Is it possible for a resistor to fail closed?
>>909526
anything is possible op
but improbable
if it fails due to heat then resistance will increase as it heats up then usually the film/wire catches fire and goes open circuit
if the failure mode is due to being dipped in hot solder for example then it could fail short circuit
>>909526
Dunno about a "real" short, but sometimes massive overvoltages can reduce the resistance several orders of magnitude.
Yep.
Source: twice in the last week (although to be fair it was the same fault on two identical boards).
Overheating seems to be a key element to it happening, green pcb had turned a nice caramel colour underneath.
I see.
>>909458
I made this in a thread that's over its bump limit (so I don't think anyone saw).
I've found a resistor in a PSU that seems to have done just that. It's a large 5w 660 ohm one (that looks like a spearmint chew).
>>909543
>green pcb had turned a nice caramel colour underneath.
I just had a look - they're suspended about 4mm above the board, so no browning there.
There is some slight browning on the resistor's underside though, and it's also "domed out" a bit underneath too. I can compare it directly with an identical resistor next to it - with a lily white concave underside.
So I can safely say this resistor is the problem?
>>909591
Well I've ordered one for a quid - what have I got to lose...
If this works it'll be my most significant fix-up yet.
>>909591
Something else is causing the resistor to fail. You need to fix that first. Resistors tend to last forever.
Also replace the resistor with a higher heat disipation value. Ie. If its 1/4w, use a 1/2w.
>>909607
The twist: I just tried the same test on it as earlier, this time with the front panel disconnected and the resistor tested successfully.
I traced a short circuit to the parallel capacitor and diode across the outputs. I removed that assembly and the PSU works perfectly.
I tested its output voltage with a multimeter and it's right on.
But I can't run it like this, without this capacitor and diode, can I?
What do you think it was? Did I damage the diode into a short circuit?
I'm trying to cancel the resistor and now ordered the 1n5402 diode. Can I run the unit with just the capacitor in the meantime?
>>909657
a diode and capacitor across the output of the a power supply are usually unnecessary extras. the diode protects the supply against you hooking it up to an external power source, and the cap is for preventing possible oscillation under some loads.
>>909526
unlikely but possible
