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power supply interference?

Discussion in 'Electronic Basics' started by jim, Sep 12, 2003.

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  1. jim

    jim Guest

    I'm hoping someone can clear up some confusion I have regarding the
    behavior of my power supply. I have a instek power supply,and it has
    in addition to the plus,minus terminals,a ground terminal between
    those two terminals. Now I notice that when I only connect the pos
    probe of my oscope to the pos terminal of the supply ,I get a fairly
    large 60 cycle signal,similaly when I connect the ground probe of the
    scope to the ground terminal and the pos probe to the pos supply I get
    the same signal, when I put pos to pos and neg to neg ,no
    interference. The issue is a nuisance because I'm using these
    terminals to run an opamp, with the ground running to pin 8 on the
    741. Any idea what's going on here? thanks for any help. jim
  2. Greg Neill

    Greg Neill Guest

    Sounds like the ground terminal is floating. Look
    for a break in the path from the terminal to the
    "true" ground of the circuit.

    What do you get when you put the ground probe on the
    ground terminal and the positive probe on the negative

  3. I expect you have a single "floating" supply - the output of the
    supply is not connected to ground. The "Ground" terminal of the
    supply will be connected to the case, and to the safety ground wire in
    the power cord, but not to the power supply output.
  4. jim

    jim Guest

    Same thing,60 cycle. I get 60 cycle even when I just put the pos probe
    on either pos or neg terminal without even hooking up the oscopes
    ground probe. jim
  5. Greg Neill

    Greg Neill Guest

    Your ground terminal is floating. That is, it's not
    connected to the circuit.
  6. John Fields

    John Fields Guest

    What you have is not a dual supply with a common ground, but a single
    floating supply, either end of which you can connect to ground (the
    ground connection). This will allow you to have a power supply with a
    positive output referred to ground if you connect the minus terminal to
    the ground terminal, a negative output referred to ground if you connect
    the positive output to ground, or a fully floating supply if you connect
    neither to ground. The ground terminal is connected to the power supply
    chassis and also (probably) to mains ground/neutral. Use your ohmmeter
    to find out for sure. If you want to use this supply to run a 741 what
    you'll need to do is connect the power supply + to the the 741's + power
    input, the supply - to the 741's - supply input, and make a
    pseudo-ground for the signal input. You can do that by connecting a
    couple of equal-valued resistors across the supply + and - inputs and
    connecting the junction of the resistors to to the - signal input of the
    741. It would also probably be a good idea to connect the junction of
    the two resistors to the supply's ground terminal so all the rest of the
    equipment you'll be playing with will be on the same page.
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