Do damaged resistors become fat at damaged end? - Page 1
Forum section: electronics / repair

author: bypassrestrictions
author_rank: Contributor
timestamp: January 26, 2022, 10:00:43 am
content: Do damaged resistors become fact at damaged end? I have green capsule like resistor on PCB, it has 22R 5% 3W S written on it, it is slightly fat at one end compared to the other end, is this a sign of damage?

author: tszaboo
author_rank: Super Contributor
timestamp: January 26, 2022, 10:06:54 am
content: Why dont you attach an image so we can see and tell you?

author: bypassrestrictions
author_rank: Contributor
timestamp: January 26, 2022, 10:19:11 am
content: Quote from: tszaboo on January 26, 2022, 10:06:54 amThe fatness is not visible in the photo but here is one anyway:

author: Mario87
author_rank: Frequent Contributor
timestamp: January 26, 2022, 10:26:00 am
content: Why not just measure it's resistance value with a DMM? Is it anywhere near 22 Ohms, or way off?

author: Ian.M
author_rank: Super Contributor
timestamp: January 26, 2022, 10:33:21 am
content: *NO*. In general, an overloaded resistor typically burns up, may blow off its coating, or even fracture its body if the overload was severe enough, or if ceramic or metal cased may spew its guts out one end. I cant think of any failure mode without exposure to harsh chemicals that could cause one end to selectively swell.For a vitrified wire wound resistor like that one, that is effectively hermetically sealed inside an inflexible vitrified ceramic coating, even harsh chemicals couldn't do it. Its almost certainly due to original uneven coating, or its orientation while the coating was fired to vitrify it.Measure its resistance - if its still 22 ohms +/-5% its still good.

author: bypassrestrictions
author_rank: Contributor
timestamp: January 26, 2022, 10:58:34 am
content: Quote from: Ian.M on January 26, 2022, 10:33:21 amQuote from: Mario87 on January 26, 2022, 10:26:00 amI checked with DMM set to 200 and it was showing around 24, etc, I'm guessing it is fine?

author: Mario87
author_rank: Frequent Contributor
timestamp: January 26, 2022, 11:25:02 am
content: Quote from: bypassrestrictions on January 26, 2022, 10:58:34 amIt looks like a 22R resistor with 5% tolerance which means it should be between 20.9 & 23.1, so if your DMM shows 24, I would say it's fine, despite being a little out of range, but that reading could be down to your DMM not being all that accurate to begin with.

author: Harfner
author_rank: Contributor
timestamp: January 27, 2022, 12:16:14 pm
content: The DMM could be even accurate. The TO did not spezify his DMM. Assuming a standard 3-1/2 digit DMM, the lowest ohm range is probably 200 ohms, making +/- 1 digit equal to +/- 1ohm. The resistor is in spec with 23ohm, and the DMM is in spec showing 24ohm.

author: Ian.M
author_rank: Super Contributor
timestamp: January 27, 2022, 12:58:55 pm
content: Yes. Also if the resistor is on the high end of its tolerance range, it doesn't take much contact resistance at the probes and at the meter sockets to push the total resistance over 23.5 ohms, so even a 3 1/2 digit meter that is 'spot on' would read 24 ohms on its 200 ohms range. Below 20 ohms an ordinary hand-held DMM is often less accurate than an old analog ohmmeter, and once you get down to fractional ohm current sense resistors, the best you can hope your DMM to tell you is whether or not they've gone open-circuit.If you've got an ESR meter its often a better choice for checking low-ohm resistors. e.g. I tend to use one of these when I need to check sense resistors:If you are serious about reading low value resistors accurately, you *NEED* a four wire (Kelvin) ohmmeter or at least a calibrated current source so you can do four wire measurements with an ordinary meter on a low mV range. Seehttps://en.wikipedia.org/wiki/Four-terminal_sensing