KX36
Junior Member level 2
Hi all,
As you can guess, I did a dumb thing.
I was testing a power supply I'm building which will be powered from a 380V DC boost PFC output. For testing, I've got an ATX power supply plugged into a 240V to 115V step down transformer for isolation. It's a building site transformer which I've disconnected the centre tap secondary earth, so earth passes straight through from input to output and live and neutral are isolated. Between probing low level signals and high power signals I accidentally probed the across 380V DC output of the PFC with a 300V 10x passive probe, sparks flew, small holes were punched into the PCB. I meant to use a 1kV 100x probe but grabbed the wrong one. I'm now unsure if that would have fared any better.
Now the probe doesn't work right, its resistance measures around 20Megohm and climing regardless of whether 1x or 10x is selected, the output to the scope is low bandwidth, sometimes the 1x-10x makes a difference to amplitude, sometimes it doesn't. It's pretty clear that the only path through it now is capacitive so something's burned open. It's unusable. The only odd thing is with my old analog scope and cheap ebay probes I used to regularly probe 4-500VDC in valve amplifiers without any problems, although those are higher impedance nodes capable of supplying only tens of milliamps.
With 5 mins testing, the scope seems OK with other probes although it did lock up the controls while I was testing different probes in different channels and fiddling with the controls on each channel, I'm hoping that's a coincidence. I really hope it's not damaged. Is it likely to be?
Really more than anything I'm curious as to what exactly blew in the probe, whether it was likely to be a breakdown right at the tip, in the cable, the resistor etc. I'd also like to know if the flaw is not just with me using a probe of the wrong voltage rating but also if there's a flaw in my isolation methodology. I was thinking that with the isolation transformer there to isolate L and N and the primary of the ATX supply not being connected to ground except through class Y capacitors in the input filter, I would have avoided the classic mis-probing ground short circuit but have I missed something? I don't have a differential probe but with the whole thing floating on the secondary of an isolation transformer I would have thought a normal probe would do. After all, we probe devices on the secondary side of 50Hz mains transformers all the time.
If anyone can clear that up, I'd be grateful.
As you can guess, I did a dumb thing.
I was testing a power supply I'm building which will be powered from a 380V DC boost PFC output. For testing, I've got an ATX power supply plugged into a 240V to 115V step down transformer for isolation. It's a building site transformer which I've disconnected the centre tap secondary earth, so earth passes straight through from input to output and live and neutral are isolated. Between probing low level signals and high power signals I accidentally probed the across 380V DC output of the PFC with a 300V 10x passive probe, sparks flew, small holes were punched into the PCB. I meant to use a 1kV 100x probe but grabbed the wrong one. I'm now unsure if that would have fared any better.
Now the probe doesn't work right, its resistance measures around 20Megohm and climing regardless of whether 1x or 10x is selected, the output to the scope is low bandwidth, sometimes the 1x-10x makes a difference to amplitude, sometimes it doesn't. It's pretty clear that the only path through it now is capacitive so something's burned open. It's unusable. The only odd thing is with my old analog scope and cheap ebay probes I used to regularly probe 4-500VDC in valve amplifiers without any problems, although those are higher impedance nodes capable of supplying only tens of milliamps.
With 5 mins testing, the scope seems OK with other probes although it did lock up the controls while I was testing different probes in different channels and fiddling with the controls on each channel, I'm hoping that's a coincidence. I really hope it's not damaged. Is it likely to be?
Really more than anything I'm curious as to what exactly blew in the probe, whether it was likely to be a breakdown right at the tip, in the cable, the resistor etc. I'd also like to know if the flaw is not just with me using a probe of the wrong voltage rating but also if there's a flaw in my isolation methodology. I was thinking that with the isolation transformer there to isolate L and N and the primary of the ATX supply not being connected to ground except through class Y capacitors in the input filter, I would have avoided the classic mis-probing ground short circuit but have I missed something? I don't have a differential probe but with the whole thing floating on the secondary of an isolation transformer I would have thought a normal probe would do. After all, we probe devices on the secondary side of 50Hz mains transformers all the time.
If anyone can clear that up, I'd be grateful.
