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Old 12th March 2008, 12:24 AM   #1
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Default repairing power supply

my friend gave me a power supply that gives +5 and +12 to portable hard drives. He said can u fix it? I had a look.

Before even opening it my guess was a bad low esr cap causing supply to die.

I opened it and there was a bulged low esr cap and two burnt out resistors on the secondary side.

On the primary side there is no controller IC just some simple 2 transistor oscillator driving the power transistor. One of these little transistors had it's face blown off .

with such a simple design what did suprise me is there is an opto coupler across the isolation barrier bit. i don't understand how such a simple design could possibly regulate itself well.

i conclude that the crappy low esr cap got excessively leaky and drew too much current and there was nothing to protect it against over-current.

without any knowledge of what the blown components type/values are this is one for the bin.


any thoughts?
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Old 12th March 2008, 01:05 AM   #2
TheMG is offline TheMG  Canada
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I've seen many computer power supplies die in the same way... resistors toasted, blown capacitors, and exploded transistors.

Not sure what causes it, but it often seems to happen without any overload. My guess is that a component failure causes the circuit to stop oscillating, sending DC into the transformer which in turn causes the transistor(s) to blow up from the excessive current, or something like that.

I've never really investigated it, since when that happens I just bin them cheap things. As for the quality industrial SMPS, well, I haven't had one die yet.

Sure enough, it is possible to create a regulated SMPS with only a few transistors. I haven't gotten into any detail about it but what it looks like is the voltage from the feedback path alters the operating frequency of the oscillator, thereby regulating the output voltage. Not the best way, but it works. I've even seen some with overload protection and a total of 5 transistors.
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Old 12th March 2008, 09:47 PM   #3
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Look here, there is a schematic of a regulated flyback converter using only 2 transistors:
http://www.deltartp.com/dpel/dpelcon...pers/S19P6.pdf
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Old 13th March 2008, 06:42 AM   #4
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yeah i know it can be done... they don't last long tho
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Old 13th March 2008, 01:31 PM   #5
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Works fine as long as the caps are fine. Most consumer products don't give the caps an easy life due to cost-cutting and failure will happen sooner or later.

This one is interesting:
http://www.electricstuff.co.uk/acadapter.html


The failure of your supply could also be because of the capacitor going open-circuit. When the adapter is unloaded the flyback voltage will increase and toast the switching transistor. This then takes out the small transistors. Source/emitter resistor on primary might be open.
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Old 17th March 2008, 03:17 AM   #6
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And again......

The same guy bought in a computer power supply saying it would work for a few seconds then shut down.

I suspected bad low esr caps.

The +12v supply and +5v supply caps were leaking electrolyte and bulged.

Replacing these capacitors got the 6+ year old supply going again.
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Old 17th March 2008, 03:51 AM   #7
TheMG is offline TheMG  Canada
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Don't you just love the abundance of poorly designed PSUs and cheap capacitors in consumer electronics?
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Old 17th March 2008, 04:30 AM   #8
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................and computer motherboards.

They always skimp on the caps. Better caps = stable computer.
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Old 17th March 2008, 11:13 AM   #9
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and yet another story. this same guy told me his motherboard died 4 years ago. He said one of the capacitors looked weird. I said to him 'doubt it's that'. i bet if i replaced them the motherboard would of worked again lol.
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Old 17th March 2008, 11:15 AM   #10
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Quote:
Originally posted by TheMG
Don't you just love the abundance of poorly designed PSUs and cheap capacitors in consumer electronics?

The beauty of the ATX supply is it will sense over-current / voltage and shut down before transformer meltdown / switching device meltdown.

The downside i guess is when you open one to investigate what went wrong there are no signs usually. eg exploded components
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