(Topic ID: 223286)

Help Reading Oscilloscope

By oldschoolbob

1 year ago

Topic Stats

  • 3 posts
  • 2 Pinsiders participating
  • Latest reply 1 year ago by oldschoolbob
  • Topic is favorited by 2 Pinsiders


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    #1 1 year ago

    Still learning to use my new toy. A while back I built a simple tester to check zero crossing on my MPU. It's just a 24 volt transformer with a bridge rectifier. When I hook it to my volt meter I get 24.36 VDC.

    When I connect it to my oscilloscope it shows 38 volts peak to peak. The scope is set to 10 volts per division - counting the divisions I show pretty close to 38 volts.

    When I check the 5 and 12 volt supply it shows 5 and 12 volts with a steady line across the scope. (The 5 and 12 volt supply is regulated)

    I'm sure the scope is reading correctly - I just don't understand what I'm seeing.

    I know the 24 volt unregulated should show the wave form like I'm seeing - But I thought the waves should peak at 24 volts.

    Can someone please explain what I'm looking at?



    pic_35_2 (resized).jpg
    #2 1 year ago

    AC is measured as RMS (root mean squared), so the reading you get isn't the peak. Unfiltered DC is just inverted AC, so the same logic applies.

    #3 1 year ago

    Anyone that doesn’t think electronics is fascinating is missing something. I wish I’d studied it more when I was younger.

    After seeing your post I thought I’d try another experiment. I opened up my tester and checked at the transformer (before the rectifier). WOW. My little 24 volt transformer is outputting 80 volts – peak to peak.

    pic_37_1 (resized).jpg

    Then I read up on RMS. I understand (sort of) what RMS is but the calculations are a bit beyond me. Thank goodness for DMMs. It looks like it does the calculation for me.

    Then with some really rough calculations I figured the 110 volts in my house is really like over 300 volts – peak to peak. No wonder they say AC is more dangerous than DC.

    Thanks for the lessons on electricity.


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