# How to determine the polarity of a piezoelectric buzzer?

Inside of a piezo buzzer is a little unimorph/bimorph disk which converts voltage to mechanical movement. To function as a buzzer, you just feed it an ac signal at its resonant frequency. However, John Alexander, the author of this project: STM Project, patented a design that uses the unimorph disk inside of a cheap buzzer as a fine-adjustment mechanism (US5866902).

Basically, if you apply a DC voltage to the across the ceramic part of the disk and the brass part, the disk will flex a very tiny amount (around 100nm/V).

My question is this: Piezo electric elements are given a polarity by applying a strong electric field that aligns the dipole moments inside of the piezo material. Thus, I cannot a priori know if the piezo disk that I remove from a buzzer will extend or contract when I apply a positive voltage bias. Keeping in mind that an application of 15V would yield a flex of around 1um, what is a clever way to test a disk for its polarity?

• One way to observe 1um is with a layer of glass (microscope slide?) over the brass surface and monochromatic light ( say from a red LED), observing the interference fringes between them.
– user16324
Commented Apr 30, 2016 at 10:24
• I ended up doing something different, as recommended by my professor: since I used the piezo for the fine adjustment of a scanning tunneling microscope, I had a wire rigidly attached to the buzzer. I brought the wire near to the sample and ran a high frequency wave through the sample. Since the sample and the wire are capacitively coupled, I could pick up the high frequency wave on the wire (hooked up to a scope). By then applying a bias the the piezo, I could observe the signal grow and shrink (grows when wire is closer to sample). Commented May 8, 2016 at 5:13

Normally when polarization is done a dot of paint or a scratch or some similar sign is written on one surface of piezoceramics to indicate positive polarity. To view the deformation probably the best system it shine a light ray on the surface and see how the reflected ray goes. Another system could be to mechanically couple a reference bimorph to unit under test and measure the output current from known unit when you give a drive to unit under test.

Another method is to apply a little pressure on the disk and observe the generated voltage on a scope.

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.

I have purchased specific piezo-ceramic brass disks in quantity. Typically, a batch is mostly polarized the same. :) However, some may be reversed. You can test by flexing the edge of the disk and observing the direction of voltage change. In my application, I allowed for either polarity by selective "strapping", wires are added to connect in either direction.

Simply connect to a multimeter set to Volts dc. Press on the centre of the disc (with a pencil eraser). The readout will go +ve or -ve very quickly (and then probably reverse) so watch the +/- sign NOT the actual volts. You need to notice the initial response not subsequent changes.

You will need to change the Vdc scale for best results

The easiest way is to hook up a digital multi meter (DMM) to it. Piezoelectric materials produce forward EMF, which means, below resonance, the generated voltage is opposite to the applied, resulting in the series resonant point (low impedance) in a crystal's response (the high imp. parallel res. point just above is due to the phase of the vibration reversing, turning the forward EMF into back EMF). Knowing that the generated EMF is opposite to the applied, just hook up a DMM to it. Suck on the hole to pull the diaphragm forward. If you get a negative pulse, you need a positive voltage to drive it forward. If you don't want to get that up close and personal, press it back with a cotton bud and you will get a positive pulse if a positive drives it forward.