SPICE model of a three wire piezo transducer? - Page 1
Forum section: electronics / projects

author: Zero999
author_rank: Super Contributor
timestamp: October 02, 2022, 09:55:26 pm
content: I stripped apart an old smoke alarm which kept going off and the thought occurred to me: how would I go about simulating the three wire piezo? I've read article about the two wire variety, but how do I simulate the third wire?The disc is about 35mm in diameter and I think is made of steel, possibly stainless as it's not that strongly magnetic.Another random question: what's the green coating on the back for? It looks like solder resist? I've not seen it before on a piezo.I don't have a use for this transducer, but I'm interested in how to model it, for educational purposes.

author: SiliconWizard
author_rank: Super Contributor
timestamp: October 02, 2022, 11:12:29 pm
content: They are called self-drive piezo transducers and are made so that a very simple circuit can make them self-oscillate at close to their resonant frequency.https://mspindy.com/technical-guides/transducers-self-drive-piezo/And yes, the green coating is solder mask. Not sure what it is for here - I've seen the same kind of piezo without that. Possibly those are meant to be mounted very close to/or right against a PCB, or otherwise in some metallic enclosure.As for a model, the Wikipedia article gives a model for a piezo transducer:https://en.wikipedia.org/wiki/Piezoelectric_sensorNot completely sure how to model those self-drive piezos though. If someone has a relevant model...

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 03, 2022, 02:29:35 am
content: You'd have to characterize it yourself to make an equivalent circuit. Generally speaking, it will be a capacitor in parallel with myriad series RLC networks, corresponding to various resonances of the disc; the impedance and Q of which are all over the place, as some resonances couple stronger than others to the parallel-plate electrodes.The equivalent circuit at the feedback electrode will be similar, and in general there will be coupling factors between respective sets of resonators. The resonances will not have the same coupling factor (or impedance) to the two electrodes, because they have different distributions with respect to the acoustic waves; but what's seen in one will largely be reflected in the other, give or take anything from simple coupling to some complicated network inbetween (equivalent to the propagation of that mode around the disc).Probably, none of the resonances are double-peaked (or more), but that would also be the most I would expect (that is, a double-peak (overcoupled) response corresponding to waves under each electrode, coupled by waves between them).The fundamental (1/2 wave "cup" shaped bending, I believe?) is likely the strongest (lowest impedance, widest bandwidth), and also the lowest frequency mode. Thus it's the most likely mode to develop from an oscillator.Tim

author: Zero999
author_rank: Super Contributor
timestamp: October 03, 2022, 08:51:05 pm
content: How can I model the coupling? Is it like an autotransformer going to the feedback, via a small capacitor to give resonance?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 03, 2022, 10:07:47 pm
content: More like some k factor (< 1/Q) between the inductors in the RLCs. But I'm not sure, again, you'll have to measure it.Tim

author: Zero999
author_rank: Super Contributor
timestamp: October 04, 2022, 04:38:07 pm
content: What's the best way to go about measuring it? The last time I measured the resonance of a piezo transducer. I took two of the same and used one as a transmitter and the other as a receiver. I connected the transmitter to a signal generator and the receiver to an oscilloscope and adjusted the frequency to find the peaks.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: October 04, 2022, 06:14:36 pm
content: You can probably find some papers/app notes on creating crystal models. Use those, but add a resistor in series with the bulk capacitance, as the piezos have more losses than crystals. The "tap" used for feedback can just be another inductor coupled to the inductor of the basic model. It could be its own resonant circuit, but it's physically small so its resonance may not be much of a factor.Some model info here-https://www.maximintegrated.com/en/design/technical-documents/app-notes/2/2127.htmlMaybe useful stuff-https://statek.com/wp-content/uploads/2018/03/tn32.pdfAlso useful-https://www.omicron-lab.com/fileadmin/assets/Bode_100/ApplicationNotes/Quartz/App_Note_Quartz_V2_1.pdf

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 04, 2022, 08:52:10 pm
content: In general, you solve for the elements of a two-port matrix, take your pick of type. The first step is measuring in a consistent impedance environment: with say 50 ohm source and load, measure the source reflectance and load transmittance. (A reflectance bridge is easy enough to make with a couple transformers, at lowish frequencies.) Then flip it around and do the same thing again. Preferably record phase and magnitude in the process. Measure at multiple frequencies in the range of interest, enough to plot a reasonable resonance, or several.Then, set up the same in simulation, and... start building a network that reproduces the results. An algorithm exists -- but you may find it faster to do it ad-hoc and by hand.For bonus points, or alternate methods: test output impedance by open/terminated/shorted; test also at different amplitudes (ferroelectrics are generally nonlinear), maybe test with different acoustic conditions as well, or if applicable. At the very least, test in conditions representative of the use case. So, if installed against a panel, maybe keep it there, etc.Tim