In my job, I’m involved with some VSD to control Motors of various capacities, but I’m not much familiar with PC power supply, even thugh it works in simmilar way (Switch Mode). and here is my problem.

I found in two or three of them a common symptom. Every time I replaced the main Fuse it blows. Is this a common problem ? Please help me by your experience and if possible lead me to resources to enrich my practical knowledge in this field

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Well that is not much to go on but I would assume that either the "surge protection" or the main power switches are shorted. Or maybe the input bridge rectifier.

What motor ratings are you working with? Various could 0.1-1HP or 1-10HP.

At my job we work with 3HP running right off the rectified line, so in that case there is no power supply in that sense, only a motor control.

D.

 

Thanks cadstarsucks for your reply. Not being clear enoug is my problem every time i post a thread.

Sorry for not making my problem clear, My problem with PC Power Supplies and not with my VSD's. I'm reading related topics found here in the forum, but still i'm in need for more practical information.

thanks again cadstarsucks and sorry for misleading you.

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Not a problem...usually PC supplies the first thing to go is the cooling fan...once that goes if you run a heavy load the next thing to happen is most likely the main power transistor(s).

D.

 

Thanks cadstarsucks for your continues interest to help me, and sorry for my broken English.

It looks that I’m doing quite systematic since I checked exactly in same sequence (Fuse, Fan, Transistors). But while testing the first two is so simple, the Transistors are making some trouble for me to check. Is it similar to ordinary transistor check procedure?! by considering that the transistor as two diodes or it needs different procedure ? by the way I've got Transistor checker in my multimeter, do it of any use for power transistors ?

Thanks for your help

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Check electrolytic capactitors too.
These fail under high temperatures when the fan fails.

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They are likely to be FETs. You should see a drain source diode in one direction and an open in the other. The gate should also be open.

D.

 

I give electrolytic Capacitors big concern in any circuit I troubleshoot, and you are correct they give allot of trouble especially in classical power supplies.

One of the things I found in SMPS is the high number of coils. I do not no how to check wither the coil o.k. or not.

Another problem is the Toroidal transformer, why is the transformer is needed while we tailor the wave shape and value we need, (or at least that’s my idea of SMPS), and again I do not know how to check them.

Thanks for all who is trying to help

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The transformer is for safety isolation. The inductors supply better filtering.

The only way to check an inductor, other than visual and not open, is to measure the inductance knowing what it is supposed to be.

The transformer you is a bit easier, you connect it to a signal generator and see what comes out the other end, but you still need to know what it is supposed to be.

D.

 

Thanks for your patiant on my silly questions, if possible i'd like to keep on questioning, may be every body else participate also.
Inductor as filter is needed in Hi freq. Switching, but Isolation transformer ... isolating what from what

what i'm suppose to use in measuring the coil (Ohm may be) and what values i'm supposed to get.

Can i vary the input voltage and measure the output ?

Thanks again

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Isolate the you from the line. Safety standards are good things for the most part.
If you isolate an oscilloscope from ground, measure it while the circuit is running, and your scope is fast enough. Not a good practice for a hobbyist in general.

A high voltage diode and capacitor might do the trick. IF there is no PFC you can look at the low side, but I think PC supplies all use universal input power factor correction these days. That would mean a PFC switcher boosting the line to 400V and a regulator switcher driving the outputs, unless they are getting creative.

D.