self charging capacitors

Discussion in 'General Electronics Chat' started by brett, May 27, 2007.

  1. brett

    Thread Starter New Member

    May 27, 2007
    2
    0
    hi every one - rather a strange question for you all - which i would appreciate any thoughts on - it may seem a tad odd - but please bear with me .

    i have a phenomia going on here that is puzzling me - that of capacitors - charging and discharging themselves - and in some cases giving negative voltage readings - i have at the moment two capacitors being used for the ongoing experiment - both of which are new - have never had a voltage applied to them ( so this will hopefully rule out capacitor soak ) - the first is a one farad /24v and the second is a 10,ooo uf /40v - the 1 farad one is connected directly to a dipole antenna of circular configuration - eg a loop insulated in the middle - with an arm of the dipole connected to each terminal - the second is a stand alone capacitor - in a plastic casing with nothing connected to the terminals - the voltage on the capacitors is monitored daily - and before any test runs are commenced each is shorted AND grounded to a proper earth ground - for some considerable time ( hours ) - but as soon as they are brought into the house they start to pick up charge - running into many hundreds of Milli volts (dc )

    today i measured the larger of the two and it had swung from a positive to a negative voltage of 45.7mv the previous reading 5 days ago ( have been busy else where ) being + 43.3 mv !

    OK - now i have been told by so called experts that this phenomena is impossible - however it is happening - other wise i would not be wasting my time or yours asking this question

    as to WHY i am doing these tests ?? - sorry guys if i told you that it would detract from the discussion

    any assistance or thoughts would be welcome on this puzzling matter

    best regards

    Brett
     
  2. cumesoftware

    Senior Member

    Apr 27, 2007
    1,330
    10
    I had the same phenomenon, but the capacitors are not charging themselves. This is related to some kind of "memory". For example, when suddenly discharging large electrolytic capacitors with low value resistors, I could read almost 0V with the multimeter, but after some time I would read a greater voltage.
     
  3. brett

    Thread Starter New Member

    May 27, 2007
    2
    0
    hi thanks - i am fully aware of the capacitor soak phenomina - however as i stated these have never been charged to their working volatge - and where specifically checked and found at zero volts before leaving the shop - much to the amusement and puzzlement of the sales people - mind they are getting used to my somewhat odd requests .lol

    surly a capacitor will only have a memory if it has been previously charged - and i take every care to totaly discharge them to ground - in the two under discussion this is the first run - so does not apply

    told you this was an odd one :)

    regards and thanks

    brett
     
  4. thingmaker3

    Retired Moderator

    May 16, 2005
    5,072
    6
    Could your meter be "leaking" somehow? Have you checked with more than one meter?
     
  5. Dennis Atwood

    New Member

    May 26, 2007
    8
    0
    that was what i was thinking, maybe your meter is imparting a tiny current into the caps. by "capacitor soak", do you mean dielectric absorption?

    do you know what the caps are made of? what's the dielectric? maybe it's acting as a sort of battery? just a guess
     
  6. cumesoftware

    Senior Member

    Apr 27, 2007
    1,330
    10
    No, I made two measures of voltage, as I said. If the meter had some problem I would get both erroneous readings.
     
  7. cumesoftware

    Senior Member

    Apr 27, 2007
    1,330
    10
    The capacitor was about 4700uF. A tiny current in the order of nA (the biasing current for my digital meter) would not charge the capacitor to that point. Don't forget capacitors by construction have a leakage resistance, normally in the order of MOhms.
     
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