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Back to Back Tantalum Caps

Discussion in 'Electronic Design' started by PaulCsouls, Feb 12, 2005.

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  1. PaulCsouls

    PaulCsouls Guest

    I was recently looking at a design that had two Tantalum caps reversed
    biased to each other to make a bipolar cap. I know tantalums can work
    down to -1v and don't need a DC bias the way aluminum electrolytic
    caps do but this looked strange. What are the pitfalls to this

    Paul C
  2. John Larkin

    John Larkin Guest

    If there's an unknown DC polarity and a small AC signal, it's OK. If
    the AC across the combination gets to be larger than a volt or so the
    the capacitance will change during each AC cycle (as the
    reverse-biased cap goes from being a capacitor to being a resistor)
    and that can cause distortion and other weirdness.

    The potential nonlinearities are complex and depend on the situation.

  3. Pooh Bear

    Pooh Bear Guest

    The cost of tantalum caps perhaps ?

    They *don't* need bias ? I rather thought they did to perform properly. In
    fact, under the right condirions, aluminium electrolytics work well for
    audio coupling under zero bias conditions. You need a big cap to ensure
    that the ac component of the signal is essentially miniscule but you might
    be interested to know that almost every single top-end modern audio mixing
    console uses unbiased aluminium electrolytics to couple signal - right at
    the very top-end too. I'm talking $100,000 + products here.

    I do know an audio company that ( many years ago ) used back to back
    tantalums for coupling that biased their centre junction to ensure biasing
    polarity. Today that would be considered esoteric overkill and financial
    suicide for a large product.

    I have used that technique with a pair of aluminium electros though, where
    the DC offset voltage at the source is subject to tolerances. Works

  4. Tam/WB2TT

    Tam/WB2TT Guest

    You might be better of putting a low Vf diode across each cap to keep the
    cap from ever conducting forward current.

  5. John Larkin

    John Larkin Guest

    Makes it more nonlinear in many situations.

  6. PaulCsouls

    PaulCsouls Guest

    I'm just following Ian Sinclair's Passive Components for Circuit
    Design. He says that Alumunium Electrolytics require a DC polarizing
    voltage to maintain the insulating film and for Tantalum Caps he says
    "Tantalum electrolytics can be used without any DC bias and can accept
    a small reverse voltage, typically less than 1.0v." But he doesn't get
    into the frequency characteristics and I would guess biasing would
    improve the usable bandwidth.

    Paul C
  7. PaulCsouls

    PaulCsouls Guest

    That's my intuition. I would pay for a bipolar cap with a data sheet
    to give me that warm fuzzy feeling of knowing what's going on.

    Paul C
  8. PaulCsouls

    PaulCsouls Guest

    Yes, or clamp the signal to +/- 0.7 volts.

    Paul C
  9. Tam/WB2TT

    Tam/WB2TT Guest

    Wouldn't the cap acting as a diode be as bad or worse? He does not say what
    this is for; so, I am assuming he wants to put 60 Hz through it, which I
    have done at 24VRMS..

  10. John Larkin

    John Larkin Guest

    Without knowing the DC bias, AC current and frequency, and the
    capacitance it's impossible to say how weird or maybe not weird this
    would behave. It's complex.

  11. PaulCsouls

    PaulCsouls Guest

    It was in an AGC circuit. Basically a low pass filter with the caps in
    series to ground two 47uF caps and 1k resistor so a 4Hz 3dB. Not very
    demanding with respect to distortion or tolerances. I was just curious
    about how the limitations of the configuration.

    Paul C
  12. Tam/WB2TT

    Tam/WB2TT Guest

    What I had done was put 2 caps back/back with diodes to get a NP 200uF
    capacitor for use with a reversible 2 phase electric motor in an antenna
    rotator. Worked fine. I know of people who have don this with speaker
    crossover networks, but as John points out, probably not a good idea.

  13. Rich Grise

    Rich Grise Guest

    Aluminums can do this all day - that's what a "non-polar" cap is -
    essentially two aluminum electrolytics back-to-back in one package.

    The OP was asking about doing that with tantalum.

    Nobody seems to know. ?:-|

    I'd say, look for followups on the performance of the circuit design the
    OP saw with that configuration, or put a couple on the bench in the
    breadboard with some kind of test setup, and see what happens.

  14. Pooh Bear

    Pooh Bear Guest

    Any link ?
    Oh well. That means almost every piece of current pro and consumer audio is
    built wrong then !
    Alumiums are good for a few 100mV reverse volts typically. Perfect for split
    supply audio apps.
    What 'frequency characterisitcs' ? Bias affects bandwidth ? Some very strange
    ideas here.

    Maybe this Mr Sinclair is one of those audiophool gurus ?

  15. Nico Coesel

    Nico Coesel Guest

    I don't even want to know. If possible, I avoid tantalum caps because
    they short out when they fail. They can also burn a hole right through
    the PCB if you are unlucky.

    When it comes to distortion, I think the best idea is to read the
    datasheet and determine how the capacitance changes with the voltage.
  16. Nico Coesel

    Nico Coesel Guest

    This may be interesting reading material:
  17. Clarence_A

    Clarence_A Guest

    This document IS interesting, and it seems to be good technically.
    It will be useful if someone can get past the poor phrasing and
    reverse orders of many statements.

    Perhaps the inserted reverse logic was intended as humor?
  18. Ian

    Ian Guest

    A better method would be to put a bias resistor to a sufficiently large bias
    connected to the junction of the 2 capacitors. That way, both are always
    correctly biased.

  19. PaulCsouls

    PaulCsouls Guest

  20. PaulCsouls

    PaulCsouls Guest

    The bias / bandwidth idea is one of my fool ideas. Ian Sinclair's book
    is recommended by Bob Pease in his Troubleshooting book, so it has
    been proofread by someone with some experience in the field.

    Paul C

    I'd be really smart, if I wasn't so stupid.
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