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Generator output waveform check with cheap o-scope

Discussion in 'Power Electronics' started by MarshSt, Jan 15, 2021.

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  1. MarshSt

    MarshSt

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    Feb 28, 2017
    I want to view the 120VAC waveform output from my generator but only have a DSO-150 Shell o-scope with a max input voltage limit of 50Vpk (100Vpp). How can I reduce the voltage without affecting the original waveform characteristics (shape/cleanliness)? I'm considering two options.
    1: Two 60W incandescent bulbs in series as a voltage divider with the scope probes attached across one bulb. Will a resistive load like a light bulb (old school filament type) have any effect on the waveform?
    2:Step down transformer- Will the transformer smooth the waveform or clean up any noise in the original signal?
    Thanks,
    SM
     
  2. Nanren888

    Nanren888

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    Nov 8, 2015
    The data seems to say the input impedance of the scope is 1MOhm, so probably easy to use a resistive divide. If your probes don't have significant reactive component to their impedance, then the distortion should be minimal.
    .
    Incanescent bulbs tend to have strange, non-linear characteristics with voltage, related to heating up, so for that option to work, you'd be relying on the match between the bulbs.
    .
    You can probably source a couple of high voltage resistors. Just do the maths to check the power handling capability you'll need for any values you choose, can get.
    .
    Scope inputs are often unballanced; that is one side is earth. Not familiar with 120VAC supplies. Maybe someone else can comment on where earth is in a 120volt systems and whether the scope you reference will cope with that.
     
  3. Bluejets

    Bluejets

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    Oct 5, 2014
    Gererator outputs normally float. i.e. no ground reference.
     
  4. MarshSt

    MarshSt

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    Feb 28, 2017
    I set it up using two light bulbs but got an imbalanced voltage reading that was outside of the scopes range so the waveform was chopped of on the positive peaks. It looks like that was caused by the floating ground as you both mentioned. I switched the input selector from AC to DC (just SWAG since the reading didn't make sense) and got a waveform that was balanced on the vertical origin so I can see the whole waveform. I did a baseline on my 120VAC wall plug and the wave is nice and smooth, the waveform from my inverter generator has a slight ripple in the trace that validates the fact that this is a smoothed out DC generated signal.
    The purpose of this experiment is to be able to show the son in law what kind of power his generator is outputting. He is concerned his might be making "dirty" power and now I have a way to actually see the output.
    Thanks for your input!
     
  5. Harald Kapp

    Harald Kapp Moderator Moderator

    10,999
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    Nov 17, 2011
  6. Bluejets

    Bluejets

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    Oct 5, 2014
    You will find if it is an older style generator from the 80's or further back it will be sine wave output.

    Newer inverter style will actually tell you if it is pure sine wave output, sadly though, if no such notice, it a rough old chopper.

    Using a $2 meter and changing things to get what you expect to see is no way of prooving what the output actually is.
     
  7. debe

    debe

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    Oct 15, 2011
    I used a 240V to 12V transformer to check the wave form on a cheap 2stroke 240v AC generator. It certainly shows some glitches in the sinewave. 2.JPG 3.JPG 1.JPG 8.JPG
     
  8. Bluejets

    Bluejets

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    Oct 5, 2014
    Cannot read the label....
     
  9. debe

    debe

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    Oct 15, 2011
    Here you go. 7.JPG
     
  10. MarshSt

    MarshSt

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    Feb 28, 2017
    Harald, that's the probe I used. Amz special! Here are the waveforms with about 120W load.
    1. 120VAC main
    2. Inverter gen
    3. Homelite 6500 about 8 years old, slip ring I think
    Not a perfect setup but good enough for the visual I was after.
    upload_2021-1-18_14-12-55.png
    upload_2021-1-18_14-16-46.png
    upload_2021-1-18_14-17-50.png
    SM
     
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