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Seeking low chemical reaction caps

Discussion in 'Electronic Design' started by Paul, Dec 21, 2007.

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  1. Paul

    Paul Guest

    Hi,

    As you may know, Electrolytic caps can generate a few hundred
    microvolts DC. I read that this voltage is due to slow chemical
    reactions. What capacitors (over 4uF) generate the least DC voltage?

    I appreciate it,
    Paul L.
     
  2. D from BC

    D from BC Guest

    What's the app. ?


    D from BC
     
  3. Henry Kiefer

    Henry Kiefer Guest

    Try polymer caps, like OS-CON. I never measured it, but their electrolyt
    is solid!

    - Henry
     
  4. Paul

    Paul Guest



    It's for a circuit that measures DC microvolts. Ceramics are better
    than electrolytics, but in terms of microvolt levels they react to the
    slightest vibration or an appreciable temperature change. What about
    Poly films? They have self healing properties, so are there chemical
    reactions occurring in Polys that may generate microvolts? As already
    stated, electrolytics are out of the question.

    Does anyone have any capacitor recommendations?
     
  5. Paul

    Paul Guest



    Thanks. The local store has some 4.7uF Mylars. I'd like to order some
    from digikey, but there are so many types of polys and the datasheets
    provide no hint of the caps chemical reaction factor -->

    ---
    B
    B32022
    B32023
    B32024
    B32026
    B32231
    B32232
    B32559
    B32613
    B32671
    B32672
    B32911
    B32912
    B32913
    B32914
    BF
    BQ
    CB
    CombiSuppressor
    E
    ECH-U(B)
    ECH-U(X)
    ECHAX
    ECHS
    ECP-U(A)
    ECQ-E(B)
    ECQ-E(C)
    ECQ-E(F)
    ECQ-E(H)
    ECQ-P(U)
    ECQ-P(Z)
    ECW-F(B)
    ECW-F(L)
    ECW-H(L)
    ECW-H(V)
    ECW-U(B)
    ECW-U(C)
    ECW-U(V16)
    ECW-U(X)
    ECW-UC(V17)
    FB
    FFV
    M
    MKP
    MKP 416-420
    MKP X1
    MKP X2
    MKT
    MMKP 383
    P
    PETP
    PPS
    UNL
    V
    X-Y
    366
    460
    462
    464
    940C
    ---
     
  6. Eeyore

    Eeyore Guest

    Can they ? Into what load resistance ? Where did you hear this ?

    Graham
     
  7. Eeyore

    Eeyore Guest

    No.

    Graham
     
  8. Eeyore

    Eeyore Guest

    There are no 'chemicals' to react in a plastic film cap.

    Graham
     
  9. Henry Kiefer

    Henry Kiefer Guest

    Every mylar should work.

    Don't mix the technology names of polymer electrolytic caps with plastic
    film caps! Even if some plastic films may be constructed out of chemical
    polymers.

    If you like a I can measure a OS-CON. But it will need a little time to
    fully discharge it.

    Tantals are also solid. Give them a try.


    - Henry
     
  10. D from BC

    D from BC Guest

    It's news to me about free electrolytics having a battery
    characteristic.

    Perhaps focus on the nature of the dielectrics.
    You can start with vacuum sealed capacitors and then work your way up
    in size until the capacitor is as big as the rest of the components.

    A vacuum is an inert dielectric but not the best permittivity
    especially compared to tantulum.

    How is the capacitor being used?
    As a filter?

    Read up on:
    http://en.wikipedia.org/wiki/Capacitor
    D from BC
     
  11. D from BC

    D from BC Guest

    Yeah.. This is new to me.. Electrolytics acting like an
    electrochemical cell ???
    I also suspect somebody is getting fooled by the bias current from
    some voltage test equipment.

    Foil impurities??
    D from BC
     
  12. D from BC

    D from BC Guest

    oopps... correction: 'work your way down in size'


    D from BC
     
  13. John Larkin

    John Larkin Guest


    Good grief, he's right. I have a dusty bin of assorted lytics,
    guaranteed not used for years, some not for decades. Six various caps
    measure, in millivolts,

    14
    180
    135
    11
    22
    350

    all in the "plus" direction.

    I shorted that last one, the 350 mV guy, for 5 seconds. It dropped to
    2 mV but is steadily charging itself back up, 10 mV now after a couple
    of minutes.

    That makes sense: aluminum plates of various purity and oxidation
    level, some sorts of leadwires, all glopped with sulfuric acid and
    additives.

    Cool.

    John
     
  14. D from BC

    D from BC Guest

    For fun, maybe put a bunch of electrolytics in series and power up a
    LCD watch. :)


    D from BC
     
  15. John Larkin

    John Larkin Guest

    More:

    Surface-mount MnO2 tantalums, right off the reel, initial charge in
    mV:

    300 33 uF 25v
    730
    820
    1.2

    150 47 uF 25v
    70
    4

    I shorted the 820 mv guy for about 10 seconds and it went to 2 mv but
    started recharging, to about 35 mv so far. Unloaded, it charges, but a
    10M load pulls it back down.

    A bunch of never-used polymer aluminum caps, United Chem-Com 100u/16v,
    had initial voltages from 200 to 500 uv. I shorted one, and it went to
    a few microvolts but is charging back up.

    I suppose I could measure the short-circuit currents too.


    So, avoid lytics in dc-sensitive circuits!

    John
     
  16. Jim Thompson

    Jim Thompson Guest

    Lecher! STOP squeezing them ;-)

    ...Jim Thompson
     
  17. Guest


    You sure your DMM isn't charging the caps while you're measuring them?

    M
     
  18. Boris Mohar

    Boris Mohar Guest

  19. John Larkin

    John Larkin Guest

    Yes.

    John
     
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