Piezo buzzer overshoots

Discussion in 'General Electronics Chat' started by Baron, Jan 17, 2010.

  1. Baron

    Thread Starter Active Member

    Jun 15, 2009
    31
    0
    Hi All,
    In my project I'm driving a piezo buzzer by CPLD.
    One side of it is connected to a square wave at 3.3v, and the other side is connected to an opposite square wave. in such way I get a higher sound volume, because, relatively to the buzzer it's operate at 6.6v.
    My question is why I see overshoots on the signal when I measure by oscilloscope? Is it real overshoots or it's fake by the measurement device?How can I be sure?


    Thanks
     
  2. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    They're real. You're overdriving the piezo. Put small caps (maybe 20pF-220pF) on either side of the piezo as bypass caps to ground, and use resistor(s) to connect your CPLD output(s) to the piezo.
     
  3. Baron

    Thread Starter Active Member

    Jun 15, 2009
    31
    0
    what do you mean "overdriving"? Why is this happen? Isn't a piezo buzzer behave like a small capacitor?
     
  4. retched

    AAC Fanatic!

    Dec 5, 2009
    5,201
    312
    You are putting too much into the piezo. With no caps to protect or resistors to limit the current, the piezo is back-feeding. The spikes are the power that the piezo is producing because it is being worked too hard.
     
  5. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    If the piezo is subjected to really fast rise and fall times, the crystal will start rapidly bending back and fourth. When crystals are flexed rapidly, they generate electricity. You don't get more energy out than you put in, but the fast rise and fall times will "kick" the crystal into producing high peak voltages.

    Adding a low-pass RC filter as I implied will slow the rise and fall times, so the piezo won't be "kicked" so hard.
     
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