Requesting help with a faded 5 watt power resistor value - Page 1
Forum section: electronics / repair

author: AstutePauciloquent
author_rank: Contributor
timestamp: December 10, 2023, 01:07:27 am
content: Hi everyone, my first post here. I am working on restoring a vintage ATX PSU. One of the power resistors on the low voltage side seems to have been overloaded, and the outer shell has chipped away. Measuring Ohms with a Multimeter shows 7.16 Ohms. My first guess of the resistor value based on the colors would be Black, Green, Brown, Gold 50 Ohm 5%. However I don’t think this is correct.My Second guess of the value would be Violet, Green, Gold, Gold. Putting the value at 7.5 Ohms 5%The resistor is 24mm long and 8mm diameter. In circuit it reaches 120C even when the PSU has no load.I have tested a 50 Ohm replacement and it is definitely triggering current protection, as it doesn’t power on on as it should. So I’m thinking that it is indeed a low Ohm resistor. But with the colors and measured values not making any sense, im at a loss. Hopefully someone can help, thanks for your time.

author: Kim Christensen
author_rank: Super Contributor
timestamp: December 10, 2023, 04:08:21 am
content: Where is the resistor in the circuit? Across the output? What voltage rail if across an output?

author: DavidAlfa
author_rank: Super Contributor
timestamp: December 10, 2023, 04:09:07 am
content: Violet, Green, Silver, Gold?

author: AstutePauciloquent
author_rank: Contributor
timestamp: December 10, 2023, 06:07:41 am
content: Quote from: Kim Christensen on December 10, 2023, 04:08:21 amHi thanks for the reply; this resistor is between Ground (Black wires going to 20 Pin ATX plug) and the +5V rail (Red wires going to 20 Pin ATX plug)

author: DavidAlfa
author_rank: Super Contributor
timestamp: December 10, 2023, 06:45:15 am
content: Seems a minimum load resistor, 7 ohms looks ok for a 5V rail in an old PSU.But there's any fault in the PSU at all, or did only fail when putting that random 50ohm resistor?Probably what's getting triggered is the overvoltage protection due a too small load.Did you recap it? Don't overlook the small ones, I've fixed lots of PSUs killed by a dry 2.2 or 10uf cap, nothing else!

author: alm
author_rank: Super Contributor
timestamp: December 10, 2023, 11:32:24 am
content: 5V across 7 Ohm = 5V * 5V / 7 Ohm = 3.6W, which sounds sensible for a 5W resistor.

author: wraper
author_rank: Unknown
timestamp: December 10, 2023, 11:47:51 am
content: QuoteAs it's working, why not just measure it? Also why would you need to replace it? It's heating, so it's working and there is not reson to replace it.

author: AstutePauciloquent
author_rank: Contributor
timestamp: December 10, 2023, 07:42:30 pm
content: Quote from: wraper on December 10, 2023, 11:47:51 amThe outer shell is brittle and cracking away. From what i gather running at 120C is normal for this resistor

author: AstutePauciloquent
author_rank: Contributor
timestamp: December 10, 2023, 07:45:43 pm
content: Quote from: DavidAlfa on December 10, 2023, 06:45:15 amYes it all works if the old worn out resistor is in circuit, and yes ive removed, tested and replaced all caps that needed it.

author: AstutePauciloquent
author_rank: Contributor
timestamp: December 11, 2023, 05:09:59 am
content: Quote from: alm on December 10, 2023, 11:32:24 amIm going to be perfectly honest here, this is the first time ohms law has ever made sense to me, totally cool, thanks!