Resistors failing without optical damage?

mr_push_pull · 24th November 2011, 11:27 PM

an amp of mine failed recently because external mains overvoltage. ok, I was reckless enough not to fit a fuse and a MOV.
there were some indicator LEDs on the front plate. they were all burnt and I replaced them only to find that they were still not working. there were 180 ohm resistors in series with all of them and upon measurement I found that ALL the resistors were damaged (circuit interrupted). NONE of the resistors showed any usual sign of physical damage. basically all the resistors that were connected to the unprotected part of the amp were damaged without any optical trace.
this is the very first time something like this happens to me, I've had tons of cooked resistors (physically burnt) in the past. and the large number of them presenting the same type of failure makes me wonder...

wrenchone · 24th November 2011, 11:33 PM

Flameproof resistors if done right expire with a tiny puff of smoke and a little pin prick in the coating, unlike their less safe siblings who either go up in an impressive ball of flame or glow cheerfully orange and set the place on fire.

jitter · 25th November 2011, 05:36 AM

I have seen this behaviour of resistors in an overcurrent fault at work once before.
When I was looking up the failed parts in the bill of materials, I found out that the resistors weren't ordinary resistors but fusible resistors.

If you have the service manual of your amp, the bill of materials should list them as fusibles, and often in the schematic an exclamation mark is printed next to them to indicate they MUST be replaced by identical type.

Elvee · 25th November 2011, 08:34 AM

I have also seen numbers of ordinary, lacquered resistors, failing open without the slightest external sign of damage.

Sometimes, the cause is known, as when it serves as current limiter in capacitive supplies, but sometimes there is no apparent reason.

simon7000 · 25th November 2011, 01:35 PM

Long term over power burns resistors. High current pulses can blown them open quickly without lots of heat so no burn marks!

mr_push_pull · 25th November 2011, 01:50 PM

yeap, the shock was quick. but I found about the existence of flameproof resistors so I guess the topic wasn't for nothing.

simon7000 · 25th November 2011, 01:55 PM

Flame proof resistors are always made in blue base color so you can tell the difference.

mr_push_pull · 25th November 2011, 01:58 PM

the failing resistors were the regular "buttery" color.

simon7000 · 25th November 2011, 02:03 PM

Then fuses are a very good idea!

mr_push_pull · 25th November 2011, 02:06 PM

Quote:

Originally Posted by simon7000

Then fuses are a very good idea!

clearly.

24th November 2011, 11:27 PM	#1
mr_push_pull diyAudio Member Join Date: Mar 2004 Location: At the output stage	Resistors failing without optical damage? an amp of mine failed recently because external mains overvoltage. ok, I was reckless enough not to fit a fuse and a MOV. there were some indicator LEDs on the front plate. they were all burnt and I replaced them only to find that they were still not working. there were 180 ohm resistors in series with all of them and upon measurement I found that ALL the resistors were damaged (circuit interrupted). NONE of the resistors showed any usual sign of physical damage. basically all the resistors that were connected to the unprotected part of the amp were damaged without any optical trace. this is the very first time something like this happens to me, I've had tons of cooked resistors (physically burnt) in the past. and the large number of them presenting the same type of failure makes me wonder... __________________ "This site gives off very confusing signals, one minute highly technical discussions that leave the truly interested learners wallowing in their own stupidity and the next finds it endlessly repeating old wives tales and cockeyed theories as fact."

25th November 2011, 08:34 AM	#4
Elvee diyAudio Member Join Date: Sep 2006	I have also seen numbers of ordinary, lacquered resistors, failing open without the slightest external sign of damage. Sometimes, the cause is known, as when it serves as current limiter in capacitive supplies, but sometimes there is no apparent reason. __________________ . .Circlophone your life !!!! . . ♫♪ My little cheap Circlophone© ♫♪

25th November 2011, 01:50 PM	#6
mr_push_pull diyAudio Member Join Date: Mar 2004 Location: At the output stage	yeap, the shock was quick. but I found about the existence of flameproof resistors so I guess the topic wasn't for nothing. __________________ "This site gives off very confusing signals, one minute highly technical discussions that leave the truly interested learners wallowing in their own stupidity and the next finds it endlessly repeating old wives tales and cockeyed theories as fact."

25th November 2011, 01:58 PM	#8
mr_push_pull diyAudio Member Join Date: Mar 2004 Location: At the output stage	the failing resistors were the regular "buttery" color. __________________ "This site gives off very confusing signals, one minute highly technical discussions that leave the truly interested learners wallowing in their own stupidity and the next finds it endlessly repeating old wives tales and cockeyed theories as fact."

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24th November 2011, 11:33 PM	#2
wrenchone diyAudio Member Join Date: Feb 2004 Location: Silicon Valley	Flameproof resistors if done right expire with a tiny puff of smoke and a little pin prick in the coating, unlike their less safe siblings who either go up in an impressive ball of flame or glow cheerfully orange and set the place on fire.

25th November 2011, 05:36 AM	#3
jitter diyAudio Member Join Date: Aug 2004 Location: Eindhoven, The Netherlands	I have seen this behaviour of resistors in an overcurrent fault at work once before. When I was looking up the failed parts in the bill of materials, I found out that the resistors weren't ordinary resistors but fusible resistors. If you have the service manual of your amp, the bill of materials should list them as fusibles, and often in the schematic an exclamation mark is printed next to them to indicate they MUST be replaced by identical type.

25th November 2011, 01:35 PM	#5
simon7000 diyAudio Member Join Date: Nov 2008 Location: Oakmont PA	Long term over power burns resistors. High current pulses can blown them open quickly without lots of heat so no burn marks!

25th November 2011, 01:55 PM	#7
simon7000 diyAudio Member Join Date: Nov 2008 Location: Oakmont PA	Flame proof resistors are always made in blue base color so you can tell the difference.

25th November 2011, 02:03 PM	#9
simon7000 diyAudio Member Join Date: Nov 2008 Location: Oakmont PA	Then fuses are a very good idea!

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