strange scope/ground issue - Page 1
Forum section: electronics / projects

author: m3vuv
author_rank: Super Contributor
timestamp: August 21, 2018, 05:27:03 pm
content: hi all,ive just noticed my scope doing wierd things when i probe the -ve lead of my bench psu,the trace showed about 10vd measured between my bench psu negative lead and my scope ground,any ideas?,also if i grab the probe tip i get about a 20v p2p reading like noise,if i grab my electric cooker chassis at the same time i get 0,is my house wireing iffy or my scope,its a pme system on the house,cheers m3vuv.

author: madires
author_rank: Super Contributor
timestamp: August 21, 2018, 05:35:02 pm
content: Which bench PSU model/type do you have?

author: m3vuv
author_rank: Super Contributor
timestamp: August 21, 2018, 06:27:29 pm
content: hi,the psu is a palstar ps-30m.i dont think the psu has anything to do with iti still get 20v p2p holding the probe tip when the psu isnt plugged in!

author: madires
author_rank: Super Contributor
timestamp: August 21, 2018, 07:38:48 pm
content: That PSU seems to be a linear one (isolated output?) and the cause of the voltage you see on your scope is possibly just your local electrosmog. When you touch the probe tip you act as an antenna. Regarding the high value I wonder if you have set the probe to x1 while the scope thinks the probe is set to x10.

author: H.O
author_rank: Frequent Contributor
timestamp: August 21, 2018, 07:57:29 pm
content: To me this sounds pretty normal although I'm not sure what exactly what you mean when you say "the trace showed about 10vd".On a typical lab power supply* the negative terminal of the output is completely isolated from the mains and therefor from your earth/ground to which your scopes ground lead is connected (provided of course you have the scope connected to a properly grounded outlet). Since there is no "real connection" between the negative terminal of the power supply and mains earth/ground it means that the power supply output voltage can be floating above (or below) ground.When you grab the probe tip the scope displays the noise picked up by you and your more or less floating (relative to ground) body. When you then grab hold of your properly grounded electric coocker chassis your body basically shunts the all the noise to ground and the scope goes "silent".* Of course there are exceptions. Your PC Power supply for example has its negative output tied to mains earth/ground so if were to probe that the scope should show pretty much nothing. Many lab power supply has provisions for optionally connecting the negative output to mains earth/ground using a metal shorting bar but I'm not familiar with the model you mention.EDIT:madiresposted while I was typing, he have a valid point on the scope probe attenuation setting, worth checking!

author: ArthurDent
author_rank: Super Contributor
timestamp: August 21, 2018, 07:59:56 pm
content: Most bench supplies have 'floating' outputs (neither + or - grounded). If this is the case then what you are seeing is capacitive coupling and kind of meaningless. Unplug the supply and check from the input ground prong to either output lead with an ohmmeter to see if there is any connection. most supplies have a shorting link that can connect the negative lead (usually) to ground. If negative is connected to ground it should act like your cooker.

author: m3vuv
author_rank: Super Contributor
timestamp: August 22, 2018, 04:28:34 am
content: when i saig 10vd ie volts per division,my scope is an old tek 453 it has no 10x on ,if i measure the current between my probe ground and the psu negative i get 1amp at 10 volts between the two.

author: madires
author_rank: Super Contributor
timestamp: August 22, 2018, 05:36:24 am
content: Is the output of the PSU isolated or tied to earth?

author: ArthurDent
author_rank: Super Contributor
timestamp: August 22, 2018, 12:12:05 pm
content: You should have followed the advice I gave in post #5 to see if the output is 'floating' or not. From what you now say, the added information would indicate that there is some connection in your supply to line ground so you will get a real voltage and current between negative and scope, or any other ground. If you set the output to zero and you get zero difference then set it to 10VDC and you read 10VDC, the positive terminal is connected to ground. If the voltage you measure appears different from the output setting then there is some connection inside the supply connected to ground. This connection may be intentional or an accidental short.There appears nothing is wrong with the scope and the supply is causing the problem. It would help if you actually checked the supply out with a continuity/resistance tester as I previously described.https://pa0fri.home.xs4all.nl/Diversen/EP925/ep925s2.gif

author: m3vuv
author_rank: Super Contributor
timestamp: August 22, 2018, 12:49:42 pm
content: well ive done some more checking,the -ve of the psu is tied to the power cord ground,i set the output of the psu to 10.14 volts then measured between the scope probe ground and the psu -ve,i get a reading of -11.89,thats with the scope switched off,i would of thought there would be no potential difference between the scope ground and the psu -ve but it reads 11.89volts,is this normal?,cheers m3vuv.

author: ArthurDent
author_rank: Super Contributor
timestamp: August 22, 2018, 01:37:32 pm
content: Whether the scope is switched on or off has nothing to do with its ground. If the scope grounding via the power cord hasn't been buggered up, the case and probe ground will be at power line ground whenever it is plugged in because that is the way the scope and all other pieces of good test equipment are designed. Perform the test I recommended in post #5 on the scope to verify this.If the power supply negative is at ground potential as you claim there can be no voltage difference between the two grounds unless they are plugged into two different house A.C. circuits and there is some weird house wiring error which is highly unlikely. Plug both the PS and scope into the same duplex outlet or the same grounded extension cord with two outlets and check for voltage differences between PS and scope ground. If it still exists, there is a problem with the internal PS connections or what you believe you are doing.

author: madires
author_rank: Super Contributor
timestamp: August 22, 2018, 02:07:04 pm
content: My guess is that the mains wiring could have an issue, like the outlet's earth tied to neutral because of a missing earth wire. You've mentioned a PME installation. A quick search shows a Protective Multiple Earthing with a PEN. There you goSince neutral and earth share the same wire (PEN) you see voltage differences caused by the current flowing through the PEN. The difference will increase with the load. Therefore the voltage difference is expected in your case.

author: m3vuv
author_rank: Super Contributor
timestamp: August 22, 2018, 09:23:42 pm
content: i think i will mod the psu and see if i can make it floating,otherwise i can see me blowing sommat up while probing with the scope if i dont take care,i wonder if sommat in the house has an earth leak.

author: t1d
author_rank: Super Contributor
timestamp: August 23, 2018, 04:04:24 am
content: Quote from: m3vuv on August 22, 2018, 09:23:42 pmYes, an actual explosion is possible. Please see Dave's video on how to not blow stuff up, with your oscilloscope.

author: StillTrying
author_rank: Super Contributor
timestamp: August 23, 2018, 01:49:27 pm
content: Quote from: m3vuv on August 22, 2018, 12:49:42 pmHow do you come to that conclusion, when it looks like the PSU's+vemightbe connected to earth ground.I'd start by unplugging the scope and check for continuity between the probe's ground and the plug's earth pin.Do the same for the power supply.

author: ArthurDent
author_rank: Super Contributor
timestamp: August 23, 2018, 03:14:21 pm
content: StillTrying - "I'd start by unplugging the scope and check for continuity between the probe's ground and the plug's earth pin. Do the same for the power supply."There have been several suggestions for m3vuv to do this which is the logical way to try to understand this problem but m3vuv doesn't seem to want to take any of this sound advice and would rather try to modify the power supply, which is probably working as intended.

author: StillTrying
author_rank: Super Contributor
timestamp: August 23, 2018, 06:23:50 pm
content: Yes AD,the whole thing makes no sense. With a 30A? PSU what could possibly go wrong.