Checking my reasoning on identifying a safe ground reference for an oscilloscope - Page 1
Forum section: electronics / beginners

author: krby
author_rank: Contributor
timestamp: December 25, 2019, 09:51:09 pm
content: 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* 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?

author: magic
author_rank: Super Contributor
timestamp: December 25, 2019, 10:03:05 pm
content: 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.

author: krby
author_rank: Contributor
timestamp: December 25, 2019, 10:51:16 pm
content: Quote from: magic on December 25, 2019, 10:03:05 pmAhh 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.

author: Ian.M
author_rank: Super Contributor
timestamp: December 25, 2019, 11:34:57 pm
content: 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.

author: vulcanian
author_rank: Newbie
timestamp: January 10, 2023, 10:04:03 am
content: Sorry to revisit this old thread but it's the clearest step-by-step for comparing grounds I've found in hours of searching.I think in following your advice, I'd attach the resistor at one end to the scope's grounded BNC shield and at the other to the DUT's candidate ground. Then I'd use my DMM in Auto-V Lo-z mode across the resistor, and if I see <8v, I'd check again with AC then DC. If all <8v, so far good to go.Then I'm caught up on the light bulb. Am I looking for voltage or current here? Why can't I just use the VMM to check this?Sorry for the noob questions, and thank you so much for this step-by-step.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: January 10, 2023, 10:24:27 am
content: I'd say make another GND lead for your scope probe and put a fuse in it. (The smallest you can get, probably 50mA or thereabouts) such a fuse may not be good enough when measuring high frequency signals, but at least you will keep the smoke inside your scope until you want to measure such signals.It's also easy to check whether the fuse still works, just connect it to the probe tip and then touch it with a finger. If you see 60Vac then the fuse does not workI usually don't ever connect my scope ground to anything other than the "ground" of the circuit under test.You can use two channels of your scope and then subtract them for a differential measurement. It's one of the joy's with a 4-channel scope. Plenty of inputs.Good differential probes are quite expensive. They can easily be more expensive then your oscilloscope. I'd say build a Instrumentation amplifier yourself from 3 opamps, and then use it as a learning experience to design a circuit. It can be as simple as a 50cent TL074 with a pair of 9 volt batteries

author: Ian.M
author_rank: Super Contributor
timestamp: January 10, 2023, 11:48:35 am
content: The reason for the incandescent torch bulb is to differentiate between ground referenced low voltage high(ish) current supplies and high impedance nodes that may have 'phantom' voltage on them due to leakage current. Its effectively the same as the test procedure with the 10K resistor but checking for low voltage rather than mains. e.g. if probing round a PC motherboard, you'd be in for a lot of grief if you clipped your scope ground to the 5V rail, with e.g. 25A of continuous current available. The least to expect would be a melted probe lead, and there's a high risk of PCB trace damage to the motherboard and/or the scope.However if you are lucky enough to have a DMM with a low impedance voltage test mode you can skip the above - with it in its low impedance mode, simply measure the voltage (both AC and DC) between the candidate node and ground. its low impedance will pull down any voltages due to leakage currents, so if you see more than a few volts AC or one volt DC don't clip your ground there. An E5 6V 'pea' bulb has a small enough base diameter to hold directly in the croc clip on your probe ground lead, then simply touch the pip to your candidate ground connection point.Doctorandus_P's idea of a fuse in series with your ground clip isn't bad*, just inconvenient. You don't want to increase the inductance any, so the total lead length should not be increased, nor should the fuse wire be spiral wound, but now you've got a hard lump in the middle of your ground lead reducing its flexibility and getting in the way. Also, as you *DON'T* want to hack up your original ground clip leads, can you actually buy spare ground leads for your probe - there is quite a range of clip styles and diameters, and while some are cheaply available on Amazon or Ebay (but they never say what dia.) others can only be special ordered from the probe manufacturer, usually at an exorbitant price, or are near totally unobtainable.* I've independently considered making up such a fused ground clip lead, but as I've yet to blow up a scope or DUT by misconnecting a ground clip, and I spent a couple of decades in the service trade, scoping potentially live equipment every day, I think I'm past the stage of needing it. If I do blow up a scope in future, I should probably take it as a sign I'm getting too old for mains bench work!