally 10? resistors had dropped down to about 5?, and stayed that way after they cooled. Whatever special properties are claimed for metal film resistors, I doubt they apply to this simple DC circuit. If you need precision, measure a bunch and select a pair that are within 1% of each other.

John Roberts has already weighed in on this subject in another forum to say that not all metal film resistors behave this way, but the ones that Amek and Crest used for their production certainly do.

Geoff
Perhaps there is another John Roberts.

I have had ROHM carbon film resistors turn red hot on my bench and not fail open circuit, while the resistance value changed some after it cooled down. If hot enough they can melt solder or come apart. But they can not be counted on to fuse or protect anything.

There are special "Flame Proof" resistors (noted as FP on schematics) designed into such applications to intentionally fail open circuit when over heated and protect system power supplies from board level shorts. Tantalum caps and even common ICs were notorious sources of dead shorts across the PS rails. In a console you want just that input strip to go dark, not take the whole console with it.

UL likes to prevent forest fires, or studio fires, or house fires.

JR
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