smps repair

Thread Starter

gim461

Joined Jan 12, 2013
11
Hi, I am trying to repair a KY-05036S-12.

It failed with a violently blown main fuse (glass broken).

I have been over the board with mm and the only obvious fault is an open circuit primary side DC filter cap (47F, 400V).

My question is, would this cause the mains fuse to blow violently? The switching is done by 5MC0380.

Thank you.
 

dendad

Joined Feb 20, 2016
3,788
Alow capacity filter will greatly increase the switching spikes and that can cause all things to fail.

If you can, start it up via an isolation transformer and add a 40W incandescent mains lamp in series.
It will save a lot of smoke, and usually will let the supply run on, but not on any much of a load.
It is a good practice when working on mains SWMPS.
 

wayneh

Joined Sep 9, 2010
17,058
...the only obvious fault is an open circuit primary side DC filter cap (47F, 400V).
Every SMPS I've opened (because it failed) had a bulging 400V cap in it. It's always the cap.
I can find or buy an SMPS cheaper than ordering new capacitors, so I have a pile awaiting repair if I ever get in the mood.
 

Thread Starter

gim461

Joined Jan 12, 2013
11
Thanks for the advice; I will buy a new cap ( ~$7), and do the light bulb trick. I'll let you know how it goes.
 

dendad

Joined Feb 20, 2016
3,788
They are. But I have saved some for this.
One way is to use a mains to 12V transformer, connect a car headlamp to the 12V winding of a second transformer, then you have an isolated current limited mains supply ont of the second transformer.
Add a switch to short out the lamp for full power.
Incandescent lamps are ideal as they allow full current on startup then their resistance rises as they heat.
Work much better than just a resistor.
And a bright glow is a fault indicator ;)
 

Yaakov

Joined Jan 27, 2019
2,442
Incandescent lamps are ideal as they allow full current on startup then their resistance rises as they heat.
Work much better than just a resistor.
And a bright glow is a fault indicator ;)
When I worked at the Boston Museum of Science, one of our exhibits was motor driven but interactive. The visitor could load it down, sometimes severely. Inside the cabinet was a 100W filament lamp which was all the protection needed. If the motor loaded down, the lamp got brighter. Worked for years to protect the motor which surely would have been burned out long before and cost almost nothing.
 

Thread Starter

gim461

Joined Jan 12, 2013
11
Ok, so the smps is fixed. I replaced the cap and then powered it up with a 75W light bulb in place of the fuse.

And good thing I did too, because the light bulb lit up on power-up and there was a cracking sound indicating arcing. I soon observed that the arcing was coming from a 56k resistor which was in contact with the aluminium heat sink. Evidently the insulation had broken down, possible from a long period of minute vibration against the heat sink. I have not replaced the resistor, but just pulled it away from the heat sink.

The bottom of the resistor is connected to the positive side of the primary DC, so +340V. The top of the resistor is connected via a diode to the Drain of the 5MC0380 which is also connected to the transformer.

The cap I bought from the local electronics shop is too tall for the smps case so I will need to get a shorter one. I will also replace the resistor.

I have tried to fix a few smps before, and this is the first time I have been successful, so feeling pretty happy!Overall.jpgArcing resistor.jpg
 
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