Author Topic: How to measure "floating" (ungrounded) voltages using two channels of a scope.  (Read 9081 times)

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Offline Rene

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Hello,

Some time ago, I was surfing YouTube when I bumped into a video that talked about how to make floating measurements with your otherwise grounded oscilloscope (at least I think that was what the video was about).

Due to my limited knowledge about all this, I am not sure if this technique applies to all oscilloscope / proves or if this only works for a particular type of oscilloscopes / prove.

Could one of the expert on this forum checkout the video and let me know if this is something that would work on one of the cheap Rigol scopes?... I don't want to try it myself before I know if this is ok to do because I am afraid of damaging the oscilloscope.

Here is the link:



Thanks.
« Last Edit: January 09, 2015, 04:10:31 am by Rene »
 

Offline devanno

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I just did this (what I call the "poor man's differential probe") using two probes on two channels on my Siglent DSO.  This is a pretty common approach (confirmed by a EE designer buddy of mine) as even the analog 'scopes ground the shield of the probe to mains ground. 

Just be sure to NOT exceed the input voltage rating on your channels.  I was working on low frequency stuff (around 8 KHz) so I just clipped the ground leads together... a better option may be to remove the ground leads from the probes completely. But find some way to insure that you don't inadvertently tie one of those ground leads to something other than ground.  You could hurt either your 'scope or the DUT.

BTW, In my case I was looking at a waveform on a model RR running Digital Command Control.  I didn't add the channels, I left them both alone, so I could actually see the waveform from both "(model RR) rails".  The waveform in question that I was measuring is a PWM AC square wave, actually. 
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Offline Paul Moir

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Yes, you can do this on almost any real multichannel oscilloscope.  While the Rigols are inexpensive, they're definitely real. 

A few things to keep in mind:
The CAT rating of your probe.  Do not exceed this.
I pull the ground leads off when I'm doing differential measurements to prevent accidents.
Know roughly your voltage to ground so you do not exceed the maximum input voltage on your 'scope.  For example you cannot measure something that's at 400v to ground with a 1x probe.
Avoid switchable probes (1x / 10x switchable).  Accidents happen.  They wouldn't be called accidents if they didn't happen.
If you want to measure a small signal on top of a very large voltage, use AC input coupling on both probes.  That way you're not wasting your ADC's resolution & input amplifier's range measuring the DC offset.
Think three times about your measurement before you make it.  If at all possible make the measurement hands free, or at least single handed.

« Last Edit: January 09, 2015, 04:30:51 am by Paul Moir »
 

Offline devanno

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-----8<------ (snip)------

Avoid switchable probes (1x / 10x switchable).  Accidents happen.  They wouldn't be called accidents if they didn't happen.
If you want to measure a small signal on top of a very large voltage, use AC input coupling on both probes.  That way you're not wasting your ADC's resolution & input amplifier's range measuring the DC offset.

Yup.. I forgot to mention about the 1x-10x.... I was stuck using switchables, and a buddy of mine, while hooking up the 'scope under the model layout (to the power bus) inadvertently switched one probe to 1x from 10x.  Fortunately, our voltage was low enough that there was no issue whatsoever, except that all of a sudden my volts/div had to be cranked up to see one of the waveforms... and the voltage reading was, well, 10x the other probe.  Took a min, but we figured it out without incident.

Paul, good point about the AC input coupling!  Added that to my mental notes. :-)
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Offline Paul Moir

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I happened to be doing some differential measurements tonight, so it was fresh in my memory.  Low voltage, but I didn't want to fry the drive I was working on and lots of deadly voltage was very near by!  :)
 

Offline Rene

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Thank you guys for responding.

I have one more question. What if my oscilloscope is a four channel scope? Can I use two of the proves to do the differential measurement while at the same time use the other two to do measurements as usual? Would this cause any issue?

Sorry if this is a dumb question, I just need to make sure.

Thanks.
 

Offline Paul Moir

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Well, if you have a 4 channel scope then dammit, you can go straight to hell!  :D

No, you can do regular measurements with the other two channels.  That won't affect anything.  No problem with the questions:  better safe than sorry.


 

Offline rf-loop

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Thank you guys for responding.

I have one more question. What if my oscilloscope is a four channel scope? Can I use two of the proves to do the differential measurement while at the same time use the other two to do measurements as usual? Would this cause any issue?

Sorry if this is a dumb question, I just need to make sure.

Thanks.

In main stream yes. And yes if all these signals have synch with each others and time scale do not differ much of.

Example if you use CH1 and CH2 for poor mans differential you also mostly use trigger for this.
 
Other channels, in this case CH3 and 4 have same trigger source.

If you want look signals what are not syncronized with this signal what is your trigger source then you see more os less garbage (with some rare exeptions)

There is some scopes what have ALT trigger and  also then more rare oscilloscopes what have ALT trigger with Dual time base (what is example in Siglent SDS1000 series with 2 channels).  I have not seen 4 channel digital scope what have 4 channel ALT and never I have seen with Quad timebase. (with some analog oscilloscopes also this is possible.)
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Offline ralphrmartin

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I've found one limitation of this approach. I want to do an X-Y trace. Unfortunately my scope wont let me feed the channel1 - channel 2 to X and the channel 3 - channel 4 to Y. Its an obvious thing to want to do - but I can't.
 

Online Mechatrommer

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you do that 2ch scope when your scope is grounded and you want to measure grounded signal in respect to another "not ground" signal. for ungrounded (floating) signal, you can do that with 1 channel of either grounded or floating scope. with floating scope you can even do 1 channel to measure grounded signal in respect to another "not ground" signal. confused? dont worry its normal, just read slooooowly.
« Last Edit: January 21, 2015, 03:24:21 am by Mechatrommer »
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Offline IanB

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Could one of the expert on this forum checkout the video and let me know if this is something that would work on one of the cheap Rigol scopes?... I don't want to try it myself before I know if this is ok to do because I am afraid of damaging the oscilloscope.

Although you "can" measure un-isolated mains voltages on a scope using just the center contact of a pair of probes, it is a good idea not to do so. If you make any mistake, if the probe slips, if something goes wrong, the result will be somewhat damaging.

If you try such a thing, it would be best to do it with a spare, back-up scope that you don't attach too much value to.
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Online tautech

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Could one of the expert on this forum checkout the video and let me know if this is something that would work on one of the cheap Rigol scopes?... I don't want to try it myself before I know if this is ok to do because I am afraid of damaging the oscilloscope.

Although you "can" measure un-isolated mains voltages on a scope using just the center contact of a pair of probes, it is a good idea not to do so. If you make any mistake, if the probe slips, if something goes wrong, the result will be somewhat damaging.
Every measurement of dangerous voltages carries some risk, un-isolated or not.
It is the operators responsibility to minimize that risk.

A few essentials to reduce risk. (others will add any I omit)

100x probes
Probe boot to isolate/insulate Gnd/Reference ring from DUT.(grabber hook removed)
Rubber footwear.
ONLY one hand used for probing, the other in your pocket.

These things I always do over ~40 V
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Offline aaronwhite30

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Before I got a differential probe, I used both channels of my scope in add mode with the second channel invterted and turned off trace 2 on a Tek 2445A. Although the diff probe I got was a high voltage 20X model, small signals are a pain, if I have to I switch back to the 2 channel method.
 

Offline rx8pilot

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What methods will work for small (mV) signals? Do you need an amplifier?

I saw this on eBay but it is a little short on bandwidth.
http://www.ebay.com/itm/171647384279?_trksid=p2060778.m1438.l2649&ssPageName=STRK%3AMEBIDX%3AIT

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Offline FrankenPC

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A tiny FYI.  I'm sure others have done this... I have a small 12V lead acid battery I keep charged for camping.  It's in a bag with an inverter and the charging circuit.  When it's home, I will sometimes plug my scope into it when I'm messing around with something that's potentially dangerous.  Completely floating.  Not portable by any stretch of the imagination, but it is disconnected from mains ground.
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Offline devanno

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A tiny FYI.  I'm sure others have done this... I have a small 12V lead acid battery I keep charged for camping.  It's in a bag with an inverter and the charging circuit.  When it's home, I will sometimes plug my scope into it when I'm messing around with something that's potentially dangerous.  Completely floating.  Not portable by any stretch of the imagination, but it is disconnected from mains ground.

All in one solution then would be like an Un-interruptable Power Supply (UPS) for a computer... just unplug it and run off the battery.  Oddly, I hadn't actually given this much thought until I read your comment.... I have UPS's literally right next to my test bench, too!   :palm:
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