Isolating my ATX from ground

Thread Starter

MachineHum

Joined Nov 3, 2014
74
Hey guys,

I use an old computer ATX PSU for my bench work. If I connect my scope to anything other then the DC-COM of the ATX supply it will short out and kill the supply. This kinda sucks because I want to measure signals wrt to the -12V or -5V rail say...

If I need 24V for a circuit I just use the -12V to the +12V, you can now see why I would want to measure wrt the -12V rail.

Obviously the scope and DC-COM of the ATX are tied to ground (Earth ground) so when I put my scopes common (actually earth ground) to -5V rail (which is -5 wrt DC-COM & Earth) current flows and messes everything up.

If I was to just rip off the ground pin on the wall plug of my supply, would this fix my problem? and more importantly will it be safe?

I know i'll probably have issues with static buildup + if something from the ATX pcb shorts to the case it wont blow the wall fuse and if I touch it i'll get electrocuted blaa blaa blaa I'm not really concerned about that.

I don't really want to buy an isolation transformer.

Thanks
 

MaxHeadRoom

Joined Jul 18, 2013
20,911
PC supplies are case grounded and also the low voltage common is also grounded, both in the supply and also in the PC itself via the MOBO ground plane.
If using the supply itself you would need to lift or isolate the ground connection on the black conductors.
Leave the service ground to the case connected.
Max.
 

dl324

Joined Mar 30, 2015
10,985
This kinda sucks because I want to measure signals wrt to the -12V or -5V rail say...

If I need 24V for a circuit I just use the -12V to the +12V, you can now see why I would want to measure wrt the -12V rail.
Can you describe the types of signals you would measure where being ground referenced would be a hinderance?

Does your scope have 2 inputs? Do you have the ability to subtract the channels?
 

GopherT

Joined Nov 23, 2012
8,012
PC supplies are case grounded and also the low voltage common is also grounded, both in the supply and also in the PC itself via the MOBO ground plane.
If using the supply itself you would need to lift or isolate the ground connection on the black conductors.
Leave the service ground to the case connected.
Max.

I believe there is a bare copper ring around one mounting hole that clamps to the case when you attach it. Add a piece of plastic milk carton between board and case - then use a plastic screw to lock it down
 

GopherT

Joined Nov 23, 2012
8,012
Can you describe the types of signals you would measure where being ground referenced would be a hinderance?

Does your scope have 2 inputs? Do you have the ability to subtract the channels?
He wants to make the rarely used but often talked about dual rail power supply - -12/+12. He can stack two supplies so he needs one not to be linked to ground.
 

Thread Starter

MachineHum

Joined Nov 3, 2014
74
I believe there is a bare copper ring around one mounting hole that clamps to the case when you attach it. Add a piece of plastic milk carton between board and case - then use a plastic screw to lock it down
Doesn't this connect the ground of the PCB of the board to the case? I'm looking to disconnect the DC-COM of the PSU to the ground of the wall.

I'm going in... off comes the ground pin on the plug.
 

GopherT

Joined Nov 23, 2012
8,012
I literally had the wire cutters in one hand and the plug in the other before I read this. What's the worst that can happen?
The chassis is grounded so that the circuit breaker pops if line (mains) voltage touches the case. The ground wire is connected to the neutral wire back at the fuse box. And connected to ground.

A broken wire or a wire falling into the power supply is very likely to happen in the shop. Just clipping a resistor and it lands in the ps. Easily energizes the chassis.
 

Thread Starter

MachineHum

Joined Nov 3, 2014
74
The chassis is grounded so that the circuit breaker pops if line (mains) voltage touches the case. The ground wire is connected to the neutral wire back at the fuse box. And connected to ground.

A broken wire or a wire falling into the power supply is very likely to happen in the shop. Just clipping a resistor and it lands in the ps. Easily energizes the chassis.
But didn't you just tell me to electrically isolate the case from ground a few posts ago?
 

GopherT

Joined Nov 23, 2012
8,012
But didn't you just tell me to electrically isolate the case from ground a few posts ago?
No,

The green ground wire connects the case to that third plug on the plug to the chassis. Your goal is to disconnect all of those black wires on the DC part of your power supply from the chassis.
 

GopherT

Joined Nov 23, 2012
8,012
The black wires (zero volts DC) connect to the chassis when the PCB is pressed tightly to the chassis when it is all screwed together. Check if there are any other places that bond the chassis to the board.
 

Thread Starter

MachineHum

Joined Nov 3, 2014
74
Oh man just did a continuity test... you're good. How did you know that the DC ground would be connected to the earth ground through the case like that? Is that some sort of standard? Theoretically you could achieve the same effect by just running the green from the plug to a pad on the PCB no?

Thanks!
 

Dodgydave

Joined Jun 22, 2012
9,206
Most Atx psus have the Black( common) 0V terminals connected to the metal case through the pcb screws, if you put a spacer between the pcb common and the case, it should remove this connection to isolate the mains earth from the dc supply. Also there may be a separate green earth wire to the pcb dc common terminal instead.
 
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MaxHeadRoom

Joined Jul 18, 2013
20,911
But didn't you just tell me to electrically isolate the case from ground a few posts ago?
The case is kept grounded for safety, also although the input side of the SMPS is galvanically isolated from the low voltage side, there are usually suppresion devices fitted on the power input side to case ground.
If the case ground is removed these are ineffectual.
Max.
 

alfacliff

Joined Dec 13, 2013
2,458
one other possablity, if you isolate the p;rimary (line) side of the supply, the supply will isolate the ac line from the case and power supply output. look for connections between the line neutral and case on the rectifyer side of the supply. that way, all black wires can still be connected to the case and ground.
 
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