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Earth ground from lab power supply. Problem? Seems like an oscilloscope short

CPBCPB

New Member
I was watching a review of a Matrix power supply. It has a green output that connects to the mains ground. Also a strap to join it with the black output in front of you want to.
I'm assuming that the black output isn't at all connected to the ground otherwise, right?
Running a device connected to the ground seems fine, but having that ground connection connected to the black seems like a real danger when using an oscilloscope.
I just bought my first oscilloscope, so I don't want to fry it or me. I don't have a power supply yet. So, it's the following correct?
Run device with ground from power supply connected. OK.
Remove ground before testing circuit with oscilloscope while it's powered by lab supply. OK?
Am I seeing this correctly or is there anything else I'm missing?
Thanks for any help.
 

Reloadron

Well-Known Member
Most Helpful Member
Normally yes, the green terminal of some power supplies is earth ground. The red and black output terminals are floated and yes, you can jumper the green and black terminals placing the negative with respect to positive output terminal at earth ground. That said I strongly suggest you consult not only the power supply manual but also your scope manual. Good user manuals will cover all of this. The scope manual will explain how to correctly take measurements and not damage your scope.

On my older line powered scopes the back shell of the BNC connectors is at earth ground. I also have used scopes with differential inputs where the inputs are isolated. Again, read the manuals for the equipment you have.

Ron
 

rjenkinsgb

Well-Known Member
Most Helpful Member
As long as you only ever connect the ground of the oscilloscope probes to the circuit ground (PSU negative) it should not be a problem.

It should become a habit that you never connect a scope ground clip to anything that could have a voltage on it, to avoid damage to both the scope and equipment. (And yourself).

If you need to view a signal relative to a non-grounded point, set the scope in differential (subtractive) mode, as Ron says.
Temporarily put both probe tips on a signal or test point and adjust the channel gains so the signal cancels out, then you can display the difference between the two probed points - without the ground clips ever being at anything but ground / 0V.

ps. One other thing a lot of new users don't realise: Set the probes and input mode selects to "x10" and leave them there.

x1 causes the circuit to you are testing to be put under some load, and whatever voltage is fed directly to the scope. Plus the frequency response is usually rather poor.

x10 add a frequency-compensated attenuator in the probe which reduces probe load & helps protect the scope inputs - plus giving a much better high frequency response.

You need to use the calibration output of the scope to adjust the compensation trimmer in each probe, to get the cleanest square wave, as near to perfect square corners as you can, & then preferably keep the same probes on the same inputs. Once they are adjusted to that specific scope, they should be OK for years; just verify the square wave occasionally.
 

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