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Ceramic capacitor polarity trick

Discussion in 'Electronic Basics' started by Jason Carson, Aug 4, 2006.

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  1. Jason Carson

    Jason Carson Guest

    Hello,

    I know that MLCCs have no polarity associated with them. However, I'm
    wondering if there's a way to make the capacitor polarized like an
    electrolytic. I believe I've seen this on the web, but can't find it
    now. I'm using the cap in an application that will not allow
    electrolytic or tantalum capacitors.

    Any help would be greatly appreciated. Thank you very much.

    Jason
     
  2. I don't quite get your question. If the app won't allow electrolytics,
    and you have a MLCC, what's the problem?

    More details would help too. What capacity, what voltage, what
    allowable leakage current?
     
  3. Genome

    Genome Guest

    Yes... this is perfectly feasible. However you have to realise that there
    are people who don't know about this stuff who will suggest that by making
    one of the terminals more positive or possibly negative (depending on the
    polarisation they think you want) then your capacitor will be polarised.
    They think they are clever but we know they are wrong.

    There are two effects that you are looking at here, one is for shielding and
    the other is for the piezo-electric effect.

    As you know your capacitor is a Multi-Layer device and, in the case of
    shielding, we are interested in the way that the 'Layers' have been layed
    down by the manufacturer and how they are connected to the terminations.

    Obviously, for puposes of shielding, it is very important that the first
    layer and the termination it is connected to must be at the lowest or ground
    potential when referenced to possible sources of interferance.

    So, if your capacitor is likely to receive interferance from the board it
    should be placed with the first layer close to the board and the associated
    termination connected to circuit ground.

    Of course, if you are looking to minimise external interference then the
    first layer should be uppermost and the associated termination connected to
    circuit ground.

    Getting this wrong is a common mistake and unavoidable when buying
    multi-layer devices from a manufacturer who does not screen and package
    their product in a manner that allows your pick and place machine to perform
    its function correctly according to your specific design requirements.

    If you require to apply a DC bias to your Multi-Layer capacitors then, like
    others who really know about this sort of problem, you are looking to
    pre-stress the devices to drive the resonant frequency where piezo-electric
    effects become apparent above common and future sampling rates to avoid the
    problems of aliasing.

    Once again, as is typical with board applied products such as
    femptodamp(TM), it is important to verify that your Multi-Layer capacitors
    are correctly oriented on the board so that the applied DC bias causes
    flexure of the device towards the board applied layer to give proper contact
    along with the required levels of stiffening and damping.

    Your Multi-Layer capacitor manufacturer should be able to help you with the
    information you require about the relevant layers, termination process and
    bias polarity required for correct flexure of the materials they use.

    DNA
     
  4. Why not just make sure the signal flow is in the direction that the
    value marking reads?
     
  5. Genome

    Genome Guest

    Here we go, another bloody presumed expert.

    Do surface mount MLCC capacitors have markings on them.... ay? I mean do
    they?

    NO, so you have to get them from the right place and then you will get TOP
    and an arrow marked on them.

    It's not as if life was any better when capacitors looked like liquorice
    allsorts and I'm damned sure you couldn't claim anything about which way to
    stick the buggers in since the stripes were horizontal!!!!!!

    DNA
     
  6. Well, of course; you could put those in either way round.
     
  7. Genome

    Genome Guest

    Sheesh.

    So how did you know whether they were wound clockwise or anti-clockwise?

    DNA
     
  8. They were all wound clockwise if looked at from the right end.
     
  9. Won't work either. A Schottky diode has a very low forward voltage
    drop. An electrolytic cap doesn't have much reverse leakage at that
    sort of voltage.

    Best regards,
    Spehro Pefhany
     
  10. Yes, sometimes you need to add a small resistor (1 ohm or something
    like that) in series with a ceramic cap for hysteretic converters.
    Also for required stability with some LDO regulators.

    Best regards,
    Spehro Pefhany
     
  11. jasen

    jasen Guest

    A non-polarised capacitor will work anywhere a polarised one is used
    correctly.

    The only reasons polarised capacitorsd are ever used is because they are
    cheaper and smaller than non polarised ones.

    They can also explode but that's generally when they are used incorrectly.

    Bye.
    Jasen
     
  12. Alan B

    Alan B Guest

    Zoloft prescription's run out, then?
     
  13. No, he's upset that his favorite lingerie store can't sell him what
    he wants.

    --
    Service to my country? Been there, Done that, and I've got my DD214 to
    prove it.
    Member of DAV #85.

    Michael A. Terrell
    Central Florida
     
  14. Alan B

    Alan B Guest

    Ah, well then, Genome, just put in your favorite Eddie Izzard DVD, have a
    good laugh, and all will be better with the world! And oh by the way, how
    do you define horizontal and vertical on a cylinder that could be placed
    any which-way? Did you perhaps mean "lengthwise?"
     
  15. Joel Kolstad

    Joel Kolstad Guest

    I wouldn't suggest it's good circuit design, but I don't really think this
    is true. My understanding is that electrolytic capacitors will look more or
    less like a short circuit when reverse polarity is applied to them -- hence
    the common suggestion of using two electrolytics "back to back" to build a
    non-polarized cap from two polarized ones. As such, I wouldn't be surprised
    if someone's circuit somewhere relies on this behavior to eliminate, e.g., a
    Schottky diode in parallel with an unpolarized cap.

    ---Joel
     
  16. I don't think so. There's more leakage, and damage if the reverse polarity
    is applied for long enough and high enough, but it's nothing like a short
    circuit. A quick check with an ohmmeter, or a look at the schematic of some
    cheapo audio gear (where polarized elcaps are often used just like
    nonpolarized ones to pass low-voltage AC signals with minimal DC bias, eg
    the output of an opamp with a bit of offset voltage), is enough to confirm
    that.
     
  17. I don't believe any elco maker would regard that as anything other than
    serious abuse.
     
  18. Eeyore

    Eeyore Guest

    Elcos used in audio coupling with zero bias perform far, far better than most
    ppl believe actually. Hence their use in professional audio in this
    configuration.

    Graham
     
  19. I had a switch-mode regulator circuit where I had to use an
    electrolytic - the chip capacitor I used had too low an impedance and
    made it unstable. It was a "hysteretic" mode buck regulator IIRC.
     
  20. It's more 'designable' to use a chip cap with a series resistor. You
    can't rely on the ESR of elcos.
     
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