monitoring leakage resistance

Discussion in 'The Projects Forum' started by MPaulHolmes, Aug 22, 2013.

  1. MPaulHolmes

    Thread Starter New Member

    Aug 22, 2013
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    I'm making a meter that will monitor a leakage resistance. I have been trying this approach:

    GROUND ---- unknown Leakage Resistance ----- OP AMP Input PLUS

    Then I set the rest of the op amp up as a voltage follower. Then I'm monitoring the output of the op amp with an A/D channel on a microcontroller.

    It works OK, but not very well. The op amp input current into the PLUS input is too unpredictable. It makes the input voltage vary too much.

    Here's what someone suggested to me, but I don't understand it. They are unavailable at this time to clarify:
    I don't even know what to ask about it. Could someone turn that into a sort of text based schematic like this:

    TEST VOLTAGE --------- KNOWN RESISISTANCE ----- OP AMP input PLUS

    etc... I can't make heads or tails of it!
     
  2. wayneh

    Expert

    Sep 9, 2010
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    Of what, under what conditions? What range of resistance values are you expecting?
     
  3. MPaulHolmes

    Thread Starter New Member

    Aug 22, 2013
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    0
    Resistance range of 0Ohms to 60MOhms. 60MOhms is to be regarded as infinity.

    It's just a black box with 2 wires to the outside world. One of the inputs is connected to the ground on the board. The other input is connected to the "thing" that is at a high resistance relative to the ground. (Well, that's between 0 and 60MOhms.) Maybe 25 degC as far as temperature conditions go. I'm not sure what other conditions to consider. Oh, it's to be assumed that there's no significant voltage difference between the 2 wires that's caused by a voltage that's off of the board.
     
  4. wayneh

    Expert

    Sep 9, 2010
    12,086
    3,024
    Ah, a high impedance ohm meter. I think 60MΩ will be tough for a DIY.
    There are op-amps with extraordinarily high input input impedances. Last time I went looking for a high impedance op-amp, I found the LMC6035 or LMC660. I never actually used either of them but I think they'd be a start for this application.

    You should read this, for a start. The device you want is called a "meager" or even a "gigger". Geez, damn autocorrect almost got me in trouble just now.
     
    Last edited: Aug 22, 2013
    MPaulHolmes likes this.
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