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I can't remember the source of this image, but I found a similar image in an Ekulit catalog. One recommended circuit shows just a resistor in series with the piezo buzzer. But this one adds a diode. What's its function?

enter image description here

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It's not so much about external sounds but mechanical vibrations. Piezo transducers are capable of quite large voltages. There may be very little current behind them, but that can still punch thru the insulation of a FET gate, for example. The voltage is high enough that a small piezo can actually draw a spark if hit right. This is how the ignitors in many outdoor grill starters work, for example.

Sound vibrations coming thru the air don't cause enough mechanical vibration to generate such high voltages, so that's not really the problem. However, placing the device on a table or dropping it on the floor can produce a short sharp mechanical impulse that can result in high voltage.

However, that's not the reason for the diode. Presumably the IC has protection diodes on it's pin, which should work well enough with the series resistance to clip the voltage. After all, the circuit shown expects this to work in one direction, so it doesn't add up that they think this protection is needed in the other direction. The polarity of external mechanical shocks can't be predicted.

The reason for the diode is that the piezo looks partially inductive to the rest of the circuit. Think what would happen if you replaced the piezo with a inductor in the circuit shown. When the IC output is high, the inductor charges up. Then what happens if the output goes low or high impedance? The inductor current could drive the IC pin voltage negative. So in short, it's a inductive kickback catching diode.

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    \$\begingroup\$ thank you. Do ignitors produce this high voltage directly? I always thought it used some kind of autotransformer to up-transform the voltage? And I thought a piezo was almost purely capacitive. \$\endgroup\$ Jun 14 '11 at 17:45
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This Microchip discussion claims that the piezo, being a transducer, could potentially create a large voltage if exposed to a loud external sound. The resistor would limit any excessive current from a positive-going excursion, and the diode would clamp any negative-going excursion. Most likely that external diode should be a schottky.

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    \$\begingroup\$ it's not exactly a Microchip article, but rather a thread on a Microchip forum. Big difference. Anyway, I wonder how much of a problem this really is. Does a piezo buzzer really generate volts when used as a microphone? \$\endgroup\$
    – stevenvh
    Jun 14 '11 at 17:28
  • \$\begingroup\$ Yes, a piezo can generate pretty high voltages. Consider cases like: Thunder nearby Jets flying by User dropping the device Movie Night with awesome stereo system \$\endgroup\$
    – Jon Watte
    Mar 19 '16 at 20:15
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If I may dig up this old question;

Eventhough people and even manufacturers calling it a piezo buzzer, that specific type is probably an electromagnetic buzzer. The diode is to protect against the reactive current in the coil of the buzzer.

Here is the ref design from another example that I encountered recently here is the ref design from another example that I encountered recently

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    \$\begingroup\$ That seems most unlikely given the crystal symbol used in the schematic. Read the accepted answer for an explanation and you might find further information on piezoelectric inductance in the datasheets. \$\endgroup\$
    – Transistor
    Mar 18 '21 at 0:13
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    \$\begingroup\$ durandal - Hi, As required by this site rule, when we include something in an answer (e.g. photo, image or text) which isn't our own original work, we need to properly reference (cite) it. That image seems to have come from somewhere else, so can you please edit your answer and add a link back to the specific original web page, to comply with that rule? Thanks. :-) (There are more site rules and guidelines in the tour and help center.) \$\endgroup\$
    – SamGibson
    Mar 18 '21 at 0:29
  • \$\begingroup\$ @sam that image comes directly out of one of my schematics, which I snapshotted for reference from one of the many (public) chinese datasheets on buzzers from LCSC.com. Feel free to browse them as I did to find it again for a link when I posted, to no avail. The snapshot has no link at least nor is it referenced on the net. I'm sure rules give comfort but not everything has a link or one that can be provided. Shall I remove the image? \$\endgroup\$
    – durandal
    Mar 31 '21 at 17:40

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