bypass cap question

Discussion in 'General Electronics Chat' started by pinnacle06, Aug 8, 2008.

  1. pinnacle06

    Thread Starter Member

    Feb 9, 2007
    16
    0
    I have a quick question. I have a board with a cap (tantalum) that is being used as a bypass cap but also to store energy for a quick burst I need later. Anyway, I need to switch the cap in and out of my circuit as it slightly affects the current drawn from a part I am testing (1 uA or so but still I need it out of there when I am measuring the current but present when I am not). Are there any draw backs from simply disconnecting the negative terminal from ground to disconnect the cap from my circuit? This would be by far the quickest thing I can do at the moment. Thanks!
     
  2. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Rather than disconnecting the cap, why not simply measure/record the cap's leakage current at various voltage levels, then subtract those known values from your readings?
     
  3. pinnacle06

    Thread Starter Member

    Feb 9, 2007
    16
    0
    This is for a test system and I need very accurate current measurements. When the part I am working with is in stand by mood, it only draws about 20 nA of current. I need the cap to be removed from the circuit and do not want to relay on leakage current estimates. I was just wondering if there are any ill affects of disconnected the negative lead. Thanks!
     
  4. SgtWookie

    Expert

    Jul 17, 2007
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    Well, whether you can remove the cap or not depends upon the device which is receiving the benefit of having the bypass capacitor there in the first place. It may be that removing the capacitor would result in the device oscillating in an uncontrolled manner, giving false high current readings.

    As far as hurting the capacitor - no, removing the ground lead won't hurt it - unless you overheat it, subject it to voltages that exceed it's rating, or to a reverse polarity charge.
     
  5. nanovate

    Distinguished Member

    May 7, 2007
    665
    1
    How big is this cap? Ceramic caps do not leak as much.
     
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