Can resistors cause a short-circuit or change value?

hello.. Can resistors cause a short-circuit?

Particularly in a powersupply? The reason i ask is that because i'm fixing one. From my thinking resistors are put in a circuit to give resistance unless they change? I have seen resitors blown but thats considered high resistance(infinit)because the resitor is basically gone and thats not a short circuit i'm talking about. A short circuit resistor is where a resitor would let most of the current through hence creating a short circuit situation is this common or even possible.


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It's quite unusual, but not impossible (given a catastrophic failure).

Normally they fail open.

And they clearly can (and do) "change value."

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Yes. Especially in a power supply where moderately high to high voltages exist, not only do you have to worry about a plain old short, but also about the resistor arcing over internally.

There are resistors that are guaranteed to not go short-circuit in a failure, they generally have a clause something like "defined interruption behavior" in their description.

Generally resistors that overheat moderately (say 5 to 10 times their rated dissipation) go up in value but unless you pay the extra money for "defined interruption behavior" then they can and occasionally do go the other way especially in extreme overloads (hundreds or thousands of times of rated dissipation).

I have seen metal film resistors get so hot that they were glowing dull red to bright orange, probably 100W being dissipated in a 1/2W package.

All that said, usually the root cause of the resistor burning up is not the failed resistor, but another circuit element going short-circuit and causing the resistor to fail. This is doubly true in a switching PS.


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I've seen carbon resistors decrease in value when failed. Most of the resistors I've been seeing presently are not carbon resistors but mostly film-type resistors. In that experience, they typically "open" upon failure.


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I've never seen a resistor fail shorted, they always go up in value or go open completely when they die.

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James Sweet


A short circuit in a resistor is certainly possible. However it would be extremely unusual unless its voltage rating was exceeded, resulting in a dielectric breakdown.

Is it a new supply? I've seen wrong-value resistors in circuits go unnoticed because of off-color bands or illegible markings. Such a power supply might appear to be OK prior to a delayed failure caused by the wrong-value resistor.


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Fred McKenzie

Not necessarily. I've replaced large value (100K 1 or 2 Watt) molded carbon resistors because they lowered in value, 1 down to 15K. The resistors were running around 220V so it was only dissipating 1/2 Watt but needed to be 2 Watts because of the high voltage. GG

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stratus46 wrote in news:

This used to happen very frequently in TEK 520 and 520A vectorscope power supplies.(drop in value) The 2W carbon comp resistors would get so hot they would drop off the PCB,or char it to carbon(or both). 39K was one of the values I remember.

Jim Yanik
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Jim Yanik


I've seen one that measured correct 33k value on meter, but when power was applied to the circuit (old transistor short wave radio) the value dropped to nearly nothing...

for what its worth.

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