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Ceramic capacitor value changes drastically with DC bias

Discussion in 'Electronic Design' started by JW, Mar 6, 2013.

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  1. JW

    JW Guest

  2. Yeah, I got acquainted with that particular problem in the 1980's.
    High-K ceramics have all sorts of non-ideal behaviour. Capacitance
    varies with temperature and bias voltage, they are piezo-electric,
    hysteretic and tend to crack. If they couldn't be made so tiny,
    no one would want to use them.

    There was one thing in your link that *did* surprise me: One
    commenter claims to have used a batch of caps that would oscillate
    all by themselves at 900 MHz when biased. I won't believe that
    until I see it.

    Jeroen Belleman
     
  3. Uwe Hercksen

    Uwe Hercksen Guest

    Hello,

    a parametric oscillator?
    If I would see it, I would try to remove all semiconductors from that
    board to see if it still oscillates.

    Bye
     
  4. Scratch, scratch... (google google)... OK I'm not sure how a
    parametric amplifer works. I 'did' a parametric oscillator in the
    past, but even that is a bit vague now. (I recall a plot of drive
    amplitude versus frequency that looked like a semi-circle, with a
    threshold.)

    Got a link (or pic) that might explain it? (It's a mixer type thing,
    no?)

    George H.
     
  5. Joerg

    Joerg Guest

    I have used the effect for moving a filter around. It doesn't get any
    cheaper than that.

    Right now I am looking for capacitors with the worst microphonics
    because I want to use them ... as contact sound sensors. But has to be
    low enough in capacitance so I can reasonably squeeze 100Hz bandwidth
    out of them.
     
  6. Joerg

    Joerg Guest

    Also, don't automatically assume that X7R is always golden and never has
    that problem. The very high density ones often do and it can be very
    tough to pry some data about that out of manufacturers. BTDT, a lot.
    Murata is pretty good when it comes to furnishing such data so they are
    currently my preferred source.
     
  7. tm

    tm Guest

    If you take a ceramic capacitor and apply the rated DC voltage across it and
    heat to above the curie point, when it cools it will exhibit the piezo
    effect.
     
  8. Joerg

    Joerg Guest

    With MLCC one never knows. All kinds of weird effects.

    When I got the Fluke 8845A, or rather after UPS (literally!) threw it
    over the backyard fence, I took an older ceramic cap and hooked it up to
    it. Turned it to AC, all 6-/12 digits of it. Every time I clapped my
    hands near that cap I could see the excursion.

    This meter is nice, no fan. But the relays clicking around when in
    ohms-mode are a bit annoying.

    They are not very good at very low frequencies, sub-Hertz and such.

    Or design one where it quits working for a minute every time someone
    says "s..t!" :)
     
  9. tm

    tm Guest

    I wonder what the feedback sounds like with that.
     
  10. Joerg

    Joerg Guest

    Like this: Dang!

    For customers in the south: Daiang! :)
     
  11. Joerg

    Joerg Guest

    Ah, that's what it's for :)

    It is a great meter but one thing I'll never understand: It's not the
    slowest on the continuity beep but if I drag the probe across a set of
    traces too fast it misses. The cheapo Chinese meter doesn't! So for
    continuity tests I'll keep using one of the Chinese meters. I wish the
    large companies would talk to their clients some more.

    I'll still have to try out LAN and PC operation. It shipped with a
    little USB-RS232 gizmo. For the LAN I'll wait for warmer weather because
    it's a crawl space job to get Ethernet there. It came with some sort of
    Fluke software but (as usual with many US companise ...) that seems to
    only be some teaser kit.
     
  12. Tim Williams

    Tim Williams Guest

    I built a little LNA (not wideband, ends ca. 50MHz IIRC) with 10uF 50V
    1206 X7R caps coupling the input and output. Thought it was reading
    strange waveforms, maybe power supply transients or something, until I
    realized they were microphonic.

    On further inspection, the transients observed from tapping the PCB have a
    frequency reasonable with respect to the acoustic length of said board.

    FWIW, that was around 10mV for a casual finger tap. Think the caps were
    TDK. Soldered with big globs on either end, which will tend to increase
    strain.

    Tim
     
  13. Joerg

    Joerg Guest

    Here in CA, when sudden weird jitters start to appear on a circuit like
    that, it's better to get the hell out of the building :)
     
  14. Jamie

    Jamie Guest

    Years ago when I used to do some repairs for my buddies, I had one bring
    me a guitar amplifier. He could not use it because it was feed
    back, even when nothing plugged into it. He also noted when tapping on
    the cabinet, it was perfectly microphonic.

    When I finally found the problem, it was a transistor in a plastic
    package. The package was fine and so was the transistor but something
    inside broke loose and was playing with the beta and made a perfect
    microphone.

    I used that transistor in an FM oscillator and it worked perfectly as
    a bug!..

    Jamie
     
  15. Grin... well can you turn the lemon into lemonade?
    some sort of vibration sensor...
    upthread tm said to heat it above it Curie temperature.

    George H.
     
  16. tm

    tm Guest

    Grin... well can you turn the lemon into lemonade?
    some sort of vibration sensor...
    upthread tm said to heat it above it Curie temperature.

    George H.

    +++++++++++++++++++++++++++++++

    With a DC voltage applied. Allow it to cool with the DC applied and it
    should become very microphonic. Put an audio tone on it and you should hear
    it sing. Usually a soldering iron will get it hot enough but the material
    makes a difference.
     
  17. Yeah I was going to ask what the curie temperature is?
    Is there a better 'flavor'?

    Thanks,
    George H.
     
  18. Tim Williams

    Tim Williams Guest

    AFAIK, barium titanate and related blends are in the 120C+ range, maybe
    200C tops. Comparable to ferrites. They're definitely annealed in
    soldering, the chips at least.

    If you have old discs that aren't measuring right, you can try soldering
    them, I suppose the mass and size of the disc may prevent the whole thing
    from heating up.

    Tim
     
  19. Nico Coesel

    Nico Coesel Guest

    Piezo elements are too expensive I presume? :)
     
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