scope grounding

Discussion in 'General Electronics Chat' started by jut, Oct 7, 2009.

  1. jut

    Thread Starter Senior Member

    Aug 25, 2007
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    I need to measure the voltage input and voltage output (of a full wave rectifier) at the same time on my scope. However, I'm not sure where to place the ground so as to not short anything out.
     
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  2. beenthere

    Retired Moderator

    Apr 20, 2004
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    As long as it's the voltages on the secondary of the transformer, use the - terminal of the bridge as the common point.
     
  3. russ_hensel

    Well-Known Member

    Jan 11, 2009
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    No where -- they do not have a common ground ( in the ususal sense )
     
  4. SgtWookie

    Expert

    Jul 17, 2007
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    What Beenthere said.
     
  5. jut

    Thread Starter Senior Member

    Aug 25, 2007
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    I don't think it makes a difference, but I'm actually using a CT transformer. I updated the original schematic.


    So if I use the negative point of the transformer, when placing a probe at Vo, I'm actually measuring the voltage across the resistor and one of the diodes? Is that right?
     
  6. beenthere

    Retired Moderator

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    As long as you are galvanically isolated from the line, any point will do. The chosen point will be the 0 voltage reference for the measured voltages.
     
  7. jut

    Thread Starter Senior Member

    Aug 25, 2007
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    Yes, got that. But can you confirm it's not possible to measure both voltages simultaneously with my scope since CH1 and CH2 must share the same common point.
     
  8. SgtWookie

    Expert

    Jul 17, 2007
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    You can get the voltage relative to the common reference point, but you must choose just one common reference point.

    If you attempted to ground one probe to the - output side of the bridge, and ground the other probe to one side of the transformer's secondary winding, you would in effect create a short circuit across one of the diodes in the bridge rectifier. That would cause maximum current flow through the diode opposite of the short during 1/2 of the cycle.
     
  9. beenthere

    Retired Moderator

    Apr 20, 2004
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    No, you can display both voltage waveforms. You only need to ground one channel for reference. The other will use that point as the reference for its display - or at least all Tek and H-P o'scopes work like that.
     
  10. russ_hensel

    Well-Known Member

    Jan 11, 2009
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    Yes you can use the center tap ( or either end ) but the scope will not read the voltage across Rload. You can in fact pick almost any point as ground, you will just not simultaneously measure the input and output voltages. I stick by my first response.
     
  11. t06afre

    AAC Fanatic!

    May 11, 2009
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    Why is it so important to measure both at the same time?
     
  12. beenthere

    Retired Moderator

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    The OP needs to state his reason, but using two traces of synchronous waveforms and a subtract function can show amplitude differences and glitches.
     
  13. jut

    Thread Starter Senior Member

    Aug 25, 2007
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    It was a requirement for our lab at school. But from what I'm hearing from you guys, it's not possible, which is OK, I just wanted to make sure I wasn't overlooking some o-scope grounding trick.

    Thanks to everyone for their input. I appreciate it.
     
  14. t06afre

    AAC Fanatic!

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    I hope you do not misunderstand us. You can do it but, but as mentioned before. By selecting a ground reference all voltages will be measured in respect to that point. Perhaps they want you to comment this in your report
     
  15. jut

    Thread Starter Senior Member

    Aug 25, 2007
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    Why do you say I can do it if the two voltage measurements I'm trying to make share different common points and my scope has its ground leads connected.
     
  16. t06afre

    AAC Fanatic!

    May 11, 2009
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    Hi Jut. Take a look at my files. I have simulated your circuit. My ground selection is the same as yours I guess. I hope it explain something for you. You will see the same traces on a scope. If you have a 3 or 4 channel digital scope you may create the a-b wave.
     
  17. beenthere

    Retired Moderator

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    To play safe, never use more than one ground lead when doing probing. If you are working on anything more complex, the possibility of connecting to a different potential and smoking the circuit/o'scope becomes all too likely.
     
  18. russ_hensel

    Well-Known Member

    Jan 11, 2009
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    By using an external trigger on the scope you may be able to get the display to have the same relative time on two different connections ( move the input line but not the trigger ). Not sure this will work, the trigger may want a common ground ( or not ) see your scope manual.
     
  19. jut

    Thread Starter Senior Member

    Aug 25, 2007
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    Thanks for simulating it and the print out. I only have a 2-channel scope though.

    It's not that important that I see both channels simultaneously, I was just interested to know if it's possible. I guess it's not with a 2 -ch scope.
     
  20. someonesdad

    Senior Member

    Jul 7, 2009
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    There is a way to do what you want, but you can't do it with the typical two channel scope because the channels' measurements are referenced to power line ground. The work-around is to find a differential amplifier for each channel and use those to measure the voltages; the amplifiers' outputs then go to the scope channels.

    With a four-channel scope, you could clip a probe to each point to measure (input voltage and output voltage) and subtract the pairs to see the input and output voltages simultaneously, assuming your scope allowed you to do such a thing (mine doesn't). You can do the same thing with a two channel scope, but you just can't view the input and output voltage waveforms at the same time.

    The old 7000 series and some vacuum tube Tek scopes could be had with differential amplifiers (as the 5000 series could); those can sometimes still be found on ebay. The Tek AM502 amplifier can also do what you need. More modern amplifier examples can be seen here.
     
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