How to drive an ultrasonic piezo transducer? - Page 1
Forum section: electronics / beginners

author: Faranight
author_rank: Unknown
timestamp: March 16, 2023, 07:03:15 am
content: Heyas. I recently bought a few of these ultrasonic piezo transducersKemo L10, and I'm wondering how to drive them correctly. According to the product page, their rated voltage is 16Vp-p. I tried measuring the resistance, and it was in the 60+ megaohm range that the instrument couldn't measure, so clearly it is not a coil. The capacitance measured at about 130nF.I'd like to feed a 50% duty-cycle square waveform (generated by a uC like ATtiny85) in a 20 kHz to 40 kHz linear sweep to this element, but I'm unsure what driving circuitry to use. For classic coil speakers I'd use a series capacitor and an amplifier, but this time the transducer is not a coil, but a capacitor. Would using an H-bridge configuration with +8.0V PSU suffice? The h-bridge will be driven so that the polarity inverts every half of a period. Yes, I know I could have also bought the driver that this manufacturer sells, but I wanted to make my own where I can control the frequency.Thanks in advance.

author: james_s
author_rank: Super Contributor
timestamp: March 16, 2023, 07:06:11 am
content: Of course it's not a coil, it's piezoelectric. You can drive it with an audio amplifier, one of the small modules you can get for a dollar or two should work. Or you could drive it with a single transistor if you really just need a square wave.

author: Faranight
author_rank: Unknown
timestamp: March 16, 2023, 08:25:56 am
content: Sorry, my first time working with piezo elements, and I'm confused about a few things.Could you please elaborate what you mean by driving it with a transistor? Because I'm not sure I fully understand or agree with your suggestion. Please correct me, if I'm wrong, but... The device rated voltage is 16Vpp. If I power it with 8VDC and use a square waveform source on a transistor/mosfet to switch it on/off, doesn't that mean that the voltage on the piezo element will only reach from 0V to 8V (8Vpp) and thus result in only 50% efficiency? If I use an H-bridge instead (like I originally intended), then the polarity will invert, and the element will see a full range +8V to -8V (16Vpp). Or should I use a mosfet in series with the element on a 16VDC supply to get 16Vp-p?

author: tszaboo
author_rank: Super Contributor
timestamp: March 16, 2023, 08:54:25 am
content: You typically drive it with a square wave, and amplified. Look up the model of it, it acts like a capacitor, so a lot of peak current will go into it. If you want the easy way out, then TI makes piezo drivers. Otherwise, MOSFETs or even FET drivers make quite good piezo drivers. You don't necessarily need to reverse the polarity. Also the drive strength depends on your application, maybe you can get away with less voltage.

author: james_s
author_rank: Super Contributor
timestamp: March 16, 2023, 08:24:05 pm
content: How loud do you need it to be? 16Vpp is the maximum, it will make sound on a lower voltage than that. If you want it to be as loud as possible then yes, the H-bridge will work. Piezo beepers are often driven with a single transistor configured as a low side switch, the sort of thing used in alarm clocks and microwave ovens.

author: Marco
author_rank: Super Contributor
timestamp: March 16, 2023, 10:36:30 pm
content: Quote from: Faranight on March 16, 2023, 08:25:56 amThis pagehas two examples of single transistor driving. With a parallel resistor you can drive it upto the frequency where the resistor can sufficiently discharge the piezo capacitance, with an inductor you can only really drive it at resonance. The resonance has the advantage that you can use a low voltage power supply, it will ring up the voltage.How you drive it really depends on what you want to accomplish, is it just a noise making indicator or is it used as a tweeter?

author: TimFox
author_rank: Super Contributor
timestamp: March 16, 2023, 10:46:47 pm
content: I looked at the device datasheet, and found it left out an important parameter: the device capacitance at 1 kHz or some other frequency or as a function of frequency.Do you own one yet, and can you measure the capacitance?

author: Terry Bites
author_rank: Super Contributor
timestamp: March 17, 2023, 07:52:57 pm
content: Single ended drive won't work. You can't just drive it with a transistor.I can well believe it looks like 130n. The actual impedance is gahstly, it has RL and C components.You're probably right to consider the H-bridge. If you want to generate the max output from a 8V source, then thats the way to go.You'll get 16Vpp on a good day. Power increases with the square of the voltage so you'll get more like 30% of the 30V power @16V.You could use a boost converter to get 15V and get the full 30V pp from a full bridge or you could boost to 30V and use half brige (AKA push-pull) to drive the transducer.One easy way... use a "logic level" mosfet to drive a complementary pair of Darlington transistors.

author: BillyO
author_rank: Super Contributor
timestamp: March 17, 2023, 08:09:38 pm
content: Quote from: Terry Bites on March 17, 2023, 07:52:57 pmYou can, it just won't be that great, but it can be and has been done.If you don't need it to be ear shatteringly loud you can drive one with a decent CMOS logic gate, like a 74ACXXX series. But that is not "single ended".

author: radiolistener
author_rank: Super Contributor
timestamp: March 18, 2023, 12:20:37 pm
content: Quote from: Faranight on March 16, 2023, 08:25:56 amif you connect it directly to transistor without reactive elements, it really cannot produce Voltage above level which is supplied to the circuit from a power supply. But your circuit can use transformer to trasform low impedance of transistor output into high impedance of ultrasonic transducer, the output of transformer can easily exceed 8 V which is consumed by your circuit from power supply. Also your circuit can use DC/DC converters to produce higher voltage for your transistor in order to allow it to produce higher Voltage swing.Transformer which is used to match driver output impedance with a speaker impedance is pretty common solution. For example it was often used for a tube receivers, where tube has high output impedance but speaker has very low impedance. In your case it's the other way around, you're needs the same transformer, but it should be connected with low impedance coil to the driver and high impedance coil to the ultrasonic transducer.

author: radiolistener
author_rank: Super Contributor
timestamp: March 18, 2023, 12:30:22 pm
content: regarding to your ultrasonic transducer, I have the same, bought it as a toy some time ago before war. At 1 kHz its not too loud, but at 4 kHz it has pretty good sound level even when feed with 8 Vpp

author: Faranight
author_rank: Unknown
timestamp: March 20, 2023, 04:27:27 pm
content: Hello, thank you for the answers and suggestions. I learned some new things here i.e. that the piezo is indeed a capacitive load.Quote from: james_s on March 16, 2023, 08:24:05 pmIdeally, as loud as possible per applied voltage, but without causing harm to the device. Thewebsiteis rather ambiguous, it says that the rated voltage is 16Vp-p, and its "Max. rated long power" is 30Vp-p. Am I correct to assume that the 16Vp-p is the recommended voltage, and the 30Vp-p is the absolute maximum rating?Quote from: Marco on March 16, 2023, 10:36:30 pmIt's going to be an ultrasonic noise emitter of programmable frequency. A square-waveform will be generated on a microcontroller and somehow fed into the driving circuit.Quote from: TimFox on March 16, 2023, 10:46:47 pmAs I've said, the capacitance was measured around 130nF. It was measured by EEVblog BM786, and I have no idea what frequency the said multimeter uses to measure capacitance. I do not have any information about it at different frequencies either. I'll see, if I can get the manufacturer to spill the beans.Thanks again for all the comments, I'll begin experimenting now. Probably the best way would be to use an audible frequency before shifting it into the ultrasonic range, so I can determine the loudness with each method. Also,this interesting ICwas posted in a similar topic that I found on this forum. Supposedly it can drive the said piezo speaker, and has a built-in boost converter to generate voltages higher than its 5VDC supply. Gonna order a few to try them out.Cheers!

author: AndyC_772
author_rank: Super Contributor
timestamp: March 20, 2023, 04:52:52 pm
content: Bear in mind that the electrical characteristics of a piezo transducer are *completely* different when it's oscillating at resonance, compared to when it's driven at some other frequency.It's not a hi-fi speaker; the frequency response is nowhere near flat, even at or around the resonant peak.

author: Faranight
author_rank: Unknown
timestamp: March 20, 2023, 05:26:35 pm
content: They do offer afrequency response charton their website.Also, here is their reply regarding the capacitance question:QuoteEDIT: Is there any way that a half-bridge driver could be repurposed to drive the piezo transducer?Because these drivers are meant to drive two n-channel mosfets, and their gates are basically capacitive loads, just like the piezo element.EDIT 2: Seems like they can.

author: james_s
author_rank: Super Contributor
timestamp: March 20, 2023, 09:19:51 pm
content: I don't see any reason it wouldn't work, a lot of those half bridge drivers can push significant current too.

author: radiolistener
author_rank: Super Contributor
timestamp: March 21, 2023, 12:15:37 pm
content: just add 1:2 voltage transformer and you will be able to put about 16 Volts into a piezo transducer with transistor powered from 8 Volt.Such approach is used very often in Chinese dog anti-barking device. Such device usually has a simple 24 kHz generator with transistor driver powered from 9 V and ultrasonic transducer which requires about 60 V amplitude is connected through step-up transformer. In such way a high impedance transducer is matched with low impedance transistor output.