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Capacitors Used in Power Supplies

Discussion in 'Electronic Basics' started by Jonathan Mohn, Feb 3, 2004.

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  1. In virtually every schematic I've seen for regulated power supplies, the
    storage capacitor that is used after rectifying the current is a polarized
    capacitor. Why is a polarized capacitor always specified? What would
    happen if I used a non-polarized electrolytic capacitor?

    Also, I've read that electrolytic capacitors are fairly short lived. Is
    there any downside to using a longer lived type of capacitor in this
    function, aside from the size advantages of the electrolytic?

    Thanks!

    -Jonathan
     
  2. John Larkin

    John Larkin Guest

    Polarized 'lytics are just denser than other cap types by pretty big
    factors: better than 2:1 over n.p. electrolytics, and roughly
    thousands denser than film caps.

    Something like 10,000 uF at 25 volts, a modest electrolytic, would be
    outrageously big and expensive any other way. Scan a Digikey or Mouser
    catalog for some comparisons. Film or ceramic caps *would* be more
    reliable.

    John
     
  3. tweak

    tweak Guest

    Yes replacing with better quality capacitors is always a good idea.
    I use epoxy sealed with the highest thermal rating I can get.
    Some of the more knowledgeable techs here can tell you how to change
    up to a better rated capacitor as well. This can eliminate blown caps
    completely
     
  4. The power supply would be twice as big and cost ten times as much.
    The two I already mentioned.

    A capacitor must contain two conductive surfaces and an insulation
    layer that separates them and stores energy as an electric field is
    applied to it. Most non electrolytic capacitors have flat plates of
    metal with flat a layers of some dielectric between them. In film
    capacitors, this structure is often rolled up to save space.
    Electrolytic capacitors have one metal plate and one conductive liquid
    plate with only the oxide layer on the metal as a dielectric
    separator. Even with a flat plate, this way of coating a metal
    surface with a uniform layer of insulation with the other plate
    intimately enclosing the metal plate is pretty space efficient. But
    the metal plate in the electrolytic is electrochemically etched so
    that it is essentially a metal sponge with a hugh surface area
    compared to the original metal foil. The nit is oxidized
    electrolytically and covered with the conductive liquid that forms the
    second plate. This process multiplies the capacitance per volume by a
    huge factor. Unfortunately, you cannot reverse the voltage or the
    oxide will be removed as atomic hydrogen is produced at the surface of
    the metal, which combines with the oxygen to form water. And you have
    to live with the difficulties of a component that contains a liquid
    component. The liquid also is not as conductive as metal, so
    capacitive current heats it quite a bit. But if you can keep it cool,
    and not expose it to reverse voltage or excess voltage it can have a
    very useful life. Something like 10 years for good quality examples.
    The short life given in the specs assumes the internal temperature is
    constantly at the rated maximum.
     
  5. Thanks for all the answers. I didn't realize that they were THAT much
    smaller. Wow!

    -Jonathan
     
  6. Thanks for the detailed response, John. That is really fascinating. I'm
    continually astonished at the sophistication of these components. There
    sure are a lot of clever folks out there!

    -Jonathan
     
  7. You are welcome. There is a lot to know. If you want to learn more
    about various kinds of capacitors, this is a good site.
    http://my.execpc.com/~endlr/index.html
     
  8. DarkMatter

    DarkMatter Guest

    Not really. Same density, it just happens that an NP cap is really
    two caps back to back inside the can. Always was... always will be.
    Where are those 10,000 uF ceramic caps when you need one!
    Oh that's right! I do NOT have a space on my board the size of a
    commercial BRICK! DOh!

    You could have at least mentioned the difference between the
    different forms.
     
  9. Thanks for the great link! I had found several sites on capacitors and
    capacitance, but I had not come across this one, yet.

    -Jonathan
     
  10. John Larkin

    John Larkin Guest

    Well, when you put the two AlO2 dielectric layers back-to-back, the
    capacitance drops in half. That gives you half the CV product for a
    given can size. Always was... always will be.
    0805 size ceramics are up to 100 uF now, but the voltage is low and
    they're still expensive.

    Oh, please tell us! Please!

    John
     
  11. DarkMatter

    DarkMatter Guest

    Exactly... so the density is the same. The size of the cap changes
    for any given value because there are TWO... two... two caps in one!

    It isn't half the CV product, it is two seriesed caps... period.
    Anybody (any electrical idiot) knows that it would take twice the
    capacitance to get the same value.
     
  12. DarkMatter

    DarkMatter Guest

    Yes, and not always the choice to make. I have several times noted
    that an 0402 0.1 uF cap, and an 0805 or 1206 0.1uF cap behave
    differently in circuit, even with the same substrate.

    We have a hard start cap for an oscillator that hates being a small
    form factor.

    I guess the answer could be that they perform differently with the
    harmonics and such present in the signal, but there is no directly
    observable reason. Still the fact remains that it will not work with
    the smaller form factors in that location.
    Don't be a fucktard, boy.
     
  13. Bob Myers

    Bob Myers Guest

    Gee, exactly what is it you think "density" refers
    to here, Dark, old man? The important "density"
    when it comes to power supply "filter" (or "energy
    storage") caps is how much charge you can put
    into a given volume. For a specific dielectric material,
    in this case AlO2, in a given volume you can either
    cut the capacitance in half but keep the working
    voltage the same (the two caps in series situation),
    or double the capacitance at half the working voltage
    (two in parallel, which doesn't do diddly for the
    NP case which was the original question). But
    since Q = CV, in either case the amount of stored
    charge HAS to be cut in half. Half the charge in a
    given volume = half the charge density.

    It IS "half the CV" product that's important here, since
    the job of these caps is to keep the current up
    (i.e., supplying charge) to "smooth out the ripples"
    that raw rectified-AC would otherwise present. Which
    is, then, exactly why these supplies use polarized
    caps - they provide the greatest capacitance, at the
    necessary working voltage, in a given volume.

    Bob M.
     
  14. DarkMatter

    DarkMatter Guest

    I read what you (or whomever) wrote, and I know what was meant. I
    am telling you that at the maximum density modern manufacturers are at
    for a given voltage, a single cap of a given value back to back with
    another to form a non-polarized cap at half that value, is going to
    displace the same volume in a single can configuration. So 2 100uF
    caps makes a single 100uF cap that takes up twice as much space, even
    if made inside a single can construction.

    The figure stated was "double" IIRC by whomever, and I am agreeing.
    Sheesh... Don't get yer bloomers in a bunch.

    That is a correct figure. The reason seems different though. I
    simply stated that it takes twice as much capacitance to do the job
    since at any given cycle period one is charging, and the other is
    conducting like a short. It isn't calculated like two caps in series
    as would true non polarized caps or polarized caps in series,
    "properly polarized".

    Is that more clear? Old man? Or have I "screwed up" again?

    right.
     
  15. John Larkin

    John Larkin Guest

    If you put two 100v-rated film caps back-to-back, you can put twice
    rated voltage, 200v, across the series pair. So energy stored per unit
    volume is the same as you'd get from a single film cap of the same
    technology. The problem is that putting two 100 volt polarized caps
    back-to-back only allows 100 volts DC to be applied, even though the
    volume has doubled. You can't back-bias an electrolytic film
    steady-state - that would destroy it - while you *can* bias a plastic
    film in any direction as long as you like.

    Nonpolar 'lytics are funny. A 100 uF nonpolar electrolytic is
    essentially two 200uF caps back-to-back. For AC, it looks like a
    (crappy) 100 uF cap. For DC, one of the caps is back-biased, so it
    leaks and discharges, so the whole thing sort of looks like a very
    lossy 200 uF. This is a *very* non-ideal part.

    John
     
  16. John Larkin

    John Larkin Guest

    I'll take that for a 'no.'

    John
     
  17. DarkMatter

    DarkMatter Guest

    There is no "back to back" with np type caps. They are just plain
    "series" configured. And yes, the voltage drops across them split, if
    they are of an equal value.

    No shit, and the capacitance math isn't the same either, as one
    appears as a short one way, and the other does the other way.
    When an electrolytic NP is made one can never "back bias" one, as
    it will conduct as short above a certain voltage, and during that time
    the other will charge. Once charged, there is no longer a current
    flowing for the "back biased" cap to have any pressure on it.

    When conducting the other way, the opposite occurs for both caps.
    It's even more complex than that, but likely a bit safer than you
    make it sound when implemented correctly.
     
  18. You don't read so well do you DimBulb. Perhaps you should go
    back to second grade, where you left off.
    DimBUlb, do you have a *clue* what an NP electrolytic is? ...I
    thought not. Yikes, you are *DENSE*.
    Huh? You are a retard. Please read what you wrote in the
    previous paragraph and tell us what it means! Jeezum!
    It's certainly more complex than you're able to comprehend.
     
  19. DarkMatter

    DarkMatter Guest

    You retarded bastard. We use them in our designs at work, and we
    also configure our own NP electrolytic storage configurations in house
    with two caps so configured.

    Ever see a laser pulser bank? I thought not. Try RIFA, and
    Maxwell.
     
  20. Then how do you explain these words (they're yours)?

    NP electrolytic capacitors *are* back-to-back electrolytics.
    Putting them in series doesn't make them NP.

    John Larkin was exactly right. You really should try to learn
    from him, rather than constantly demonstrate your incompetence.
    Again you bring up irrelevant issues to try to wiggle out of you
    ignorance.
     
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