hi all....

i got one psu unit.. the problem here when i plug 115v and then the fuses blown.. how to identify what part should be replace??? i've upload the picture please tells me where to start 1st... http://img444.imageshack.us/my.php?image=dsc00332tb0.jpg

thanks...

 

Trace the circuit. You'll find a full wave bridge and my guess is the bridge will have at least one shorted diode.

 

First, look at the schematic carefully and list the possible causes of your over-current condition.

 

It could be the bridge rectifier or the main switching device. Desolder the bridge and check it for shorts. If the switching device is a transistor of some kind it will be easy to check Emitter to Collector or Drain to Source. If it's an IC then trace back to the bridge/filter capacitor and find the two pins that go to the main DC voltage and ground. If they are shorted the device is bad.

The most endearing quality of switching power supplies is that you can find a blown bridge and a bad switching device and fuse, replace all of these and have some pathetic small component blow all of them again.

The 2nd most endearing is that they are pretty easy to replace with an off the shelf generic if you know the voltages.

 

thank for the reply...

now i've remove the bridge rectifier.. but i donno how to measure the bridge shot or not.. i upload the picture, and please guide me how to measure the bridge.. http://img155.imageshack.us/my.php?image=dsc00333ut1.jpg

thanks...

 

You use a meter on the diode check function. The + and - terminals will check to the AC input pins, but with reversed polarity. If there is any reverse conduction, the bridge is bad.

Examine other components as well. Look for signs of heat, like darkening of either the component, or the board under it. If the FET controlling current through the transformer primary has failed, of the drive circuit is bad, than that will also cause the fuse to blow.

The FET will shao a diode junction source to drain, wiht a polarity in reverse to the normal operating voltage. If bad, it will be shorted and show 0 ohms S to D, and D to S.

Without a test fixture to test the PSU on, troubleshooting is somewhat a matter of luck. They are cheap enough that it's not worth much time and effort. Hint - if this is out of a computer, it's usually less to get a case and PSU combo, than jus the power supply. You're also sure the new PSU fits the case.

 

thanks for reply...

i'm totally newbie about this thing.. here i give the result from my measuring.. 1st when i put multimeter into diode mode... and red probe into + and black probe to - what i got is 0L, then reverse red probe in - and black probe to + it measured 0.786...

from what i understand in your description this diode is shorts... correct me if i'm wrong... i let's u reply me 1st then i continued to other part..


tq...

 

That's ok. The reverse conduction is 0, and the foreward is .786 mv. That is how a silicon diode should check. If it were shorted, the measurement would be an overload condition both directions. Try measuring a length of wire to see what a short looks like.

 

Measuring between those two points (+ and - ) you cross two diode junctions.

The meter should have read 1.4V or possibly OL in the forward direction, depending on the meter.

There is a diode shorted in the bridge.

To isolate the diodes, you have to measure forward and reverse between (+) and (AC), (-) and (AC), and then repeat the tests for the second AC.

You will find one or more diodes shorted.

 

Oops, didn't see it was a bridge being measured.

 

I would follow MrMeval's advice and check the switching transistors that followed the bridge.

The quick check would be to place the meter across the big capacitor ... red lead to + and black lead to - ... I would not expect to see low resistance. If it's low, then troubleshoot and find the next shorted device.