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Difference between polarized and non polarized capacitors

  1. May 2, 2013 #1
    Hello,

    I googled the difference between polarized and non polarized capacitors and all i found was differences in construction and the material that they are formed from.

    I want to know why there is 2 types of capacitors, what are the differences (cost, technical, etc), why there is not only non polarized?

    thanks
     
  2. jcsd
  3. May 2, 2013 #2
    Non-polarized capacitors (the ones we learn about in physics at first), at hard to manufacture at a small size compared to their capacitance (they become very big if we want a big capacitance). Also, they can be quite expensive when they become bigger.

    Polarized capacitors give us a way to manufacture smaller capacitors, but with a high capacitance. The drawback however, is that they are polarized.

    Some places with high requirements, it is not allowed to use polarized capacitors.
     
  4. May 2, 2013 #3

    NascentOxygen

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    Staff: Mentor

    Electrolytic capacitors are a way to manufacture a high capacitance relatively cheaply and in a small sized package. They are definitely a compromise, and have a finite life.

    You can connect two electrolytic capacitors back-to-back (electrically) so as to form a non-polarized electrolytic capacitor. Obviously, it will be about 4 times the cost of a single polarized capacitor giving the same capacitance.
     
  5. May 2, 2013 #4

    jim hardy

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    Science Advisor
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    2016 Award

    The answer to your question is:
    Electrolytic capacitors pack thousands of times more capacitance into a given volume than you can get with film dielectric types.
    But - electrolytic capacitors are necessarily polarized because of how they're made..

    capacitance= area / distance.

    Area is the area of the foil

    Etching an aluminum foil gives it more surface area per square inch. The microscopic hills and valleys have more surface area than would a mirror flat sheet. Remember the old saying "Colorado would be bigger than Texas if you ironed it flat" ?
    Etching gives a large [STRIKE]denominator [/STRIKE] NUMERATOR (golly i'm doing everything backward today !)
    Distance is the thickness of the dielectric.

    In electrolytics the dielectric is not a plastic or paper film but a thin layer of aluminum oxide on one the aluminum sheets. It's only a few atoms thick so you have a very small denominator.
    This little paper by Cornell-Dubelier explains it well.

    http://electrochem.cwru.edu/encycl/misc/c04-appguide.pdf

    old jim
     
    Last edited: May 2, 2013
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