Troubleshooting PCB current sensing.

Discussion in 'The Projects Forum' started by drevilz, Feb 17, 2013.

  1. drevilz

    Thread Starter New Member

    Feb 13, 2013
    12
    0
    Hi everyone,

    I picked up someone's project and the PCB current sensing part is not working properly. I need help from you great people to isolate the problem.

    The circuit uses a dc to dc converter to isolate the high voltage reference (COM 2 in CKT) and uses an optocoupler to re-reference the opamp output back to the initial reference node.

    When I turn on the power I get DC voltage from the output opamp (U11) and it doesn't work regardless of input. However, if I remove an opamp that is isolated to a different reference node (COM 1) and put it back in the powered circuit it works with a high amount of dc voltage in U11. The current sensing worked on the breadboard according to the previous person working on it.

    What problems do you guys think might be causing this behavior? The circuit is attached. The "output" opamp is U11 and the isolated opamps are U9, U10, U12.

    I checked the PCB for shorts or missing connections but it all seems to work properly. When I turn on the +-12 supply voltage I get a DC reading on the output opamp U11. This DC voltage does not change regardless of input unless I remove and place the opamps back as stated above.

    Thanks!
     
  2. drevilz

    Thread Starter New Member

    Feb 13, 2013
    12
    0
    The initial reading on the output opamp (U11) is -8v, when I remove the opamp and place it back into the powered circuit it goes down to -7v and I get proper readings according to the input.
     
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