Resistor Identification Problem

Thread Starter

grant1842

Joined Aug 21, 2010
7
I have a EVGA atx 600w power supply that went down.
I found 2 resistors that are shorted.
I am having problems figuring out what value they are .
Looks to be Black, Green, Silver , Gold, Green.
Can not seem to come up with a value, any help is appreciated.
 

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ebp

Joined Feb 8, 2018
2,332
These are oddballs with a leading zero, which is rare for color coded parts.

black green silver is 0 5 x 10^-2 ohms (i.e. 05 then shift the decimal point two places left) - 0.05 ohms = 50 milliohms
gold is ±5% tolerance
I have no idea what the green band means

These would read as shorted with any common ohmmeter. You really need a good "4 lead" type to measure such low resistance.

Judging by the holes in the PCB, there were two in parallel. They are almost certainly current sense resistors, which typically fail open-circuit if something goes wrong (most commonly they are in series with a power FET which is the main switching element in the circuit and FETs usually fail short-circuit, putting huge current through the sense resistors and causing them to blow like fuses - usually visibly so).

Switchers are very hazardous to work on. Everything on the primary side of the transformer must be considered to be connected directly to AC mains HOT. There will be large capacitors that will hold sufficient charge to give a very nasty shock. You can take a wholloping great DC shock without being killed, but you can still be badly injured when you jerk your and back and punch yourself in the face, or involuntarily throw a power supply across the room. The caps can hold charge for many minutes if the switcher isn't working.

If the main switching FETs haven't failed (and they probably haven't, since those resistors seem OK), the next most probable, and very highly probable failure is electrolytic capacitors. If the supply is fairly new this is unlikely. If it has been in use for a long time it is very likely.
 
Looks like a wirewound resistor, as their package commonly comes to stand high power ratings. I agree with @ebp that they would read as shorted. I don't think they are faulty, and the physical appearance doesn't suggest that. A rare color code though. .
 

Thread Starter

grant1842

Joined Aug 21, 2010
7
Ok thanks for your reply . I was having a hard time. I will look around some more the power supply starts up for a second and shuts down so something in the protection area must be shutting it down (OCP, UCP, OVP, POWER GOOD SIGNAL,ETC)
 

Dodgydave

Joined Jun 22, 2012
11,285
Ok thanks for your reply . I was having a hard time. I will look around some more the power supply starts up for a second and shuts down so something in the protection area must be shutting it down (OCP, UCP, OVP, POWER GOOD SIGNAL,ETC)
Never known current sense resistors go s/c usually they go open, and take out the mosfet too, what is the smps chip number?
 
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