Why parallel inductor with piezo transducer plate?

Thread Starter

Willen

Joined Nov 13, 2015
333
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
 

bertus

Joined Apr 5, 2008
22,270
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
 

BR-549

Joined Sep 22, 2013
4,928
And in another context, you can think of it as a choke, supplying dc power to the transistor.
 
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Thread Starter

Willen

Joined Nov 13, 2015
333
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.
 

BR-549

Joined Sep 22, 2013
4,928
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.
 

Thread Starter

Willen

Joined Nov 13, 2015
333
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)
 

DickCappels

Joined Aug 21, 2008
10,152
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.)
 

Dodgydave

Joined Jun 22, 2012
11,285
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.
 
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