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Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Well, hats off to the EEVBlog guy for trying to explain it. I've tried many times but the electronic carnage continues!
Dave's explanation of the plug packs is a little oversimplified. The switched mode ones have a capacitor between one of the plug pins and the negative DC output line. The capacitor is for EMC purposes, but if that plug pin happens to go into the live socket hole, the DC side of the unit can dish out a nasty zap to sensitive circuitry.
The scope probe ground clip is for clipping to your circuit's ground. When working on switched mode power supplies or SSTCs, if I want to see waveforms on the primary side, I use an isolation transformer, and clip it to the DC bus negative.
Registered Member #1928
Joined: Mon Jan 12 2009, 01:51PM
Location: Reidsville, NC
Posts: 10
The video cleared up what I was possibly doing wrong, along with the responses here. I am going to test (powered down) the switch-mode supply to see if I was putting the ground of probe to point of non-ground potential, which I believe was the case. As soon as it cools off , 103 today outside and 90 in lab even with air running.
I found the reason the power supply died. A trace to the half bridge vaporized, obviously a dead short. More research , viewing waveforms after the GDT is done with a CT. Or directly with a HV differential probe . Started pricing them out, not cheap .
Update: Confirmed , the 320vdc from half wave multiplier is ground referenced on (-) dc bus (120vac neutral = gnd) So I was creating a short . So a HV differential probe is in the works to make working on half and H - bridges easier.
Registered Member #509
Joined: Sat Feb 10 2007, 07:02AM
Location:
Posts: 329
Steve Conner wrote ...
Well, hats off to the EEVBlog guy for trying to explain it. I've tried many times but the electronic carnage continues!
Dave's explanation of the plug packs is a little oversimplified. The switched mode ones have a capacitor between one of the plug pins and the negative DC output line. The capacitor is for EMC purposes, but if that plug pin happens to go into the live socket hole, the DC side of the unit can dish out a nasty zap to sensitive circuitry.
The scope probe ground clip is for clipping to your circuit's ground. When working on switched mode power supplies or SSTCs, if I want to see waveforms on the primary side, I use an isolation transformer, and clip it to the DC bus negative.
Ive seen a vehicle with a few computer systems in it that were powered from 230 VAC from a grounded generator, but the power strip they were plugged into was wired improperly. Ground was broken and line and neutral were swapped. Everything ran fine, but leakage through the RF/EMC capacitors made the vehicle live at ~120 VAC to ground. The tech connecting a BNC from our instrumentation to theirs got a light shock while he was trying to hook it up. No one injured, and no equipement damaged, but we had to call in the electricians in to check out stuff once I metered vehicle to ground and read 120VAC.
Grounding is definitely very important to observe.
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Oscilloscope probing Do's and Do Not's ???
