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Power Supply Failure

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Joel Rainville

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I have a defective computer power supply : 250W, typical +3.3, +5 and +/-12 voltages.

When I opened it, two big 200V 330uF dielectric capacitors had leaked a little electrolyte. I replaced those, but it didn't do the trick. I was getting voltages in the mV on all outputs. So I just bought a new power supply and moved on.

In the past few days, I have slowly salvaged various parts from the blown PS, transistors, caps, resistors... Everything works fine.

What is the most likely part to blow up in a power supply? Any guess as what part will be defective?
 
Well, 2 minutes after posting this, and I've found a bad solder on a small cap (cracked, with loose component lead). I think I'll try to rebuild the whole thing to see if that was the main reason for failure.

But the question still remains. What part should be checked for damage when a computer power supply, or any power supply, blows up?
 
I assume you're talking about a computer PSU? They're switch-mode power supplies. Electrolytic capacitors are favourites for causing problems, especially after a few years of use. The advantage of electrolytics is you get high capacitance, small package size. The disadvantage is they're longetivity - they tend to show signs of stress after a few years of use, particularly if subjected to a significant amount of stress.

Usually replacing the defective capacitors solves the problem, assuming you managed to find all the defective ones. If electrolyte has been leaked all over the board it's also advisable to give the area a very good clean and inspect the PCB for damage. If you've still had no joy, you really need to determine what kind of fault the switch-mode PSU has. You mentioned that all of the outputs showed millvolts - this means that none of the secondaries are up and running. The fact that none of them are up and running suggests to me (although it is by no means certain) that your fault is more likely to lie in the primary stage of the power supply. Assuming you've checked the internal fuses, I would tend to check the chopper transistor next. Make sure you fully discaharge the resevoir cap before you do this! If the chopper transistor is short circuit, it's definately worth checking any other surrounding components to see if they are damaged. Sometimes a chopper transistor can fail of it's own accord, but other times it fails because some other fault has caused it.

Other than that, without a schematic of the power supply you're quite limited. You could try checking the start-up section of the power supply, assumung you're easily able to recognise it.

Brian
 
in all honesty it will prolly be less hassle just to replace it. You can get a 1/2 decent computer PSU for £30 with more kick you have at the moment

if you want to fix it though, that another thing, just bear in mind you get something wrong you could end up frying yr MoBo and everything else, so someting that would have cost £30 will now cost £400
 
Thanks guys,

I'm just looking for small "projects" to learn electronics basics. I was not going to actually put it back in the computer. As I said in my first post, there's already a new PSU there!

A few questions :

- There's a coil of big gauge wire around something that looks like a magnet, all wrapped in plastic (well, was wrapped in plastic ;)). Is that an inductor?

- Two huge diodes are tied together on one end and enclosed in a silver metallic case. Any idea why? Is it just a hacked up heat sinking device?

- It looks like what I thought were voltage regulators are actually TO-220 sized transistors, at least a few of them. (Googling the markings on them brings up nothing) Are these the transistors that make it a switching power supply? Any links to documents explaining the theory/physics going on in a switching power supply would be great!

Thanks
 
The inductors are for output filters.
The TO220 packed regulators only for -5 and -12V 500mA outputs.
Most of case used two diodes with small cooler-plate for +12V out.
The two switching transistors mounted on bigger plate.
 

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  • atx12v_power_supply_2v0.pdf
    420 KB · Views: 764
I've been reading the specs sheet. I stumbeld upon this sentence: "The power supply shall not latch into a shutdown state when PS_ON# is driven active by pulses between 10ms to 100ms during the decay of the power rails." on page 23. What exactly does that mean?
 
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