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Piezo Element

Discussion in 'Electronic Design' started by Jon Slaughter, Oct 25, 2009.

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  1. I've seen several piezo driver circuits and claims. One required 200+ volts
    to drive the piezo element and others used 3-5V.

    I have a piezo element and have used a simple method to drive it described
    in some document I can't find now. A resistor is in parallel with the piezo
    and a low side switch is used to ground the element. This allows the element
    to be switched between VCC and GND. I've tried both 5V and 12V but the sound
    is quite low(using several frequencies from 1k to 3k). Sounds like a
    mosquito near your ear.

    Obviously with some type of acoustical amplification it is significantly
    louder. I know one can get the elements in a housing that is designed for
    amplification. My question is, how loud should they be when there is no
    acoustical amplification? I've seen greeting cards that have them built in
    and they are about 10x louder than what I have and they do not use an
    housing(that I am aware of).
  2. Les Cargill

    Les Cargill Guest

    It's really more acoustical impedance matching than
    amplification. Making them louder... might not be a
    good thing.
  3. GregS

    GregS Guest

    Can't figure out what your talking about. Element needs a baffle, and
    some add a horn. Bare element ?? Is it housed ?

    Most speaker drivers have a limit under a 100 volts.
    I have driven elements with hundreds of volts to produce
    movement of 5 mm or more.

  4. Nemo

    Nemo Guest

    Jon Slaughter writes
    In general you try to keep the drive volts below 30V ac or you run into
    regulatory issues to do with safety...

    But that usually only applies to the supply. Some drive circuits use the
    piezo element (which is quite capacitive - did you take this into
    account in your drive circuit?) as part of a resonant circuit. It can
    end up with well over 30V ac across it that way. This resonant technique
    is only good at one frequency. Most of the piezo sounders I've come
    across have been used as alarm sounders so were optimised for one tone.
    People used every trick they could to maximise (apparent) volume:
    Helmholtz resonators, differential drive, careful choice of drive
    frequency, voltage doublers, clever attention-grabbing cadences... sine
    waves generally give loudest apparent volume, followed by square waves.

    Basically you won't get much volume at 5V unless you use a magnetic
    sounder, but that will take tens of mA.

    I don't know how greeting cards combine decent volume with a broad
    frequency response and small size.
    The most common resonant frequencies for piezos seem to be 2048 and
    4092Hz. This seems to be an informal standard. It varies a few percent
    depending on ambient air density (humidity, pressure etc).
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