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Oscilloscope Ground and Common question

  1. Jul 26, 2012 #1
    Hello all, I'm new to this forum. I have a scenario that I don't quite understand with my oscilloscope, a tektronix 2246. I'm using it to see the waveform of a square wave frequency generator circuit. The circuit basically has two stages: The first is a 555 timer astable oscillator circuit that is used to generate a square wave from approximately +7V to ground. It is powered by a regular 9V battery. This output then goes into an inverting amplifier circuit, using an LM675 op-amp to output a wave from approximately -23V to ground. This circuit uses a dual power supply; the +9V from the 9V battery and a regulated 24V DC power supply, hooked up "backwards" to get -24V for the -VEE pin of the op-amp. When leave the -24V supply disconnected, and attach the probe common to my circuit ground, and the probe itself to my 555 output, I get a beautiful square wave at the exact frequency and voltages it should be. When I connect the -24V supply, and attach the probe common to my circuit ground, sparks fly. Literally. A huge spark emits from the connection point, so I instinctively pull the clip away from the point of contact. Does anyone have any idea what could be going on here? Using a much higher capacitor value to slow the wave down and read it with a multimeter, I find that the wave is in fact going from about 0.6V to -23.4V, so the wave is working properly, I just would really like to see it on the scope. Any help or suggestions would be much appreciated.

  2. jcsd
  3. Jul 26, 2012 #2
    Ah! seems like I got it. I Had similar issue recently.
    So, you are attaching the +24 volt terminal of the DC power supply to your circuit Ground, and the 'ground' of the DC power supply to -Vee.
    When you attach the probe common to the ckt ground, you get sparks!!!.
    The most probable reason (this I say because it was the case for me) is because, the Ground of the DC power supply and the Common probe of the Oscilloscope are internally connected <most probably through the grounding wires>
    So, try instead to put the Oscilloscope common to the -Vee (i.e. ground of the DC power supply) and the other lead to Op-amp output.
    You won't get 0 to -23V this way, but 0 to +23 Volt instead, but you can accept that, don't you?

    P.S. Welcome to Physics Forums
  4. Jul 26, 2012 #3


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    Science Advisor

    It should be pretty obvious that the 24V power supply has the zero volt terminal grounded (earthed) and so has the scope. In this case you cannot connect the scope probe ground clip to any point in the circuit other than the power supply zero volts (which in your case is -24V relative to the battery negative).

    So either

    1. Connect the probe (common) to your -24V. OR

    2. See if there is a ground link on either the power supply or the scope that can be removed. OR

    3. Get an isolation transformer and use it to power either the scope or the power supply.
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