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Registered Member #3637
Joined: Fri Jan 21 2011, 11:07PM
Location: Buffalo, NY
Posts: 1068
Okay, so I recently acquired an ATX PSU and was using it to supply power to my ZVS driver and other things. It works fine, but when I try to scope something on my circuit, the scope seems to be acting as a ground for the current, and the oscillations stop working, and the ATX psu seems to "short" out. Without the atx psu plugged into any aspect of the circuit it works fine, and I can measure anything on the circuit with no problems.
Registered Member #33
Joined: Sat Feb 04 2006, 01:31PM
Location: Norway
Posts: 971
The ground lead of your oscilloscope is connected to mains ground. The casing of the PSU and the output ground is also connected to mains ground. So the scope ground clip is connected to the negative output of the PSU, and you will short out the PSU if you try to connect the scope ground clip to any of the PSU outputs.
Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
Anders M. wrote ...
The casing of the PSU and the output ground is also connected to mains ground. So the scope ground clip is connected to the negative output of the PSU, and you will short out the PSU if you try to connect the scope ground clip to any of the PSU outputs.
Wait! Im confused, Ive always seen theATX case as Ground to third prong mains, the black wires to the mainboard are not ground, but really the negative pole ( really a zero V ), for the various other positive voltages. the oscilloscope thus should not be shield probe connected to the Black ATX wires.
Thus, the ATX case, is not equivelent to its own Black wires.
Maybe you could tie the black wires and ATX case together then you would lose isolation safety from the mains, but gain a measureable circuit path.
is this what you meant Anders, or am i totally psychotic?
Registered Member #2628
Joined: Fri Jan 15 2010, 12:23AM
Location:
Posts: 627
I havent touched a PC powersupply in years.. let alone any of the ATXs. IIRC some of the outputs are split rail (+V, 0V, -V). you should note that a negative voltage is not the same thing as a zero potential ground. Im pretty sure the common ground for all these sources (be it +/-) would be chasis bonded, and mains grounded. most PSU's ive gutted had a little lug on the side of the box attached to mains ground, and the common grounding on the board itself connected to that.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Patrick wrote ...
the black wires to the mainboard are not ground
In every ATX PSU I've poked around, the black wires are connected to the case and the third prong of the power cord. This used to cause me trouble with ground loops in my old home recording setup.
Registered Member #1334
Joined: Tue Feb 19 2008, 04:37PM
Location: Nr. London, UK
Posts: 615
Steve McConner wrote ...
Patrick wrote ...
the black wires to the mainboard are not ground
In every ATX PSU I've poked around, the black wires are connected to the case and the third prong of the power cord. This used to cause me trouble with ground loops in my old home recording setup.
Seconded - just checked about 10 ATX PSUs I have here from HP, Compaq, Qtech etc. & all have COM connected to chassis & mains earth.
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ATX PSU weirdness
