What maybe causing this Chopper to short ?

[budm]

Re: What maybe causing this Chopper to short ?

One thing can be that the U3845 (as shown in page 9) DR+ has the output stuck HI instead of turning those two Optos on and off.

[sam_sam_sam]

Re: What maybe causing this Chopper to short ?

That very good manual most of them are not very good

[SM-Piyes90]

Re: What maybe causing this Chopper to short ?

perhaps pictures of the board would help , take it you have ~220vac mains.
Perhaps use the dim lamp trick - a 75watt filament bulb in line wth the + mains
saves blowing up components

Hi thanks for the comment , ireally don't know about the light bulb trick , but i got this in mind

You can take a look at the PDF service manual , it contains schematic and picture of the PCB .

[selldoor]

Re: What maybe causing this Chopper to short ?

perhaps pictures of the board would help , take it you have ~220vac mains.
Perhaps use the dim lamp trick - a 75watt filament bulb in line wth the + mains
saves blowing up components

[SM-Piyes90]

Hello everyone here .

Am right now trying to repair a switching style power supply , this one converts low input current to pretty much 160 amps to be mainly used as a MMA welder .

Anyways i checked it step by step , started with the primary side , the primary caps got 300 Vdc on them , now arriving to the chopper circuit witch consists of 2 power IGBTs , that are making the input signal chopped at a very high frequency with the command of a pulse width modulation chip " UC3842 " .

So i checked the power transistors with a continuity tester , and they are both shorted , also found 2 shorted A3120 optoisolators , four 18 v zeners .

I then changed them all with new parts , in the hope to save this unit but again all the parts that i' ve replaced have again fried .

I guess something else is causing them to burn , i hope i get some ideas from people who encountered similar issue .

Here is a Service manual of the SMPS welder :