Why parallel inductor with piezo transducer plate?

Discussion in 'General Electronics Chat' started by Willen, May 1, 2016.

  1. Willen

    Thread Starter Member

    Nov 13, 2015
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    I deassembled a digital wall clock to see a temperature sensor. I saw little unusual another thing there. There was a piezo transducer plate as beeper. Exactly on the plate there was a 5mm thick inductor was added in parallel (I don't know its inductance).

    What is the purpose of the inductor there? Maybe that is high value inductor otherwise audio signal would be attenuated. Seems unusual to me. Google shows same thing I found today inside the clock: http://www.mbedded.ninja/wp-content...isc/piezo-drive-circuit-parallel-inductor.jpg
     
  2. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    A piezo transducer can be seen as a capacitor.
    With the inductor in parallel a resonant circuit will be made.
    This might help to boost the volume of the piezo.

    Bertus
     
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  3. BR-549

    Well-Known Member

    Sep 22, 2013
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    And in another context, you can think of it as a choke, supplying dc power to the transistor.
     
    Last edited: May 1, 2016
  4. Willen

    Thread Starter Member

    Nov 13, 2015
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    I guessed the sound will be loud because of inductor's back EMF (high volts than Vcc)? Read somewhere that the piezo sounds better around 40V to 60V.
     
  5. BR-549

    Well-Known Member

    Sep 22, 2013
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    Most circuits are at least 2 circuits happening at the same time. A DC and an AC.

    So that inductor is doing at least 2 jobs at the same time.

    So it depends on how you look at it. You can also look at it from the piezo point of view too. That's a third job or function I should say.
     
  6. Willen

    Thread Starter Member

    Nov 13, 2015
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    Hi,
    If the piezo transducer is as a capacitor then how it has a fix resonance frequency? All piezos have same resonance frequency?

    What is the resonance frequency of a dynamic speaker? (confused little)
     
  7. DickCappels

    Moderator

    Aug 21, 2008
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    A piezoelectric transducer's resonant frequency is a matter of mechanical resonance, not electrical resonance. To a driver circuit it looks like a reactive load with peak(s). The resonant frequency varies as a function of the mechanical dimensions, materials used, etc.

    Dynamic speakers follow the same rules as far as resonance is concerned. The larger a piece, the lower the frequency, the greater the stiffness of a piece, the higher the resonant frequency. (You audio guys, please check me on that last one.)
     
  8. Willen

    Thread Starter Member

    Nov 13, 2015
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    Is there any simple calculation to choose the value of the inductor?
     
  9. DickCappels

    Moderator

    Aug 21, 2008
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    Not that I know of :)
     
  10. BR-549

    Well-Known Member

    Sep 22, 2013
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    Won't the frequency be determined by the buzz signal?

    I would sweep the input and see what you get.
     
  11. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    The capacitance of the piezo and the inductance will form a tuned circuit and will have maximum output voltage at resonance, use the F= 1/2pi root LC formula to calculate resonance.
     
  12. ian field

    Distinguished Member

    Oct 27, 2012
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    If there is; it might be in the Murata catalogue.
     
  13. blocco a spirale

    AAC Fanatic!

    Jun 18, 2008
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    You are correct, this has nothing to do with resonance, it's simply a way of getting more volts across the piezo.
     
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