Signal/voltage measurement with oscilloscope - Page 1
Forum section: electronics / beginners

author: YST
author_rank: Newbie
timestamp: May 15, 2021, 03:53:45 pm
content: Dear Sir/MadamPlease kindly help to assist me for the following question.I got a riveting machine, which has a counter on it.The counter used is line seiki MCF-4X, with voltage of AC220Vhttps://www.lineseiki.com/products/electromagnetic/MCF/It is found sometimes the counter not responding.I want to measure the two wires linked to the counter (to ensure the signal is normal).Can I use oscilloscope to measure directly? I am worried about damage my oscilloscope. (not sure if it is earth grounded) Two wires power source (like two-prong electrical outlet?) can't be grounded, right?or what I can do to double confirm?If it can be measured directly, I need to measure it with counter connected, right?Thank you very much in advance.Best RegardsSimon

author: Terry Bites
author_rank: Super Contributor
timestamp: May 15, 2021, 04:21:55 pm
content: With the power disconnected.Check if either side is grounded with a DVM. Test again with your sense switch open and closed. If it is you can probe directly with your 10x at the highest volts/div and slow time base 1s/divThese counters cant go above 10 clicks per second.Is it an AC or DC model?

author: YST
author_rank: Newbie
timestamp: May 15, 2021, 07:34:45 pm
content: Thank you, Terry.and it is AC model with 220V.So, does that mean, as long as probe directly with 10x, the device under test and oscilloscope won't be damaged, regardless of grounded or not?Thanks!Best RegardsSimon

author: bdunham7
author_rank: Super Contributor
timestamp: May 15, 2021, 07:44:57 pm
content: No, definitely not! He mentioned 'with power disconnected'. We don't know the power level of the counter input. Also, what model of oscilloscope do you have?The only safe way to do this, IMO, would be to connect two probeswithout any ground connections, make sure they are set to 10X and then set the scope initially to 50V/div (with 10X attenuation set) and see what the signals look like. You can then use MATH, CH1+CH2, to see what voltage is present. Since you only want to know if your counter should be incrementing or not, this should be good enough.To repeat, it is unsafe to make any ground connections to this signal without being sure of what it is and whether it is ground referenced.

author: Ian.M
author_rank: Super Contributor
timestamp: May 15, 2021, 07:49:15 pm
content: Quote from: YST on May 15, 2021, 07:34:45 pmDon't bet on it, not even with someone else's scope and someone else's money!Clipping a scope non-isolated probe ground to anything other than a floating node of a fully isolated circuit, or to a node at true earth ground potential can blow up your scope.Even if you *think* its a ground node, if there's a large area ground loop, there can be enough voltage offset at a low enough impedance to drive a large enough current through your probe ground clip to melt your probe or burn ground tracks off the scope PCB! Mains Neutral is a particular hazard as the voltage offset between it and ground can vary unexpectedly due to switching of high current loads at other consumers in your neibourhood.Misguidedly floating your scope to try to get around this limitation can kill you and has killed other experienced engineers who should have known better.Bdunham's suggestion of two probes on two channels then using MATH to display CH1-CH2 (or visa versa) is safe enough if you are using identical x100 HV probes with their ground clip leads removed. If you are using fixed x10 probes its fairly risky due to mains supply spikes that may exceed the probe voltage rating. Keep tests brief (to minimize exposure) and use a known good surge suppressor in line with the mains supply. Do *NOT* handle any connections with the mains supply present. With cheap switchable x1/x10 probes, you are playing Russian roulette with your scope, and if you handle them with power on, with your life.Get an isolated differential probe with a Cat II or better rating for your mains voltage, its cheaper than a new scope or a funeral.

author: rstofer
author_rank: Super Contributor
timestamp: May 15, 2021, 07:50:45 pm
content: Viewing mains voltage with a scope is a really bad idea. You know this, that's why you posted the question. Yes, it can be done but it has to be done very carefully knowing exactly which wire of the circuit it grounded and which is 'hot'.You can avoid all this risk by just using a 220V/6V transformer to reduce the voltage and remove the ground reference. Then when you hook up the scope probe ground clip to one side of the transformer secondary winding, you will ground one side of the 6V circuit and that's not an issue. Connect the probe to the other side of the secondary winding. The primary winding connects across the counter leads.There must be a transformer laying around somewhere...Another option: You are measuring a junk signal, precision isn't necessary. One side of the 220V circuit is likely already at earth ground potential. Remove the probe ground clip, don't just leave it dangling. Now probe away! The scope and device neutral/grounds are already close enough in voltage levels that the lack of a probe ground won't affect anything.

author: bdunham7
author_rank: Super Contributor
timestamp: May 15, 2021, 08:03:03 pm
content: Quote from: Ian.M on May 15, 2021, 07:49:15 pmYes, thank you, I forgot that part. Hands off only with that type of setup. I'd still like to know what type of scope, though.

author: YST
author_rank: Newbie
timestamp: May 18, 2021, 02:28:29 pm
content: Terry, bdunham7, Ian, and rstofer, Thank you all!!!Really appreciated!The oscilloscope I got is "Hantek DSO5202P", not a high-end one.I did think about measure the signal through a transformer, guess I better get one transformer for testing it.The counter malfunction every once in a while. As this is the 2nd counter malfunction (as it appears), I am thinking about replacing the counter with a Arduino or raspberry pi for courting purpose. But thinking I should check whether the signal is normal or not when the counter acts abnormally.Thanks again!Best RegardsSimon

author: ledtester
author_rank: Super Contributor
timestamp: May 19, 2021, 05:53:53 am
content: Here's the datasheet:https://www.lineseiki.com/files/manual/MEMCUMCFmanual(E).pdfAnd here is what they say about the connection:What about using a current transformer around one of the connection lines? Then you could monitor the signal as well as the counter operation.Or could the transformer work in parallel with the counter (which would accomplish the same goal)?