How does Piezo works

[xmen_xwk]

So I needed a buzzer for my fun project and I found piezo, it seems very interesting thing. It expands when we give it some current and generates reverse current back when collapses. Something like inductor or coil. Right ?

Now my question is, if I give it power and it expands, and then I cut the power using a transistor or anything else. Will it stay expanded or it will go back to previous state itself ?

 

[Audioguru]

You do not have a piezo buzzer. Instead you have a piezo transducer (a piezo speaker). A piezo buzzer has a built-in transistor oscillator to make it beeeeep when a DC voltage powers it. A piezo transducer is attached to a metal plate and it expands which causes the assembly to bend in one direction when voltage is applied and it bends in the other direction when the DC is reversed. It becomes straight again when the voltage is removed. It bends back and forth when AC is applied.

 

[xmen_xwk]

You do not have a piezo buzzer. Instead you have a piezo transducer (a piezo speaker). A piezo buzzer has a built-in transistor oscillator to make it beeeeep when a DC voltage powers it. A piezo transducer is attached to a metal plate and it expands which causes the assembly to bend in one direction when voltage is applied and it bends in the other direction when the DC is reversed. It becomes straight again when the voltage is removed. It bends back and forth when AC is applied.

Indeed but I do not have it yet. And if I give it DC PWM, will it make sound? I mean do I have reverse the pulse to pull it back to normal state or it will come back itself when PWM period is down.

 

[c_mitra]

It expands when we give it some current and generates reverse current back when collapses.
Now my question is, if I give it power and it expands, and then I cut the power using a transistor or anything else. Will it stay expanded or it will go back to previous state itself ?

It does not depend on current, it is purely a voltage dependent matter. When a voltage is applied, the positive and negative charges in the crystals move in opposite directions and the crystal changes its dimension. As long as the voltage is present, the change remains. When I apply pressure, the same thing happens in reverse: opposite charges are produced on the two faces. This is a feature of all crystals not belonging to the cubic system.

Only current it consumes is the displacement current - just like a capacitor. The crystal is otherwise non-conducting. The quartz is the classic example but there are so many other now.

The buzzer works by resonance- the ac voltage applied is corresponding to the natural frequency of vibration of the crystal- just like any quartz crystal.

 

[xmen_xwk]

As long as the voltage is present, the change remains.

Alright so it goes back to the normal state when voltage is 0V?

 

[Audioguru]

Alright so it goes back to the normal state when voltage is 0V?

Yes but it has a strong resonance at about 4kHz when it will be much louder then ring like a bell for awhile after it has been fed 4kHz.

 

[c_mitra]

has a strong resonance at about 4kHz

That depends.

The natural frequency of a crystal depends on its elastic properties. For silica, which is anisotropic, it also depends on the cut. In addition, there are different modes of vibrations possible in a solid and they each have different frequencies. It is also possible to have surface waves excited in a piezoelectric crystal. Similar piezoelectic disks are also used in ultrasonic cleaners- they work around 40KHz.

 

[Audioguru]

Here is a horrible frequency response curve of a typical cheap piezo audio transducer:

 

[c_mitra]

Here is a horrible frequency response curve of a typical cheap piezo audio transducer:

This one is optimized for audio frequencies and the natural frequency of vibration is suppressed by suitable mounting techniques- just like a common speaker. The transducer is optimized for wide frequency range- and it responds to forced vibrations and the natural frequency is highly damped. Similar materials are used also for filters (many radios use crystals for narrow band filtering) and other purposes. We also get piezo transducers that are unmounted and they have a sharper peak at their resonant frequency.