Piezo polarity-true or BS?

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Mbira

Well-known member
Joined
Jun 4, 2004
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Location
Austin, TX
A friend wrote me this the other day, but I think it's BS:
One problem that most people do not take into account with Piezos is that they have random polarity. I often use as many as ten piezos wired together and the trick is to make sure they all have the Ceramic as the +. I use a oscilloscope to test but you may be able to use a good volt meter. If you put piezos of different polarity in line you get automatic cancellation.

He is using several piezos in a row on a piece of wood, and my thought is that the sound waves traveling through the wood may reach the piezos at different times-thus creating the polarity problem, but this is not an issue in the piezos themselves...am I missing something?
 
PS: please note, that he's not just talking about making sure you hook things up correctly, but that you have to actually sort through piezos to make sure they are the same polarity and then wire them up.
 
It's plausible... I think.  Piezo elements tend to be substrates with the piezo stuff sprayed on, right?  On the other hand there's "getting squeezed" and the material producing a voltage.  It's a good theory question.

My own experimentation with similar things would indicate that it's important to get polarity correct.  The vast frequency range in a musical instrument would have local high frequencies not mattering wrt to polarity, but low frequencies being the same through the whole instrument.
 
I don't have any extra discs around here or I'd run some DC through them and test.

BTW, we're talking about these type:
ab2040b.jpg
 
From time to time, I actually design the things.

The picture you have shown is called a unimorph. It's a layer of typically soft PZT (lead zirconate titanate) ceramic conductively glued to a thin brass disk.

These are used in audio buzzers basically at one frequency, 2-7 kHz in a resonant mode...like opening and closing an umbrella slightly.

Yes they do have a polarity. And yes the buzzer elements tend to be random. Polarity matters if you are using multiple units together.

I can tell you how to sort them if you really need to... but the method depends on what kind of gear you have.

Les
L M Watts Technology
 
RE: gear-I have a scope and the typical setup, though I don't have a benchtop power supply at this time.
 
I did a bunch of foolin around with piezo's as pickups.  I found right about the time I got them sounding schweet, I crushed them.  It seems like you gotta apply just enough pressure to get bass out of them.  I'm curious how you're mounting them?
 
Hi,
Yes please!  I use several strung together regularly...

Ok. At frequencies below resonance their impedance is capacitive...about 20,000pf avarage. Bigger ones are higher.

Easiest way is if you have something like a charge amp. A fet input follower (unity gain) op amp will do. My lab scopes are very high impedance so I can just hook the piezo directly to the scope.

Hook  input/ grd to the piezo.  Then just lay them on a support (like a little ring about 2/3 the diameter) and press on it with a pencil eraser or something. See if the op amp  (or scope) output goes positive or negative. Even with a fairly light pressure you should see 5-20 volts.

For this to work RC has to be a second or so, so that means the op amp  (or scope) input has to be at least 50 meg ohms . A bit higher would be better. The input impedance of the amplifier and the capacitance of the element form a high pass filter.


Take care soldering to these things. High heat will depole them.

I buy the PZT material in large sheets with the poling direction marked with a dot on one side. I still double check them though.

Digi key sells the  buzzer elements alone. They used to be 21 cents, but the price just went up to about 90 cents. Buy a bag of the ones that resonate at 7 or 9 kHz....They are thinner and make better pickups.

I design special microphones with similar materials used for energy harvesting....using sound and vibration to generate power to run electronics.

Les
L M Watts Technology
 
Butterylicious said:
I did a bunch of foolin around with piezo's as pickups.  I found right about the time I got them sounding schweet, I crushed them.  It seems like you gotta apply just enough pressure to get bass out of them.  I'm curious how you're mounting them?

http://rattletree.com/phpBB3/viewtopic.php?f=19&t=30

I buy the PZT material in large sheets with the poling direction marked with a dot on one side.

Do you have a link to a supplier?  Do you then glue the material to a copper plate or something?  Thanks for answering these questions!



 
Do you have a link to a supplier?  Do you then glue the material to a copper plate or something?  Thanks for answering these questions!

More than you would ever want to know (check the tutorials):

http://www.morganelectroceramics.com/piezomaterials/index.html

Honestly the buzzer elements are best for hobby use though. With care, you can even take them apart.
The research materials I use cost an awful lot, but we are making high volume stuff. I am an electroacoustics engineer and do this (and other things) for a living .

The tutorials show how to make a bender by gluing 1-3 material to brass or similar to make a unimorph, or gluing multiple layers to each other to make multimorphs.

For musical instrument pickups, I highly recommend a FET source follower very close to the elements to lower the impedance.

Read the tutorials on the morgan site....much is highly technical though. I'll try to answer any questions. Great fun playing with the stuff!! Enjoy!

Les
L M Watts Technology
 
hi Les,
Thanks for the links...I, too, do this for a living-just on the other side of the fence... ;D

I'll try and get 10 or 20 more of the discs here soon so I can do those polarity tests.  This is very interesting indeed! 

Best regards,
Joel


 
Hello,
I was thinking about building a large tank for ultrasonic cleaning ( patchbays, consoles parts )
Would these transducers be worth for such an application ?
What type of signal exciter do they need ?
Any experience/link of DIY an ultrasonic cleaner tank ?
Best,
Guy
 
Hi Guy

Sorry I didn't see your post...

I have actually designed a unit, but can't talk about it much. Client owns the intellectual property.

In general though:

The piezo buzzer elements would not work for an ultrasonic cleaner. The cleaner elements are larger and acoustic impedance matched to water, and operate at a much higher frequency. 50 to 100 watt units are typical, at 30-100kHz.

You could certainly build a cleaner though. I have seen piezo cleaner transducers on ebay. They are resonant High Q devices with a purely resistive impedance well under 100 ohms...often much lower.

Some are low enough impedance and frequency  to be driven by an audio amp...I have seen cheap class d car amplifiers used. The transducers usually have a feedback sensor that can be used to lock in an oscillator to the resonance frequency.

Plenty of experimental stuff available:
http://shop.ebay.com/items/?_nkw=ultrasonic+transducer&_armrs=1&_from=R18&_ipg=

Les Watts
L M Watts Technology
 
Hi Les,

Sorry I did not see your post in reply, too !

Thanks for the information about the recommended type of piezo transducers and power amps to drive them.
On the link you provide there is a 70W transducer : is it the typical unit that I could use for the DIY of an ultrasonic cleaner tank ?

http://cgi.ebay.com/70-Watts-ULTRASONIC-TRANSDUCER-FOR-ULTRASOUND-CLEANER_W0QQitemZ160295277711QQcmdZViewItemQQptZLH_Default

Thanks again !
Best,
Guy
 
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