Author Topic: Checking my reasoning on identifying a safe ground reference for an oscilloscope  (Read 354 times)

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Offline krby

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I'm fairly new to oscilloscopes, and just got a SDS-1202X-E as a gift. I've watched Dave's video "#279 How not to blow up your...." and I've read the often quoted Scope Probe Reference Material doc. What I'm asking here is to check my reasoning about ground references points to see if I understand things completely before proceeding.

My setup.
* My scope is earth grounded. I'm going to leave it that way  :D
* For now, I'm using the OEM 1x/10x single ended probes. I'm going to research and buy differential probes soon.
* In the sort term, some of my to-be-tested circuits (I guess the proper term is DUT) will sometimes have USB attached, sometimes not, others could have earth ground.

Thinking thru this, I believe I could use a DMM to check if a a given ground point is safe. I'm hoping you all can critique and tell me why I'm wrong or right:

Let's say I have a circuit which might be tied to ground (USB, some power supply, etc). I choose a point to attach the ground clip of my probe. I have chosen carefully, and think  this point is at earth ground potential, but I want to verify before I hook up the scope.

Using a trusted DMM, I check the voltage across a the outer BNC ring on my scope and the selected ground point. If the DMM reads zero (or just a few millivolts) does that mean I can safely attach the probes ground clip? My thinking is that 0 or next to zero volts tells me a dangerous amount of current won't flow thru the ground clip and the scope. Am I on the right track, also does it matter if I use the VAC or VDC  setting on the DMM?
 

Online magic

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That's not a great idea because any random point which is not ground may be at 0V when you do the measurement and change to something else later. Or it may be at 0V average but have some AC voltage on it and a DMM will show 0V if it is set to DC volts.

You really should check for continuity from the point you want to clip to some ground pin on a known connector on the device. Then you are sure that the point is hardwired to the circuit's ground and there will be no surprises.
 
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Offline krby

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You really should check for continuity from the point you want to clip to some ground pin on a known connector on the device. Then you are sure that the point is hardwired to the circuit's ground and there will be no surprises.

Ahh ok, so since what I'm trying to do is make sure the chosen point I'm going to attach probe's ground clip is at earth ground then I could use the DMM to do a continuity or Ohm measurement between that chosen point and earth ground (I would use either the BNC shield on the scope's probe input or the ground pin on a socket next to the AC plug the scope is using for power.
 

Online Ian.M

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That doesn't help when you are dealing with non-grounded SMPSUs as their leakage current from mains line in to output side 0V can be as high as 0.75mA RMS.  Also, if the SMPSU PCB is exposed, it can be non-obvious which parts of it are live, so in that case you should treat all parts with the same respect you would an exposed live terminal, until you have established that they aren't live.

You should check with a 10K 2W resistor* connected from the node you want to ground to true ground, and measure voltage across it on both AC and DC ranges.  If its over 8V AC or DC or the GFCI (RCD) on your mains supply trips, STOP! 

If there is under 8V across the 10K resistor for AC and DC, take a small incandescent torch bulb (e.g. 6V 1/2W), and test it with a battery to be certain its good. Next, connect one terminal of the bulb to true ground and touch the other terminal briefly to the node you want to ground.  If it lights, there's probably a low voltage DC supply there so you cant connect scope ground to that node.  Next hold it on the node and if it doesn't glow at all, and retesting the bulb with a battery shows the bulb's still good, you can then connect scope ground to that node.

Of course, with experience, you'll be able to identify safe nodes to connect scope ground to by inspection, without the above procedure, or by simple continuity testing, but if in any doubt, the above procedure will save your scope and minimises the risk to the device under test.

* Resistor wattage chosen so direct connection to 120V mains supply wont burn it out.  In 220V-240V countries use a 10K 7W resistor.
« Last Edit: December 25, 2019, 11:37:46 pm by Ian.M »
 
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