Requesting help with a faded 5 watt power resistor value

[AstutePauciloquent]

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.

[Kim Christensen]

Where is the resistor in the circuit? Across the output? What voltage rail if across an output?

[DavidAlfa]

Violet, Green, Silver, Gold?

[AstutePauciloquent]

Where is the resistor in the circuit? Across the output? What voltage rail if across an output?

Hi 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)

[DavidAlfa]

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!

[alm]

5V across 7 Ohm = 5V * 5V / 7 Ohm = 3.6W, which sounds sensible for a 5W resistor.

[wraper]

The resistor is 24mm long and 8mm diameter. In circuit it reaches 120C even when the PSU has no load.

As 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.

[AstutePauciloquent]

As 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.

The outer shell is brittle and cracking away. From what i gather running at 120C is normal for this resistor

[AstutePauciloquent]

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!

Yes it all works if the old worn out resistor is in circuit, and yes ive removed, tested and replaced all caps that needed it.

[AstutePauciloquent]

5V across 7 Ohm = 5V * 5V / 7 Ohm = 3.6W, which sounds sensible for a 5W resistor.

Im going to be perfectly honest here, this is the first time ohms law has ever made sense to me, totally cool, thanks!