Can resistors cause a short-circuit or change value?

[[email protected]]

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.

Thanks

 

[CJT]

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.

Thanks

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

Normally they fail open.

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

 

[[email protected]]

Can resistors cause a short-circuit?

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.

Tim.

 

[Guest]

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.

b.

 

[James Sweet]

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.

Thanks

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

 

[Fred McKenzie]

hello..
Can resistors cause a short-circuit?

Particularly in a powersupply?

Mikuz-

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.

Fred

 

[[email protected]]

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

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

 

[Jim Yanik]

[email protected] wrote in

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

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.

 

[me]

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.

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.