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Old 23rd July 2008, 12:58 PM   #1
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Default Powering a feisty oscilloscope

Heres the story in brief,

I acquired a Lecroy 9354AM 500mhz 4 channel scope with a blown part of the PSU, fixed PSU (so i thought) blew again this time taking the control I.C. with it!

End of old PSU!

Ran up scope on bench power supplies after contacting Lecroy for power requirements... Scope works perfectly!

Scope requires +5V @ 15A, -5V @ 15A, +15V @5A, -15V @ 5A.

Bought some antec basiq 350watt PC PSU's and stacked them up for +/-15V @ 20A FOOLISHLY not checking the mains earth isolation. Their grounds are not mains earth isolated and this doesnt work!

Is there anything i can do? can i simply isolate the grounds myself? will this affect the SMPS operation?

I dont have enough money to buy off the shelf PSU's sadly (is it worth it though?)

Its like having a Ferrari snatched from under your nose!
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Old 23rd July 2008, 10:50 PM   #2
jaycee is offline jaycee  United Kingdom
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You should be easily able to isolate the mains ground from the 0V line inside the PC PSU. They are usually coupled using two Y-rated capacitors or such. I've disassembled enough dead ones and the connection is usually pretty obvious.
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Old 25th July 2008, 12:41 PM   #3
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Excellent! cheers! thats what i hoped, ill open em up and have a poke. ( i didnt want to open them up for nothing as they are brand new and the warrenty will be void).

I still worry that grounds on the PCB will go to the chassis of the power supply via standoffs, this will cause a short circuit too but hopefully ill be ok.
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Old 2nd August 2009, 04:42 PM   #4
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Digging up my old thread here.

I have opened up the pc supplies. The 0V output of the SMPS is indeed connected to the chassis and earth.
Im leaving the chassis connected to earth for safety.
The PCB 0V and some small filter capacitors are grounded via the mounting holes on the PCB.
I can insulate the PCB mounting holes from the chassis to prevent connection of the two grounds.

My questions are:

There are 4 screws holding the PCB into the PSU chassis. two of the the screws are not connected to the other two or each other by any track on the PCB, they are grounding some fat blue disc capacitors that look a bit like inrush limiters.
If I remove the ground connections from these low pass filters will this be ok? or will i need to connect them to my 0V?

Am I overlooking any safety here other than fishing about inside an SMPS?

Let me know if im not being clear with my descriptions
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Old 2nd August 2009, 10:50 PM   #5
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Yes,Those caps should be connected to Earth.

Do you have any pics of the SMPS's? Maybe someone will spot an easy way to modify them.
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Old 2nd August 2009, 11:52 PM   #6
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Thanks for your reply, here is a photo of the two mount holes in the PCB (top corners) that are independantly connected to the chassis.
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File Type: jpg dsc01040shrunk.jpg (98.0 KB, 180 views)
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Old 2nd August 2009, 11:53 PM   #7
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The lower two mounting holes can be seen here, they connect to the 0V track of the output of the SMPS.
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File Type: jpg dsc01042shrunk.jpg (66.8 KB, 168 views)
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Old 2nd August 2009, 11:54 PM   #8
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The blue capacitors that use the chassis ground:
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Old 2nd August 2009, 11:55 PM   #9
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and finally one of the whole thing
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Old 3rd August 2009, 02:58 AM   #10
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Okay,the holes with the blue caps need to remain connected to the case,and to earth ground.

The ground you want to lift is on the output side. You'll only need to do this to one supply,the one you use for the -V output.(It's positive will be connected to 0V,and it's negative will be -15V.)

You could maybe cut the trace on the PCB,on either side of the hole,and then join them with a short jumper wire,isolating the grounding pad on the board.
Or maybe put an insulating washer of some type under the PC board,and perhaps even use a plastic/nylon screw on that corner.

Maybe other people will have some other ideas.
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