| Electronics > Repair |
| Blew bench switching PSU, did I make a mistake probing? |
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| rwgast_lowlevellogicdesin:
I have a couple cheap switching PSU's and I have been working on trying to quite them down. I don't know a lot about how larger switching supplies are designed, I believe this type is a flyback supply? I had done a pretty decent job of getting most of the common mode noise off the output and dampening the ripple. Today I decided to try adding gate resistors and maybe some caps to the FETS. I wanted to probe the FETS to see how bad they ring. The AC side of the supply ties the ground directly to chassis, does not touch the circuit board at all. That means everything on the PCB is isolated correct? I connected my scopes clip to the chassis/earth ground and then probed the gate of one of the FETS and the whole PSU died on me. Admittedly I might have slipped and shorted the gate and drain with my probe, which could have killed the supply. So did I kill the PSU because I probed something wrong, or because my hand slipped? |
| wraper:
--- Quote --- That means everything on the PCB is isolated correct? --- End quote --- Nothing is isolated on primary side. You should be happy you (supposedly) did not blow your oscilloscope. Not to say the vast majority of oscilloscope probes cannot probe voltage this high, even if you used isolation transformer. Most of them top at 300V DC, that drops with frequency increase. Flyback PSU can easily have over 500V at MOSFET drain. |
| rwgast_lowlevellogicdesin:
Ok but my clip is on earth ground, so everything is at the same ground reference? Why would there be any issues? Also if the mains earth lead does not connect to the board wouldnt that mean its not using earth ground as its reference? The dc ground post is definatly floating. |
| wraper:
--- Quote from: rwgast_lowlevellogicdesin on July 08, 2024, 01:39:35 am ---Ok but my clip is on earth ground, so everything is at the same ground reference? Why would there be any issues? Also if the mains earth lead does not connect to the board wouldnt that mean its not using earth ground as its reference? --- End quote --- I edited my post to add more info while you typed this. No, it's not OK even by attaching earth ground (not that you'd get any useable measurement anyway with such probing) |
| rwgast_lowlevellogicdesin:
I measured the voltages on the fet legs and they were at about 160v referenced to earth. Why wouldnt i get usable measurements with the ground lead on earth ground, and if my voltages arent swinging above 400v wouldnt i be good? Btw there were no sparks no smoke nothing the psu just turned off and wouldnt come back on. My bad i guess the primary side wouldnt be floating due to the neutral ground bond, but still im not understanding why an earth referenced probe would cause an issue. |
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