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Substituting electrolytic capacitor.

Discussion in 'Electronic Basics' started by Dave.H, Nov 17, 2007.

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  1. Dave.H

    Dave.H Guest

    I need to replace a 1.2 uF electrolytic capacitor, and the closest I
    can find is 1.0 uF. Is it OK if I install this one?
     
  2. If you are worried stick another one in parallel to make up the total.

    R
     
  3. Dave.H

    Dave.H Guest

    This is probably a stupid question, but what value should the second
    capacitor be? The power supply is 240 volts, I can only find a 350
    volt electrolytic. I've heard the electrolytics don't like too low a
    voltage, would this still work?
     
  4. Tim Wescott

    Tim Wescott Guest

    1/10 the rated voltage may be a problem, but 2/3 is about right for long
    life.

    Capacitors add in parallel, so a 1uF in parallel with a .2uF makes up
    1.2uF.

    --
    Tim Wescott
    Control systems and communications consulting
    http://www.wescottdesign.com

    Need to learn how to apply control theory in your embedded system?
    "Applied Control Theory for Embedded Systems" by Tim Wescott
    Elsevier/Newnes, http://www.wescottdesign.com/actfes/actfes.html
     
  5. Dave.H

    Dave.H Guest

    I was thinking a .2uF capacitor would be about right. Thanks for the
    help, appreciate it!
     
  6. Dave.H

    Dave.H Guest

    Another stupid question, but would I be able to use two .01 capacitors
    in parallel with the electrolytic? I can't seem to find any .2 caps.
     
  7. Hello!


    20 times a 0.01 ;), yes.

    2 times a 0.1, off course!



    Best regards,

    Daniel Mandic
     
  8. Dave.H

    Dave.H Guest

    Hello!
    Sorry, I don't understand what you are saying. Please excuse my
    almost non existent knowledge of elctronic circuits, I'm still
    learning .
     
  9. Dave.H

    Dave.H Guest

    I have tested the original capacitor again, this time disconnected
    from the circuit completely, and returned a more accurate reading of
    1.248 uF. I think being connected to the circuit last time, was the
    reason for the mulitmeter giving me a reading of 91 nF. The capacitor
    is of the old metal cased type, made in 1966, installed in the base of
    an old electric fan. I'm guessing that since it returned an accurate
    reading, I should just leave it.
     
  10. default

    default Guest

    Sure. Many electrolytic's are specified at +200% to -20% capacity
    (common aluminum types in particular)

    The designer should have taking the tolerance in account when the
    circuit was designed and allowed for the large variance.
     
  11. redbelly

    redbelly Guest

    He was correcting a typo of yours.

    You're looking to add 0.2 uF, or the equivalent, in parallel, yes?
    Then you just need a bunch of caps that add up to 0.2 uF, all wired in
    parallel.

    Two 0.1 uF caps will add up to the necessary 0.2 uF.
    Twenty 0.01 uF caps, though a rather silly thing to do, would also add
    up to 0.2 uF.

    Your suggestion (as typed) was to use two **0.01** uF caps, which
    would only add up to 0.02 uF. Not what you want. Most people reading
    this are probably assume you meant to say 0.1, and just typed 0.01 by
    accident.

    Mark
     
  12. John Larkin

    John Larkin Guest

    The context of your problem isn't clear, but since elec caps have at
    best +-20% tolerance, and sometimes a lot worse (like -0, +50%, or
    even GMV, guaranteed minimum value) it will probably be OK.

    John
     
  13. Eeyore

    Eeyore Guest

    Use 0.22

    0.2 isn't a standard value (except from some ancient US suppliers).

    Graham
     
  14. Eeyore

    Eeyore Guest

    Are you sure you mean +200% ? More like +50% max I'd have thought in the case of
    electrolytics.

    Graham
     
  15. Fred Abse

    Fred Abse Guest

    That rang a warning bell.

    This wouldn't be a motor start/run capacitor would it?

    They're special reversible types for AC use and should only be replaced
    with a type characterized for that application.
     
  16. Fred Abse

    Fred Abse Guest

    See my earlier post about motor capacitors. "Regular" electrolytic
    capacitors will not do, in fact will probably fail spectacularly, leaving
    innards over everything.

    At that vintage, it may well be a paper dielectric type, not an
    electrolytic.
     
  17. terryS

    terryS Guest

    ----------------------------------------------------------------------------------------
    Since the original posting does not specify the type of use
    we may have been assuming (I certainly did intially) this is an
    elctrolytic cap. operating on a DC supply of 240 volts?????
    Are we sure it IS an 'electrolytic'????
    1.2 mfd seems a small value for an electrolytic. At that size it is
    not difficult to use a 'non electrolytic' as long as space is not a
    series criterion? I have some old DC capacitors for example that are 4
    mfd!
    Could the capacitor possibly be an AC rated 'motor start' type
    capacitor?
    If the input IS 240 volts, RMS alternating, the peak voltage will be
    in the region of 340 volts. Even if the capacitor is attached to the
    out[put of a rectifier? the peak voltage it will encounter will be in
    the same range of voltage?
    It really would be best to know what is the application is! If an
    incorrectly installed electrolytic exploded it could cost someone
    their sight or perhaps skin burns!
     
  18. default

    default Guest

    You're right, wikipedia gives it at -20 +50%
     
  19. John Fields

    John Fields Guest

     
  20. John Fields

    John Fields Guest

     
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