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Astable / scope problem

Discussion in 'General Electronics Discussion' started by Peter Bullock, Jun 15, 2017.

  1. Peter Bullock

    Peter Bullock

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    Mar 9, 2015
    Hi,

    I have constructed an astable circuit and I want to use the scope to display the voltage across the capacitors. The problem is, when I use
    both channels of the scope connecting one channel to each capacitor the astable stops working and both LEDs stay lit. It's ok
    when I use just one channel on each capacitor but not with both. Does anyone have any ideas what this might be ?

    FC.
     
  2. BobK

    BobK

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    Jan 5, 2010
    Are you connecting the ground leads of the two probes to different places? If so, you are creating a short between them.

    Bob
     
    HellasTechn likes this.
  3. Peter Bullock

    Peter Bullock

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    Mar 9, 2015
     
  4. BobK

    BobK

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    Jan 5, 2010
    You cannot do that. The grounds on the two probes are connected together, so you are altering the circuit when you connect the two grounds to different spots in the circuit.

    Bob
     
    HellasTechn likes this.
  5. Peter Bullock

    Peter Bullock

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    Mar 9, 2015
    Hi Bob,

    This could probably have been expressed more simply as : Where do I place the probes in the circuit to measure the voltage across each capacitor ?

    PJ.
     
  6. duke37

    duke37

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    718
    Jan 9, 2011
    I do not see your circuit.
    Many scopes need a common earth so you cannot measure the voltage across the two resistors.
    You can measure the voltage relative to common. Each channel can measure the voltage at one end of the resistor and some scopes can subtract the two values to get the voltage across the resistor.
     
  7. BobK

    BobK

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    Jan 5, 2010
    Which is basically what I asked you in my first post. When you are asking for free advice, complaining about how it is expressed seems a bit petty to me.

    Bob
     
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