Computer Power Unit Failure

[VirajKoth]

I have an Amacrox AX550-S power unit. Yesterday when um going to power on(before the pressing start button) the PC by connecting power leads to AC, there came a sparking sound from the power unit & cut off the trip switch in the panel-board in my home knowing a short circuit.

So i took the PSU out of the case & powered it again, still the same(no any sparking sound, but the trip went off)
So i removed the case & watch for any burn marks in its PCB. But nothing in PCB. But there was a burn mark on the coil of an Inductor. That was the only burn mark that had.

But when i checked for continuity in power unit for short circuit, nothing short circuited.
meter reads it as open circuit. But why is the trip cut off, meaning that there is a short circuit in the PSU. (means without a load it is open circuit, but with a load its short circuit)



Thank You.

 

[VirajKoth]

Please someone help me... do replacing that burnt inductor will solve this problem(short circuit) ??

 

[Raven Luni]

There is no indication of anything being burnt in the pictures. Have you tested the inductor with a meter (use the ohms setting on your multimeter)?

 

[Raven Luni]

Also note that if its an ATX power supply it will not power on without a signal from the motherboard (I remember the first time I built a PC thinking I had a faulty power supply)

 

[(*steve*)]

It doesn't really look "blown" to me.

Is it open circuit? (you seem to suggest it is)

Does it have shorted turns?

For this sort of fault I would suspect the transistor(s) on the primary side of the transformer.

 

[Jeffpepin]

it look like a component? or it look a brush With shrink over it ?

can you roll ''white cylinder'' with your finger ?

 

[duke37]

You can find where the mains power comes in. It will or should have a filter consisting of some capacitors and inductors. If there are capacitors across the mains and one of them goes short circuit, then excessive current will flow. Check the mains input for ohms and disconnect any component to see if the short goes.

After the filter there will be a bridge rectifier. Check that this is OK with an ohm meter.

Then there will be a big electrolytic capacitor of about 450V working, check this for short.

Then the feed goes to a switching transistor on a heat sink, remove and check for shorted junctions.

Put a bulb in series with the mains input so that the power does not trip and see if you get 350V or so across the electrolytic.

If all this is OK, then the fault could be in the switching transformer, in the secondary circuits or the voltage feeback. Put it in the bin.

There are very high voltages involved so if you do not know how to deal with them, then DONT.

 

[KrisBlueNZ]

What kind of cutout was it that tripped? Was it a standard circuit breaker that trips on overcurrent, or was it a "residual current device", aka "earth leakage circuit breaker", "earth fault cutout", "current imbalance detector" etc? If it's the latter, there may not be short circuit in the power supply; it may just be a leakage path.

 

[VirajKoth]

Thank you very much for all the replies.

 

[VirajKoth]

There is no indication of anything being burnt in the pictures. Have you tested the inductor with a meter (use the ohms setting on your multimeter)?

yes u`re correct. I thought it has burnt. But it`s not. That`s the way it came, i guess.

 

[VirajKoth]

Also note that if its an ATX power supply it will not power on without a signal from the motherboard (I remember the first time I built a PC thinking I had a faulty power supply)

no....never....
As i said the issue is coming from before powering on the PSU. means when i connect the PSU to the AC supply.
Also any PSU can jump start by contacting the switching wire(usually the Green Wire) to the ground of the PSU

 

[VirajKoth]

It doesn't really look "blown" to me.

Is it open circuit? (you seem to suggest it is)

Does it have shorted turns?

For this sort of fault I would suspect the transistor(s) on the primary side of the transformer.

yeah it`s not.
it`s(the PSU) do open circuit when there is no load.(when not connected to AC power)

 

[VirajKoth]

You can find where the mains power comes in. It will or should have a filter consisting of some capacitors and inductors. If there are capacitors across the mains and one of them goes short circuit, then excessive current will flow. Check the mains input for ohms and disconnect any component to see if the short goes.

After the filter there will be a bridge rectifier. Check that this is OK with an ohm meter.

Then there will be a big electrolytic capacitor of about 450V working, check this for short.

Then the feed goes to a switching transistor on a heat sink, remove and check for shorted junctions.

Put a bulb in series with the mains input so that the power does not trip and see if you get 350V or so across the electrolytic.

If all this is OK, then the fault could be in the switching transformer, in the secondary circuits or the voltage feeback. Put it in the bin.

There are very high voltages involved so if you do not know how to deal with them, then DONT.

yeah...there have some inductors connected series and suppression capacitors connected in parallel in the very first(there the main power inputed to the circuit) of the circuit. Right after them, there are two bridge rectifiers connected in parallel. I got that out from the PCB and checked them. Nothing wrong with them too. Knowing that the fault is in further more ahead.

yeah.. there have a big big electrolytic capacitor(420uF 420V). I checked it with ohms as my multimeter(my one is range from 4nF to 400nF) is not enough to check higher Farads.


anyway when i went further ahead(after those bridge rectifier) there have some switching transistors with the heatsink. wait i have to check those ones now....
I`ll tell what happened.
Also can i power the PSU(connecting to the AC power) by removing any component and see whether the power unit is not short circuited.? Then if it`s not short circuited i can determine that the fault is coming from that removed component. eh ? (i know that removing any component connected parallely will not understand where the fault is)

 

[VirajKoth]

What kind of cutout was it that tripped? Was it a standard circuit breaker that trips on overcurrent, or was it a "residual current device", aka "earth leakage circuit breaker", "earth fault cutout", "current imbalance detector" etc? If it's the latter, there may not be short circuit in the power supply; it may just be a leakage path.

yeah it`s a normal circuit breaker.

No, it(the power unit) cannot be a earth leakage. Also i powered the PSU without connecting it to the Ground(the earth wire).Still the same. Trip went off.

And also sometimes it will not cut of the trip, it will blow a fuse in the plug of the AC connecting wire. Knowing not a earth leakage but a short circuit. That`s because of a faulty burnt part.

 

[VirajKoth]

oh yeah...
finally figured out that one of the switching transistors(part no - F25NM60N) is gone.
Will replace it and say what happened. (Also there might be any other faulty parts that left too. Will see !!)



thank you all in Advance....

 

[(*steve*)]

finally figured out that one of the switching transistors(part no - F25NM60N) is gone.

As I said, this is what I'd be looking at first with a fault like this. I hope my suggestion helped.

 

[VirajKoth]

For this sort of fault I would suspect the transistor(s) on the primary side of the transformer.

Then the feed goes to a switching transistor on a heat sink, remove and check for shorted junctions.

yeah thank you guys......these were very helpful, also other ones too

 

[VirajKoth]

guys believe me.... seriously i couldn`t find that "STF25NM60N" transistor in here, Sri Lanka. I searched it for everywhere but still no luck.
I thought of replacing it with a same characteristic(equivalent) transistor. Guys any suggestions please ?
What is the best equivalent transistor for that "STF25NM60N" ?

Please help me dudes...Thank You.

 

[KrisBlueNZ]

The specifications of the original STF25NM60N are:
STMicroelectronics "MDmesh II" N-channel MOSFET
Vds(max) = 600V
Id(max) = 21A
Rds(max) = 160 mohm @Id=10.5A and Vds=10V
Gate capacitance 2.4nF @Vds=50V
Power dissipation = 40W
Package = 220FP (fully insulated)

It seems it's no longer manufactured.

The closest fit I can find is the STF34NM60ND "FDmesh II" device, which is available from Digikey: http://www.digikey.com/product-detail/en/STF34NM60ND/497-12248-ND/2827094 and Mouser:
http://mouser.com/ProductDetail/STM...Z2EgPHpcW4Rs4ITw02tp9kRM3NBiVNnC8jvKT4jlIng==

 

[VirajKoth]

Thank you so much dude.....what about these ?
Firstly "STF25NM60ND"
then this "MTP20N10" or this "MTP20N10E"

those can be found here easily....
if neither of them are not recommending by you then i`ll go for your "STF34NM60ND" one


THANK YOU again