## Piezo 'tweeter' power handling AVR question

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Hi all,

I am planning on driving a piezo tweeter from an AVR.  The construction of the transducer is a piezo element glued to a paper cone.  That assembly is bolted to a plastic dispersion horn as shown…

This tweeter is rated at 50 watts.

Question is… How do I calculate the max square wave FET voltage I can give it?

Being piezo, there is no coil resistance to do the math.  The terminals show near infinity.

I am thinking even the duty cycle plays a part in this, but let’s say 50% duty cycle, how much P-P voltage can I give this thing, and what is the math behind it, being we have a piezo (infinity) resistance.

My thinking is this:  50 volt square wave? With almost zero resistance, there is little current.  Any help is appreciated.

Thanks, all…

Good information here:

http://www.sonitron.be/useruploa...

My thinking is this:  50 volt square wave? With almost zero resistance, there is little current.  Any help is appreciated.

Your thinking will blow up whatever you are generating 50 volts with.  I am thinking you meant to write:

My thinking is this:  50 volt square wave? With almost infinite resistance, there is little current.  Any help is appreciated.

In the case of a piezo there is AC resistance and THAT is what determines the current through the unit.

The link I posted above goes into this a little.  I myself don't know the answer well enough.

Jim

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Last Edited: Wed. Dec 30, 2015 - 05:02 AM

perhaps post a specification of the piezo tweeter, then people have a bit more information.

on audio level the wattage is mostly specified with a given load (2ohm, 4ohm, 8ohm..........)

that way you can calculate the max current/voltage you can apply to the tweeter.

DC resistance is 'nice' but does not say a thing in the audio world.

A capacitor has several meg ohms DC resistance, but can be a short circuit at specific frequencies.....

An inductor has almost zero resistance DC wise, but can turn into an open at specific frequencies......

note that a conventional speaker is a inductor and your piezo tweeter is a capacitor...........

My 2 cents:

Piezos are basically capacitors, they use AC power and block DC current -that's why your multimeter goes off scale.

Some math:

Voltage_RMS2 = Output Power x Impedance

So for, say, a typical 4ohm /50W tweeter from loudspeakers, you'd need some 14V RMS

Now if the piezo is driven by a single transistor like this:

The max RMS voltage (duty = 50%) is : Where Vp is the peak DC voltage across the piezo.

So back to the example, the tweeter would need like 40V on top of that circuit.

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Piezo-tweeter drivers are funny things and they do not have any resistance that you can measure with an ohmmeter.  You can only measure the IMPEDANCE as a function of frequency and you will find that it resembles a capacitor much more so than a resistor.  So, as expected, you will have much more current flowing at higher frequencies than at lower ones.

Here is an example from a Motorola piezo-tweeter driver: http://www.pispeakers.com/KSN1041.htm

You'll note that the impedance get very low above 20KHz so you need to be careful that you don't burn it out with your square wave.  Square waves have strong ODD harmonics and so a 5KHz square wave will have strong harmonics at 15KHz and 25KHz and you could easily destroy the tweeter with a 50V square wave.  In spite of what you have stated, you WILL have a large current flowing at those higher frequencies and AC current CAN and WILL melt the thing.

You need to find the actual impedance graph for your tweeter driver and then look at the impedance at the frequency you are planning on driving it at.  You will then want to ensure that you limit the voltage so that the maximum power rating of the tweeter is not exceeded.