1. Forum
  2. Usenet
  3. SCI.ELECTRONICS.BASICS

  • Scope use question

    From Bob Engelhardt@21:1/5 to All on Tue Aug 15 10:42:27 2023
    I'd like to use both traces on my scope to look at 2 voltages that are
    without a common reference. E.g., across R1 & across R3:

    ________R1____
    |
    R2 ...
    |
    R3
    ____|______

    I know that there are isolation probes to do this ($$$), but I'm
    wondering if there is some clever other way to do it. In my minimal
    experience I can't see one, but I'm hopeful that someone more clever
    than I has one to share.

    Thanks, Bob

    --- SoupGate-Win32 v1.05
    * Origin: fsxNet Usenet Gateway (21:1/5)
  • From Michael Terrell@21:1/5 to Bob Engelhardt on Tue Aug 15 13:36:11 2023
    On Tuesday, August 15, 2023 at 10:42:33 AM UTC-4, Bob Engelhardt wrote:
    I'd like to use both traces on my scope to look at 2 voltages that are without a common reference. E.g., across R1 & across R3:

    ________R1____
    |
    R2 ...
    |
    R3
    ____|______

    I know that there are isolation probes to do this ($$$), but I'm
    wondering if there is some clever other way to do it. In my minimal experience I can't see one, but I'm hopeful that someone more clever
    than I has one to share.

    Thanks, Bob
    You could do it with a four channel cope that allows combining two channels in summing mode, like the cutom probes do.
    Can you just use two voltmeters, instead?

    --- SoupGate-Win32 v1.05
    * Origin: fsxNet Usenet Gateway (21:1/5)
  • From Jasen Betts@21:1/5 to Bob Engelhardt on Wed Aug 16 05:46:34 2023
    On 2023-08-15, Bob Engelhardt <BobEngelhardt@comcast.net> wrote:
    I'd like to use both traces on my scope to look at 2 voltages that are without a common reference. E.g., across R1 & across R3:

    ________R1____
    |
    R2 ...
    |
    R3
    ____|______

    I know that there are isolation probes to do this ($$$), but I'm
    wondering if there is some clever other way to do it. In my minimal experience I can't see one, but I'm hopeful that someone more clever
    than I has one to share.

    Thanks, Bob

    With that exact circuit you could do it because the voltage on R3 is
    some fixed multiple of the voltage on R2+R3 but in the general case,
    no, an ordinary 2 channel scope can only measure voltage between three
    points (one of them being ground)

    --
    Jasen.
    🇺🇦 Слава Україні

    --- SoupGate-Win32 v1.05
    * Origin: fsxNet Usenet Gateway (21:1/5)
  • From Chris Jones@21:1/5 to Bob Engelhardt on Wed Aug 16 22:57:05 2023
    On 16/08/2023 12:42 am, Bob Engelhardt wrote:
    I'd like to use both traces on my scope to look at 2 voltages that are without a common reference.  E.g., across R1 & across R3:

    ________R1____
        |
        R2           ...
        |
        R3
    ____|______

    I know that there are isolation probes to do this ($$$), but I'm
    wondering if there is some clever other way to do it.  In my minimal experience I can't see one, but I'm hopeful that someone more clever
    than I has one to share.

    Thanks, Bob


    You could buy or build a differential probe.

    If you only wanted to see one trace (e.g. the voltage across R1), you
    could put the scope into summing mode and invert one channel. This
    doesn't work all that well because the gain of the two channels and
    probes isn't all that well matched.

    --- SoupGate-Win32 v1.05
    * Origin: fsxNet Usenet Gateway (21:1/5)
  • From Bob Engelhardt@21:1/5 to All on Wed Aug 16 15:56:52 2023
    Could I use a pulse transformer for isolation? E.g.: https://www.digikey.com/en/products/detail/DA101C/3438644

    It would put some inductance in the signal path, but I don't deal with frequencies higher than audio. Any other limitations?

    --- SoupGate-Win32 v1.05
    * Origin: fsxNet Usenet Gateway (21:1/5)
  • From Bob Engelhardt@21:1/5 to Michael Terrell on Wed Aug 16 16:13:29 2023
    On 8/15/2023 4:36 PM, Michael Terrell wrote:

    You could do it with a four channel cope that allows combining two channels in summing mode, like the cutom probes do.
    Can you just use two voltmeters, instead?


    I need to see the waveforms.

    --- SoupGate-Win32 v1.05
    * Origin: fsxNet Usenet Gateway (21:1/5)
  • From Bob Engelhardt@21:1/5 to All on Wed Aug 16 16:17:51 2023
    You could buy or build a differential probe.

    If you only wanted to see one trace ...


    Differential probe: too much (this is just a lower-level hobby). Whole
    point is to see both signals.

    --- SoupGate-Win32 v1.05
    * Origin: fsxNet Usenet Gateway (21:1/5)
  • From Jasen Betts@21:1/5 to Bob Engelhardt on Fri Aug 18 11:54:19 2023
    On 2023-08-16, Bob Engelhardt <BobEngelhardt@comcast.net> wrote:
    You could buy or build a differential probe.

    If you only wanted to see one trace ...

    Differential probe: too much (this is just a lower-level hobby). Whole point is to see both signals.

    Is this a repetitive signal, perhaps you could use the trigger input
    and probe the other two signals separately?


    --
    Jasen.
    🇺🇦 Слава Україні

    --- SoupGate-Win32 v1.05
    * Origin: fsxNet Usenet Gateway (21:1/5)
  • From Nick Bowler@21:1/5 to Bob Engelhardt on Fri Aug 18 18:34:56 2023
    On 2023-08-16, Bob Engelhardt wrote:
    You could buy or build a differential probe.

    If you only wanted to see one trace ...

    Differential probe: too much (this is just a lower-level hobby). Whole point is to see both signals.

    I think a big reason differential probes cost so much is because they are usually designed for mains voltage applications and are therefore built to
    meet various electrical safety standards (CAT ratings, etc.).

    If you don't care about this safety aspect (e.g., you're probing some low voltage electronics) there appears to be some mass-produced ~US$50 option readily available on aliexpress (no clue about its quality).

    If you are not too fussed about probe performance then maybe you can just
    hack together something on the bench. Perhaps a very simple opamp
    differential amplifier circuit like this one will fill the bill:

    https://www.electronics-tutorials.ws/opamp/opamp_5.html

    --- SoupGate-Win32 v1.05
    * Origin: fsxNet Usenet Gateway (21:1/5)
  • From Chris Jones@21:1/5 to Bob Engelhardt on Sat Aug 19 23:21:28 2023
    On 17/08/2023 6:17 am, Bob Engelhardt wrote:
    You could buy or build a differential probe.

    If you only wanted to see one trace ...


    Differential probe:  too much (this is just a lower-level hobby).  Whole point is to see both signals.



    I did suggest you might build one. Depending on what your requirements
    are, it might be cheap and easy to do. I made a simple one, I think from
    an AD830.

    --- SoupGate-Win32 v1.05
    * Origin: fsxNet Usenet Gateway (21:1/5)
  • From Bob Engelhardt@21:1/5 to Nick Bowler on Sat Aug 19 10:33:09 2023
    On 8/18/2023 2:34 PM, Nick Bowler wrote:

    [...]
    If you are not too fussed about probe performance then maybe you can just hack together something on the bench. Perhaps a very simple opamp differential amplifier circuit like this one will fill the bill:

    https://www.electronics-tutorials.ws/opamp/opamp_5.html


    "Not too fussed about performance" - that's me! Low frequency, low
    voltage, low precision is my world.

    The differential amplifier looks good - thanks.

    --- SoupGate-Win32 v1.05
    * Origin: fsxNet Usenet Gateway (21:1/5)