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Difference between Bipolar and Non-polar caps

Discussion in 'Electronic Basics' started by aman, Apr 11, 2005.

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  1. aman

    aman Guest

    Can anybody please give me the difference between Bipolar and Non-polar
    capacitors ?
     
  2. The first are polarized, the second are not.

    It has everything to do with getting sufficient capacitance into
    a small enough package. For larger values, electrolytics are really
    the only means of doing that. But electrolytics by the construction
    are polarized.

    For most places electrolytics are used (because of a need for higher
    capacitance), polarized capacitors are not a problem. Either they
    are filtering power supply lines where they are seeing pretty much
    a single polarity signal, or they are carrying AC where one terminal
    is clearly more positive than the other.

    Rare is the case where high values of capacitance are needed and
    they must be non-polarized. But for those few occasions they can
    manufacture electrolytics so they are not polarized, and there you
    go. In effect non-polarized electrolytics are the equivalent of
    two electrolytics in series inside the package:

    ---||-||---
    + - +
    which of course can be emulated by putting two discrete electrolytics
    in series with the proper polarity (and remembering that it will be
    half the capacitance).

    Note that most capacitors are "non-polarized". But since they don't
    come in polarized versions, since the construction doesn't make them
    polarized, they don't need to be referred to as "non-polarized".
    We are talking about ceramic, paper, polystyrene and any other type
    of capacitor except for electrolytics and tantalums. YOu will also
    find that it's rare to find these types of capacitors in anything
    much larger than 1uF or so.

    Michael
     
  3. PeteS

    PeteS Guest

    Well, there are times one will use large capacitance that is
    non-polarised. Note it's not because it's non-polarised, but because of
    undesirable effects of electrolytics (and tantalums).

    Large capacitance at a high voltage is the realm of electrolytics, of
    course, and anything over about 100uF is too, but expect the high
    values of ceramics to increase.

    The issues with electrolytics (and to a certain extent tantalums, which
    have other undesirable traits) is the high ESR (Effective series
    resistance), their poor stability vs time and their horrendous
    stability vs. temperature.

    If you are filtering a large chip (let's say a power hungry processor),
    it's distinctly possible there's up to 100A going on, but at 0.9 -
    1.2V. As ripple performance is dominated by ESR, you need ultra low ESR
    as well as some bulk. There are 100uF ceramics available at 6.3V that
    are perfect for this sort of thing (and as the bulk output of power
    supplies, replacing 1000s of uF of electrolytics) and they are cheaper
    too. Of course, you could use bulk electrolytics with lots of small
    ceramics, but you still don't get all the benefits of that ultra low
    ESR as soon as you put an electrolytic in (it will still source/sink
    the bulk currents).

    As noted, switch mode power supplies are another example of where I
    need low ESR (although it significantly complicates the loop stability,
    but that's another issue) to maintain the output stable in the presence
    of impulses, be they on input or output.

    A bonus is the size - 100uF ceramics come in a 2220 case (0.22 x 0.2
    inch) and very low height, and the fact they are MUCH cheaper than
    electrolytics.

    (One of the issues with tatalums is the chemistry involved - if they
    overheat, they give off toxic fumes. One large company we OEMd
    equipment for insisted we could never use electrolytics in the
    equipment destined for them).

    The following manufacturers make large ceramic values in MLCC
    (MultiLayer Ceramic Capacitors) and there may be others:

    Panasonic
    TDK
    Kemet

    Cheers

    PeteS
     
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