Transformer vs. H-Bridge Driver for Piezoelectric Transducer

Hello all. I would like to receive clarification and opinions on driving bolt clamped (Langevin) piezoelectric transducers. These transducers are most abundantly found in ultrasonic cleaning equipment. The same transducers are also used in industrial tank, reservoir, and boat hull anti-fouling applications (this latter application is what I'm interested in).

I have acquired transducers spec'd at a 40khz resonate frequency and 50w operating power consumption. The specifications also state that at resonance, this transducer presents a 10-20 ohm impedance (this is pretty much the average resonant impedance for most of these transducers). In determination of the drive voltage required, and conservatively using the 20 ohm upper-end of the rated resonate impedance, Watts Law states that the voltage required to drive the transducer at 50w is just under 32v. And this is where my concerns begin...

I had originally determined to step-up the voltage to 36v and then use an H-Bridge gate driver IC to drive a transducer at resonance with a 40khz square wave.

However, after much time spent on Google, I'm beginning to second guess this approach and am more confused than ever. Nearly all of the circuits I've found, and all the applications I've found (for both ultrasonic cleaners and anti-fouling) are using transformers to directly drive a transducer at (as quoted from one source) "250VAC (about 700V peak-to-peak)".

Forgive me but I have no experience with piezoelectric transducers or AC circuits in general, and it frankly seems I'm overlooking something. I've calculated 32v to drive a 50w 20ohm impedance load, but in most applications, the same or similar transducers with equally matched specifications are driven at nearly 10x that voltage.

What am I missing?

gabenix: What am I missing?

You have failed to cite the specification sheet for the transducers.

Since they are in fact, ultrasonic, then you obviously could not drive them from the 250 VAC mains - because it is 50 Hz, definitely not ultrasonic. :grinning:

If you propose to drive them with a transformer, the transformer would have to be specially made for the job (i.e., 40 k*H*z) with the correct turns ratio and a centre-tapped primary to be driven "push-pull" with two FETs.

I don’t know… Your calculations are correct* (for a resistive load) and if the manufacturer’s specs say 10-20 Ohm’s I’d believe that.

The tricky part could be the complex impedance of the load. A piezo transducer has a large capacitive component and pure capacitance doesn’t dissipate any power. i.e. with 20 Ohms of pure capacitive reactance (or any pure capacitive reactance) the voltage & current would be out-of-phase and no power would be consumed by the transducer. But obviously there is a resistive component to the transducer because it has to consume power to convert electrical energy into mechanical energy to “do work”.

…And, I don’t know where you’d find the special high-frequency transformer with the “right” impedance ratio.

  • It can get tricky with sine & square waves but 36V into a bridge driver is +36V half the time and -36V half the time so it’s the same amount of power/energy as 36VDC and the straightforward V2/R calculation works (ignoring the slight voltage-loss across the bridge driver and assuming a resistive load).

P.S.
I have an ultrasonic “mist maker”. It’s sealed so I don’t know anything about the electronics but it runs from a 12VDC (maybe 10A?) power supply so there is no “direct” connection to the power line.

DVDdoug: A piezo transducer has a large capacitive component and pure capacitance doesn't dissipate any power.

This was specified to be at resonance.

Mechanical resonance doesn't have to match LC resonance. So the resonant impedance could easily be highly reactive (in fact piezo's are basically the same material as certain classes of ceramic capacitors).

This is a useful search I think: https://www.google.com/search?q=piezo+impedance+resonance&client=firefox-b&tbm=isch&tbo=u&source=univ&sa=X&ved=2ahUKEwj4wo6qr_jfAhUBQhUIHaANAX0QsAR6BAgBEAE&biw=1284&bih=762

Thanks for the replies so far. Let me address them specifically.

Paul__B: There is no datasheet available. These transducers fall off the boat from China en-masse.

The specs I do have are as follows: 40khz / 50w / 4,100pF Capacitance / 10-20 ohm impedance ( at resonance )

DVDdoug & MarkT: I'm gathering that you both are eluding to a point that concludes that the 10-20 ohm impedance will not be realized in real-world conditions. That it will always appear higher considering the attachment of the transducer to another body ( a boat hull or reservoir tank ) will attenuate the transducers reactivity in general, and or, modify the actual resonant frequency of the circuit to a range of higher internal impedance of the transducer. Is this correct?

gabenix:
Paul__B: There is no datasheet available. These transducers fall off the boat from China en-masse.

Ah well, … :sunglasses:

Hi,
Check this link, It is to Silicon Chip Magazine here in Australia, it is an archive of images from thier Ultrasonic Anti-Fouling Device.
It is only images, but shows schematic and how to wind the transformer.
It uses a PIC12F765 and its code may be available if you go digging, otherwise check the Silicon Chip download site.

http://archive.siliconchip.com.au/cms/gallery/article.html?a=112107&i=10

Sample of image;
112107_17mg (1).jpg

Tom… :slight_smile:

Hey TomGeorge, that's a great find and really helpful! I found a great blog post where someone had built this circuit but had added support for an additional transducer. They go into some brief detail of the programmatic operation explaining that the drive is done in short pulses. You can see the link below.

Part 1: http://nordkyndesign.com/ultrasonic-antifouling-system/ Part 2: http://nordkyndesign.com/ultrasonic-antifouling-system-part-2-fighting-algae-growth/

This design was also made into a product on a website "Jaycar" with specifications listed as:

• 12VDC • Suitable for power or sail • Works with aluminium and fibreglass boats (not suitable for timber, ferro cement or fibreglass foam sandwich construction) • Could be powered by a solar panel/wind generator • Output frequency range: 19.08 - 41.66kHz in 14 bands with 200ns steps. • Frequency sweep in each band: 12 frequencies ranging from 80Hz spacings at 20kHz to 344Hz steps at 40kHz • Signal burst period: 600ms at 20kHz, 300ms at 40kHz (12000 cycles/ burst) • Output drive. 250VAC • Supply Voltage. 11.5V to 16V maximum • Current drain: 220mA average at 12V driving a 3nF load

All of this still has me curious. The impedance of the transducers are highly variable depending on frequency and resonance characteristics, basically driving the transducer at the specified wide range of frequencies would require differing levels of voltage to achieve the same output energy. I notice that there doesn't seem to be any feedback or current control in this schematic - it's sort of a "spray and pray" approach. It will be under powered at certain frequencies and perhaps overpowered at other very low impedance resonate bands. And maybe for this application, at these low impedance resonance frequencies, very high voltage pulses exceeding the rated wattage doesn't matter given the low duty cycle. But, ultrasonic cleaning equipment appears to operate at the targeted resonant frequency continuously at these high voltages - so things still do not seem to add up completely...

I do want to target alternate resonant frequencies besides the primary 40khz. But I want to take a much more exacting approach that utilizes an initial programmatic calibration where the IC sweeps each transducer between 20khz and 70khz while sensing current draw, then identifies resonate areas that meet designated criteria. The IC would then utilize an algorithm to determine operation in only these specific bands of resonance working in duty cycles that will keep overall power consumption within specifications while also permitting external power control settings. By doing so, the transducer would be driven most optimally in only the frequencies it responds to most efficiently while allowing for precise user settable power consumption control - perhaps 25w during the day and 10w at night.

The programming part is a snap, that's my vocation. The right kind of driver circuit is where I'm struggling, but the evidence for transformer drive is becoming overwhelming. Questions...

How is transformer design/specification related to load and AC frequency? So my transformer would operate in a frequency band between 20khz - 70khz and my load would be a piezoelectric transducer with 4100pF capacitance and would need to drive 250VAC(?). Would this transformer need to be custom made as Paul__B hints at or can I find a ready made part, maybe a SMPS transformer?

quote: How is transformer design/specification related to load and AC frequency? So my transformer would operate in a frequency band between 20khz - 70khz and my load would be a piezoelectric transducer with 4100pF capacitance and would need to drive 250VAC(?). Would this transformer need to be custom made as Paul__B hints at or can I find a ready made part, maybe a SMPS transformer? unquote.

Take a look at the old TV flyback transformers. They operate at 15 kHz and use ferrite cores. Yours will need the same. But you need to research the correct "mix" of material for those frequencies.

Paul

can I find a ready made part, maybe a SMPS transformer?

That is a good idea, but you might still need to rewind it.

I would check out a couple of scrounged SMPS power supplies in operation. A 240 VAC to 12 - 25VDC supply that operates at 50 kHz might yield a transformer that could be salvaged and used (at least the HV primary).

Paul_KD7HB & jremington, thank you for your input! I took a look through SMPS transformers on Digikey and found one potential prospect that may work for me. Digikey has no filters or columns referencing the primary/secondary winding current, so this was a bit of an arduous process having to open every datasheet.

The single prospect I found was the Wurth Electronic 750313417 Flyback, 6:1 ratio, 400uH @10kHz, operation range of 70-130kHz, and spec'd at 3.43A @ 19v on the secondary winding according to the datasheet.

Datasheet here: https://katalog.we-online.de/icref/datasheet/750313417.pdf

This would be less of a step-up than the custom wound option in TomGeorge's post, but I suppose driving it with a full H-bridge (12+/12-) would compensate for this in comparison to the driver circuit referenced in the TomGeorge post???

How would this transformer's inductance (400uH @ 10kHz) affect a driving circuit? Will the driver side need a LC tuning/balancing component, and if so, how does that work with the need to drive in a fairly wide frequency range (20kHz - 70kHz)?

Is the "primary" nomenclature used in transformer descriptions always considered the higher voltage side, regardless of the driver/source/step-up/step-down application?

Is there any obvious reason this transformer won't work for this application?

Thanks again for your input and help!

"Primary" refers to the power input side of a transformer, but of course they are interchangeable.

AC voltage output = (turns ratio)*AC voltage input.

With 6:1 and 24VAC in, you would get about 144 VAC out.

gabenix:
Paul_KD7HB & jremington, thank you for your input! I took a look through SMPS transformers on Digikey and found one potential prospect that may work for me. Digikey has no filters or columns referencing the primary/secondary winding current, so this was a bit of an arduous process having to open every datasheet.

The single prospect I found was the Wurth Electronic 750313417 Flyback, 6:1 ratio, 400uH @10kHz, operation range of 70-130kHz, and spec’d at 3.43A @ 19v on the secondary winding according to the datasheet.

Datasheet here: https://katalog.we-online.de/icref/datasheet/750313417.pdf

Is there any obvious reason this transformer won’t work for this application?

It looks reasonable - so long as you don’t saturate/overload the core it ought to handle the power and frequencies involved. I suspect that 70–130kHz range is for optimal efficiency, it will work at lower
efficiency over a wider frequency range. At lower frequencies derate the power handling in proportion.

Hi Gabenix, did you get this anti fouling system working? It cleams that it works over the fulll range from 20Khz till 40 Khz.

Yes Lammens, I solved all issues encountered with this project. You do indeed need drive the transducers at high voltages (at least 400v peak-to-peak) using a transformer. Unfortunately there are no commercially available transformers that will provide the required step-up and current rating within the target frequency band. You will need to wind custom a transformer or have some manufactured.

Hi Gabenix, I used the transformer as given in the “Silicon Chip Magazine” (from China) but can not get it working over the full range of frequencies with the transducer connected. So please be so kind to tell me how you made your transformer. Regards.

I'm confused as to what you mean when you say it is not working over the "full range".

Also, are you saying that you "purchased" a completed transformer, OR did you wind a custom transformer according to the directions given in Silicone Magazine? If you purchased a transformer, can you post a link or part#?

Are you saying that it is not working "efficiently" at lower frequency (ie. 20KHz), OR is it not permitting enough current at higher frequency (ie. 40kHz)?

How are you driving it and controlling frequency output to the transformer?

first: this is the link for the transformer https://www.aliexpress.com/item/32557223885.html?spm=a2g0s.9042311.0.0.58574c4dBtGufU What I mean with the full range is that the transducer only resonate around the 20Khz and a litle bit at 40Khz. Alle the frequencies in between give no, or verry little transducer output.(what is normal for this kind of transducers). So what is the use for a system with a full range of frequencies between 20 and 40 Khz? without the transducer connected the cicuit looks working more-or-less fine, but with the transducer connected it works only at the resonance frequency. As far as I understand this kind of anti-fouling will not work. There are tests done with poor results. So where I´m working on is a system with multy transducers connected with resonance frequencies of around 20, 30, 40 and 60Khz.

Okay I understand what you are saying...

Your transformer is probably functioning properly. The results you are seeing is because the transducer will only have one primary resonate frequency - you state this in your post. Transducers have other minor harmonic (fractional) frequencies where they will resonate at lessor amplitudes, however there is only a couple between 20KHz and 40KHz (maybe around 27KHz and 33KHz) for a 40KHz transducer.

Concerning effectiveness, the only data I've found within published scientific papers are for function in the low 20KHz range for use repelling barnacles, probably as an acoustic deterrent. Operation at higher frequency for algae/biofilm is mostly speculation.

I believe these systems sweep through the other frequencies for no valuable reason except to pass time because driving at resonance continuously would require too much current and power...

  • 20KHz deters barnacles
  • 40KHz (with 40Khz transducer) may provide enough mechanical surface motion to shed/repel micro-organisms.
  • Everything else in-between just passes time at lower power output

Good luck with your project!