How to measure piezo impedance

Thread Starter

Eric cc

Joined Jul 26, 2020
15
I’ve a 4-40 KHz square signal into the piezo device, but I can’t found resonant.
I’m only a oscilloscope and a multi-meter to found impedance.
But when I used multi meter to measure current. Couldn’t found the maximum current.
If frequency increase and current will be increase.
In my knowledge the minimum impedance means current will be maximum.
Seem like I’m mistake......
So if only oscilloscope and multi meter could found resonant frequency?
Thanks for your help....
 

andrewmm

Joined Feb 25, 2011
537
your meter is measuring an "average" current, the current the crystal uses is going to be none DC, and small, so your meter wont register it.
 

Thread Starter

Eric cc

Joined Jul 26, 2020
15
your meter is measuring an "average" current, the current the crystal uses is going to be none DC, and small, so your meter wont register it.
I’m using AC mode to measure it.
Oh! But if I stop at resonant frequency dose average current should be higher than other frequency?
I used button to control frequency when each step is 100HZ, but still couldn’t found current different with other.
 

KeithWalker

Joined Jul 10, 2017
1,149
There are very many different types of piezo devices. Depending on what it is, it could resonate from a few Hz to many MHz. Can you give us any information about it? Without that I can't offer any helpful suggestions.
Regards,
Keith
 

Papabravo

Joined Feb 24, 2006
14,259
Here is some info on how to measure crystal parameters. It is not as simple as you imagine.


Haven't come across anything better -- recently.
 

Audioguru again

Joined Oct 21, 2019
1,779
A multimeter is designed to measure 50Hz and 60Hz and might not correctly measure higher than 200Hz.
An audio piezo transducer has a few resonant frequencies.
 

andrewmm

Joined Feb 25, 2011
537
Check your meters spec, its going to be rated for AC at 50 / 60 Hz, anything outside that range its not going to be registering
what frequency are you expecting your crystal to work at ?

Do I assume you have the meter in series with the crystal ?
This is a rather unusual question, and its got me thinking,
Just applying a sine wave across a crystal, and looking at the current, Im not certain is going to work,
The meter is going to have a relatively large inductance, which is going to swamp the impedance change of the crystal.

What you would normaly do with a crystal, is make an oscilator,
involving capacitors, the crystal and a feedback circuit.

https://www.electronicdesign.com/technologies/test-measurement/article/21800437/simple-tester-provides-readout-of-crystal-frequency

Thinking about it, what seems to be a way is to not measure the current, but the voltage.
At resonance, the voltage will be at a peek,

https://hackaday.com/2018/10/20/the-crystal-testing-method/

I just dont think your method with a current meter is going to work


BTW: Just had one other thought, some Pizo , are DC powered, you apply power and they transmit.
 
Last edited:

KeithWalker

Joined Jul 10, 2017
1,149
Eric, you still haven't let us know what is the piezo device you want to test.
Is it a transducer or a resonator?
Is it just the piezo-electric element or does it have electronic conditioning and amplification?
A piezo-electric element is a very high impedance device. The AC signal it would generate at resonance would be far too small to measure the current with a multimeter.
If it is a low frequency piezo element (<1MHz) and you had an AC signal generator, you could connect the generator and your oscilloscope across the device. As you tuned through resonance you would see the voltage increase and then drop rapidly.
If the resonant frequency is higher than that, to get accurate results, you would have to take into account the loading (capacitive and resistive) of the generator and scope.
That's the best I can do with the very limited information you have given us.
Regards,
Keith
 

Thread Starter

Eric cc

Joined Jul 26, 2020
15
Sorry, I’m replied too late, my piezo is a resonant only used to vibration.
I have a gate driver to driving 3.7v to 16v square signal and duty is 50% into piezo and it material spec I need to confirm tomorrow.
 

Thread Starter

Eric cc

Joined Jul 26, 2020
15
Eric, you still haven't let us know what is the piezo device you want to test.
Is it a transducer or a resonator?
Is it just the piezo-electric element or does it have electronic conditioning and amplification?
A piezo-electric element is a very high impedance device. The AC signal it would generate at resonance would be far too small to measure the current with a multimeter.
If it is a low frequency piezo element (<1MHz) and you had an AC signal generator, you could connect the generator and your oscilloscope across the device. As you tuned through resonance you would see the voltage increase and then drop rapidly.
If the resonant frequency is higher than that, to get accurate results, you would have to take into account the loading (capacitive and resistive) of the generator and scope.
That's the best I can do with the very limited information you have given us.
Regards,
Keith
Does Input wave type will be effect to driving?
 

Audioguru again

Joined Oct 21, 2019
1,779
Since he is driving it with an AC signal then it is a piezo transducer (speaker). He said 'Vibration" so maybe he is fastening it to something to vibrate the something.
 

KeithWalker

Joined Jul 10, 2017
1,149
From the information you have given:
You are driving some kind of piezo electric device with a variable frequency square wave that has a fundamental frequency which is variable from 4 to 40 KHz at an amplitude of 0 to 16 V.
I could ask you post oscilloscope waveforms but I very much doubt whether they would tell us anything about the resonant frequency of the piezo element that was not very obvious to you.
There are three ways that the resonant frequency could be determined:
1. By taking the piezo device out of the circuit, exciting it with a variable frequency sinewave generator and measuring the amplitude of the sinewave across the device with an oscilloscope. The amplitude will be maximum at the parallel resonance frequency an minimum at the series resonant frequency.
2. By exciting it with the existing circuit, varying the frequency from 4 to 40 KHz and measuring the mechanical vibrations of the device with an accellerometer. The vibrations will be of maximum amplitude at resonance.
3. By exciting it with the existing circuit, varying the frequency from 4 to 40 KHz and recording the of oscilloscope waveforms measured across the device, then doing Fourier analysis of the waveforms. The frequency domain result should show higher amplitude at the resonant frequency.
If the squarewave drive circuit, a scope and multimeter are your only resources, then I don't know what to suggest.
Regards,
Keith
 

Thread Starter

Eric cc

Joined Jul 26, 2020
15
Thank
From the information you have given:
You are driving some kind of piezo electric device with a variable frequency square wave that has a fundamental frequency which is variable from 4 to 40 KHz at an amplitude of 0 to 16 V.
I could ask you post oscilloscope waveforms but I very much doubt whether they would tell us anything about the resonant frequency of the piezo element that was not very obvious to you.
There are three ways that the resonant frequency could be determined:
1. By taking the piezo device out of the circuit, exciting it with a variable frequency sinewave generator and measuring the amplitude of the sinewave across the device with an oscilloscope. The amplitude will be maximum at the parallel resonance frequency an minimum at the series resonant frequency.
2. By exciting it with the existing circuit, varying the frequency from 4 to 40 KHz and measuring the mechanical vibrations of the device with an accellerometer. The vibrations will be of maximum amplitude at resonance.
3. By exciting it with the existing circuit, varying the frequency from 4 to 40 KHz and recording the of oscilloscope waveforms measured across the device, then doing Fourier analysis of the waveforms. The frequency domain result should show higher amplitude at the resonant frequency.
If the squarewave drive circuit, a scope and multimeter are your only resources, then I don't know what to suggest.
Regards,
Keith
Okay I know it, thank you!
I’m trying use Fourier analysis but I can’t found anything with square signal....
 

Audioguru again

Joined Oct 21, 2019
1,779
Its impedance is fairly high so it matters only on the amount of sound or vibration it produces at a resonant frequency. It will have a few resonant frequencies. A piezo works poorly at low frequencies, it is a shrieker.
 

andrewmm

Joined Feb 25, 2011
537
point of note, why you stopping at 40 Khz ?
point of note, the resonance frequency Q factor is quiet likely to be such that a 1 khz step would miss resonane.

can you provide your plot of current and voltage aginst frequency you already have please.
also a picture of your test set up
 
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