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Registered Member #15
Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068
ScotchTapeLord wrote ...
To scope something, you need to ground one side of the load and probe the other. If you ground any part of the AC bit of an H-bridge, you short the top transistor of that leg to the ground!
More importantly, the GND clip of a scope probe is tied directly to EARTH GND through the scope.
Better to measure output differentially using two scope probes.
Use two probes. Set oscilloscope to ADD, and one channel to INVERT. Don't tie GND clips to anything - leave them floating. Be sure not to exceed voltage rating of probe / scope.
Registered Member #1403
Joined: Tue Mar 18 2008, 06:05PM
Location: Denmark, Odense C
Posts: 1968
EastVoltResearch wrote ...
ScotchTapeLord wrote ...
To scope something, you need to ground one side of the load and probe the other. If you ground any part of the AC bit of an H-bridge, you short the top transistor of that leg to the ground!
More importantly, the GND clip of a scope probe is tied directly to EARTH GND through the scope.
Better to measure output differentially using two scope probes.
Use two probes. Set oscilloscope to ADD, and one channel to INVERT. Don't tie GND clips to anything - leave them floating. Be sure not to exceed voltage rating of probe / scope.
For maximum safety when scoping on live circuits I would use a isolation transformer and ground the negative rail on the DC bus. This lowers the short circuit current significantly and might save your ass since most you can easily find are under 500VA, good for low voltage testing.
Registered Member #2478
Joined: Mon Nov 23 2009, 03:24AM
Location: Texas A&M University
Posts: 47
EastVoltResearch wrote ...
ScotchTapeLord wrote ...
To scope something, you need to ground one side of the load and probe the other. If you ground any part of the AC bit of an H-bridge, you short the top transistor of that leg to the ground!
More importantly, the GND clip of a scope probe is tied directly to EARTH GND through the scope.
Better to measure output differentially using two scope probes.
Use two probes. Set oscilloscope to ADD, and one channel to INVERT. Don't tie GND clips to anything - leave them floating. Be sure not to exceed voltage rating of probe / scope.
When scoping a circuit this way, should the ground leads of the two probes be clipped together?
Registered Member #2478
Joined: Mon Nov 23 2009, 03:24AM
Location: Texas A&M University
Posts: 47
It is my concern that large electric fields could be capacitatively coupled into the shields and then into the center conductors. Clipping the leads together would force any such interference to be common-mode up to twice the frequency of only connecting the shields at one end.
Registered Member #15
Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068
Mads Barnkob wrote ...
EastVoltResearch wrote ...
ScotchTapeLord wrote ...
To scope something, you need to ground one side of the load and probe the other. If you ground any part of the AC bit of an H-bridge, you short the top transistor of that leg to the ground!
More importantly, the GND clip of a scope probe is tied directly to EARTH GND through the scope.
Better to measure output differentially using two scope probes.
Use two probes. Set oscilloscope to ADD, and one channel to INVERT. Don't tie GND clips to anything - leave them floating. Be sure not to exceed voltage rating of probe / scope.
For maximum safety when scoping on live circuits I would use a isolation transformer and ground the negative rail on the DC bus. This lowers the short circuit current significantly and might save your ass since most you can easily find are under 500VA, good for low voltage testing.
Good point. And keep in mind a variac is NOT isolated. I see lots of individuals hooking up variacs thinking they provide isolation - they do not!
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