Shorted resistor, is this possible?

[fastline]

Working on a sprinkler controller. We already know that one the field solenoids is shorted and caused the system to pull maybe about 8A where it should be drawing less than 1A. There is no fuse (smart ones) on the controller. There is a power resistor (maybe 2watts) inside the device among other things that got VERY hot. It actually cooked all the coating off and desoldered itself. Because the bands are gone, there is no chance of determining the value.

Upon testing it, it reads about .3ohms. The leads both conduct with the metal barrel or case. I have never had a resistor short so I am curious if this is a possibility or if this seems normal?

 

[Nigel Goodwin]

Resistors going S/C is VERY, VERY rare - but it can happen very occasionally on specific types of resistors.

 

[fastline]

That is exactly what I thought and makes me wonder if this was actually a zero ohm resistor fro the start. I do have another board I can look at to verify because I have never seen one short either.

 

[JimB]

Resistor going short circuit?
I dont think that I have ever seen that.

However you say:

The leads both conduct with the metal barrel or case.

Does this mean that this is a metal cased resistor, the type with fins and can bolt on to a mounting plate?
If that is so and there is a conduction path between the wires and the case, the resistor is truly broken.
Often the metal cased resistors will explosively blow their ends out when severely overloaded.

JimB

 

[fastline]

No, this is a typical axial lead metal oxide type resistor with a metal case. No heat sink or anything of that nature.

 

[fastline]

Well, I inspected a known good device and the resistor in question apparently is NOT shorted. Other one was reading near the same. That being said, I cannot find a reference for the band markings anywhere. Here is the banding colors from one end to the next. Green, Red, Silver, Gold. What am I looking at?


OK, I finally found my more complete chart and looks like this is a .52ohm resistor. Actual measurement with the ESR meter is .35ohms so it IS slightly out of tolerance.

 

[KeepItSimpleStupid]

Google Image Result for https://www.williamson-labs.com/images/resistor-color-code-all.gif

0.52 ohms 5%

You won't be able to measure this accurately without using a 4 terminal technique.

Metal oxide resistors will usually just open. They are designed to do that. Cabon composition resistors usually increase in value ad WW resistors have a tough time breaking.

 

[fastline]

My ESR meter is a low ohms meter but does test at 100khz. it is my understanding though that it does not matter about the frequency when testing a resistance? .35ohms is concerning me a bit that it will just overheat again even though the original problem was an overloaded circuit.

 

[KeepItSimpleStupid]

What does a good one measure? I would expect R to go up if overloaded.

It's probably used for a reliable fuse.

 

[fastline]

Uh, as a fuse, it did not work....lol The PCB is a little smoke, roasty and took out an isolation transformer with it. I am adding a fast acting fuse in the system now... I already put it together with the old resistor so I guess we will see what it does...