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Why can't we use non-electrolytic capacitor ?

Discussion in 'Electronic Basics' started by mowhoong, Mar 29, 2005.

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  1. mowhoong

    mowhoong Guest

    We knew that all the Aluminium Electrolytic capacitor having shelf
    life of 5 year
    according to the mfe standard.( Due to crystaline in the material). Is
    that mean any electronic product if you do not use more than 5 years
    will not fuction properly ? Hence why can't we use non- electrolytic
    capacitor to substitute ?
    Can any person know the reason ? Thanks.
     
  2. Properly derated, good quality electrolytic capacitors, (temperature,
    temperature rise, voltage) can last a lot longer than 5 years. Poor
    quality caps that are running at several of their maximum ratings can
    have a very short life.
    One of the wear out mechanisms is loss of electrolyte through the seal
    material. I do not know what you are referring to by, "Due to
    crystaline in the material".
     
  3. You misread the situation. People don't use electrolytics becasue
    of some special characteristic, they use them because that's the only
    feasible way to get larger capacitances. Find a non-electrolytic with
    a value of 1uF or more, and it gets rather large. But switch to electrolytic
    and they become much smaller. It's the construction of the electrolytic
    that allows the higher capacitance within a reasonable package.

    So you will find that virtually all the capacitors over 1uF or
    so are electrolytic. The exceptions are when there is a very specific
    need for something else, such as specific value (electrolytics don't
    come in tight tolerances). There is little other reason to justify
    the size and cost of non-electrolytics in those larger values.

    And once you have that path, then people simply live with any drying
    out of electrolytics.

    Michael
     
  4. mowhoong

    mowhoong Guest

    Hi thank all for the response, I read from some book ,is just like lead acid
    battery if not use for a long period the plate in the battery will be polarize.
     
  5. It is like a battery only because it involves chemistry. In
    electrolytic capacitors, one of the plates is metal (aluminum for
    most, tantalum for some) and one plate is the conductive electrolytic
    solution surrounding that metal. The insulation that separates them
    is a thin layer of oxide that is made of the metal and oxygen released
    by driving current through the electrolyte in one direction during the
    manufacturing process. If the unit sits around for years, there is
    some degradation of the oxide, reducing its insulating properties.
    There is also a possibility that the electrolyte will escape and there
    will be no conductive plate opposite the metal. Both these aging
    problems are much improved since I started electronics, 40 years ago.
    But long periods of hot storage or operation will eventually wear out
    any electrolytic capacitor. It is only a question of how hot and how
    long. That is why capacitors are life expectancy rated based on
    several conditions (temperature, voltage, ripple current). Sitting on
    the shelf, they last much longer, generally.
     
  6. mowhoong

    mowhoong Guest

    Thanks John for your explaination. I have another question. A ceramic
    disc
    capacitor is cheaper than any of the polyester film capacitor, If
    both having the same value why can't i use the ceramic disc instead of
    polyester
    film capacitor if the cicuit is user in a 20w fluorescent light dc to
    dc
    driver by car battery, is this some thing to do with working frequency
    of the capacitor ?
    Best Regards

    same value o
     
  7. It may be possible to use the cheaper capacitor. No capacitor is
    perfect, and you have to decide if the imperfections of a particular
    type will be a problem in a given application. Ceramic capacitors
    made to be small and cheap have very strange dielectrics that change
    capacitance with temperature and also with applied voltage. They also
    act as transducers that produce voltage when force is applied to them
    and change dimensions when voltage is applied to them. They also tend
    to get warmer than film capacitors when charged and discharged rapidly.

    Here is a web site that explores many of the details of the various
    kinds of capacitors.
    http://my.execpc.com/~endlr/index.html
    There is a lot to know.
     
  8. mowhoong

    mowhoong Guest

    Hi John
    Thank for your reply, it take sometime to understand the capacitor topic
    Best Regards
     
  9. JeffM

    JeffM Guest

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