Piezoelectric buzzer polarity ?

MisterBill2

Joined Jan 23, 2018
7,004
If the device is only an enclosed piezo device then it can function with either polarity. One polarity will cause the element to bend in one direction, the opposite will cause bending in the other direction. So polarity will only matter in phasing multiple devices.
Since the piezo material will also generate a voltage when sound flexes it, polarity will also matter if multiple devices are being used as a detection array,
 

Thread Starter

DarthVolta

Joined Jan 27, 2015
509
Yeah it's sealed, and I was too lazy to test it with applied pressure on a DMM.

What about the big disc kind with a brass ring or backplate, they always seem marked with preferred DC polarity. I remember briefly doing physics problems w/ piezocrystals, I'll have to have a glance at their basics again.


I have a few transducers from over the years, like out of weigh scales, that's 1 of many things I've yet to play with, but soon will.
 

Audioguru again

Joined Oct 21, 2019
1,910
They also make a piezo buzzer (beeper) that has a transistor oscillator inside and it makes a continuous high frequency tone when DC is applied with the correct polarity.
 

MisterBill2

Joined Jan 23, 2018
7,004
They also make a piezo buzzer (beeper) that has a transistor oscillator inside and it makes a continuous high frequency tone when DC is applied with the correct polarity.
Certainly ALL of the devices with internal electronics demand a specific polarity. And those WITHOUT any internal electronic components might prefer one polarity but not demand it.
 

Bernard

Joined Aug 7, 2008
5,648
Data sheet on first is for external drive so no polarity, maybe same for second. My sounder with internal drive has + on side & beeps with 3V.
 

Tonyr1084

Joined Sep 24, 2015
5,052
Speakers (not piezo's) are marked for polarity. The purpose is for phasing. If you want two speakers in parallel and pushing at the same time then you pay attention to how you wire them. But speakers are analog AC devices. If you have one speaker pushing while the other is pulling they tend to cancel each other out. I imagine pure piezo's are the same way.

I have a piezo sonic alert on my freezer door. It has its own driver internally. Polarity is critical. You have to put V+ on the V+ side or it won't work; and could possibly be damaged if reverse polarity is applied. That's the only time I know of where a piezo would be polarized.
 

ThePanMan

Joined Mar 13, 2020
41
A piezo takes an alternating voltage. Since the piezo can act like a capacitor, current will flow till the piezo is fully charged and distorted in one direction. When the voltage changes polarity current will again flow until the piezo is fully charged in the opposite direction and distorted in the opposite direction. A piezo element is just a crystal (or manmade crystal) that distorts with electrical charge. They don't flow current, they distort and hold a charge.

You can use a pulsating charge as long as when the pulse is gone you are shorting the piezo to drain off any stored voltage. The piezo will not be as loud in this manor. Also, a piezo has a natural resonant frequency depending on the size and structure of the element. Larger piezo's have a lower tonal resonance. Smaller piezo's are higher in frequency resonance. Try operating a piezo on a different frequency from its natural resonance and it will perform poorly. Try to imagine a pendulum swinging. The longer it is the slower it swings from side to side. Attempt to swing it at a different rate and it is far less efficient, and will take more energy.
 

MisterBill2

Joined Jan 23, 2018
7,004
Certainly piezo elements do have a resonant frequency and now I am wondering as to how stable it is and would that work like the frequency determining element in a radio frequency crystal. Interesting thought there, folks. possibly even useful.
 

MisterBill2

Joined Jan 23, 2018
7,004
Withthe sharp frequecy response peaks it might work well as a frequency determining device. Consider the narrowly peaked response of a quartz crystal. So now is this a useful invention, I wonder? Possibly an interesting one, at least.
 

Wolframore

Joined Jan 21, 2019
1,789
All of my passive piezo buzzers are marked for polarity like this:
1593251702228.jpeg

I have project where multiple buzzers work in unison and wouldn’t want to have them phase cancelling.
 

Tonyr1084

Joined Sep 24, 2015
5,052
Withthe sharp frequecy response peaks it might work well as a frequency determining device. Consider the narrowly peaked response of a quartz crystal. So now is this a useful invention, I wonder? Possibly an interesting one, at least.
In alarms such as a smoke detector (since I'm currently messing with one) the sonic alert has a resonant frequency at which it will produce the greatest sound pressure (loudness). The design takes this into consideration. For instance, in @Audioguru again 's example, a 41mm diameter piezo has its best response at about 5500 Hz, and therefore, a designer would use that as a driven frequency, since the piezo will naturally resonate best at that frequency. Off hand I don't know what diameter the sonic alert in my smoke detector is (doesn't really matter) it has a resonant frequency in the upper audio range of human hearing. Just a guess, but it's probably around 15KHz. Loud and high enough frequency to be piercing.

So, yes, the natural resonance of a given size crystal or piezo will be used as part of the frequency generating device. And yes, you can use a piezo as a frequency generator. As long as it's the size and frequency you are shooting for.
 

Audioguru again

Joined Oct 21, 2019
1,910
A piezo beeper that has an oscillator transistor in it which is powered from DC. It has a piezo with 3 terminals. The extra terminal is used for positive feedback so the piezo and its enclosure operate at the loudest resonance.

Beepers operate at about 3kHz, far from 15kHz. The horizontal scan and high voltage of an old CRT American TV operated at 15.7kHz.
 

MisterBill2

Joined Jan 23, 2018
7,004
A piezo beeper that has an oscillator transistor in it which is powered from DC. It has a piezo with 3 terminals. The extra terminal is used for positive feedback so the piezo and its enclosure operate at the loudest resonance.

Beepers operate at about 3kHz, far from 15kHz. The horizontal scan and high voltage of an old CRT American TV operated at 15.7kHz.
I have seen a number of piezo beepers with only two terminal elements. Just as there are oscillator circuits without taps or separate feedback windings, so there are piezo oscillators . Sime circuits are simpler than others.
 

MisterBill2

Joined Jan 23, 2018
7,004
A piezo with only 2 terminals is a transducer (speaker).
A piezo does not have a tap or winding.
From Google, here is a piezo with 3 terminals and a transistor oscillator:
I had a box of 100 of them, each beeper had only two terminals, but they were certainly beepers, not speakers. OR were you meaning a piezo ELEMENT with only two terminals? AND typical crystal oscillators only use two crystal terminals.
 
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