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I have been working on a pulse generator circuit which will be used to drive an ultrasonic probe. In the circuit below, the 50Ω resistor is modeling the probe.

enter image description here

I have researched the piezoelectric effect, but different sources explain it in different ways. Some have said that they are activated by current, and others have said they are activated by AC.

Since I am using DC voltage pulses to control the transducer, I am wondering if this will operate as I require. PSPICE outputs the following graph for the circuit shown above, measured from the node above 50Ω resistor.

Am I correct in thinking that the piezoelectric crystals in the probe would only generate ultrasound during the change in voltage? Or would there be some ultrasound emitted during the high of 220V as well?

PSPICE Graph

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  • \$\begingroup\$ You should model the piezo by a capacitor instead. \$\endgroup\$ – Janka Nov 24 '18 at 23:17
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Piezoelectric crystals appear electrically as capacitors (since they basically consist of two electrodes surrounding a dielectric material. However, at their resonant frequency they behave closer to a pure resistance. In any case, as with any electrical impedance, applying a voltage to them will cause a current to flow. Similarly if you force a current through them, they will develop a voltage equal to the product of the current and their impedance. Usually, though, piezoelectric crystals are considered to be voltage driven because their reaction to electrical stimulation is easier to describe and measure in terms of voltage, rather than current. The current will vary considerably with frequency as the crystal's resonant frequency is approached. In any case, if you want to generate ultrasound you will need to drive the crystal with an AC signal preferably at or near its resonant frequency to get the best efficiency (its also easier to drive as, mentioned earlier, it appears close to a pure resistor at its resonant frequency). Driving it with the waveform you show is probably not a good idea as only the spectrum of the waveform near the crystal's resonant frequency will be useful. The fast rise and fall times mean high harmonic energy which could excite higher modes in the crystal and cause problems with your drive circuit as the harmonic currents could get large.

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Obviously some ultrasonic transmitters are available which need only DC or mains supply voltage. Internally those devices still must have an oscillator circuit and a speaker made for the operating frequency. The speaker can be based on piezoelectricity or it can be magnetostrictive.

Piezoelectricity means that a crystal is stretched or bended when it's under external electric field. The crystal is in the place of the insulator in a capacitor. AC voltage is applied to that capacitor.

Magetostrictive materials do the same in magnetic field. A piece of magnetostrictive material is inside a coil which has AC current.

Piezo devices also suck some AC current under AC voltage. The coil of a magnetostrictive device can have AC current only if there's AC voltage between the ends of the coil.

Both devices work best at certain frequencies where they most easily vibrate mechanically. It's the resonance.

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