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I have some 40kHz ultrasonic transducers (the same type as which you find in the common Arduino ultrasonic distance sensors.) All information comparable to a datasheet I got was:

enter image description here

I don't get how to read it or how to calculate the current drawn, which I need to know when making or buying a amplifier. It does show the capacitance at 1kHz. I guess they use that as a worst case scenario or such, however they don't show graphs showing how it behaves at different frequency ranges, at different voltage ranges, and above all it doesn't seem to show units.

Since it is a very commonly used ultrasonic transducer perhaps someone knows how to calculate the maximum current draw at a specific frequency and voltage, and/or a better datasheet or information or a close enough estimation of what current it will draw or what unit the value is in.

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    \$\begingroup\$ Use the capacitance to calculate the impedance at the frequency you're driving it at. Of course, the capacitance they give you is only at 1kHz so it will be different at other frequencies but at least you can ballpark it. You can't do much more without curves. You can also measure it with an LCR meter if it is in front of you. \$\endgroup\$
    – DKNguyen
    May 14, 2022 at 17:28
  • \$\begingroup\$ @DKNguyen Hi, I found this site: allaboutcircuits.com/tools/capacitor-impedance-calculator based on what you said, so I can just use that formula on that page? XC=1/2πfrequencyCapacitance. \$\endgroup\$
    – TeD van Loon
    May 14, 2022 at 19:21
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    \$\begingroup\$ Yes, that is the equation. Note that it is actually an imaginary number. It's not a real impedance like Ohms for resistance. There is normally a j or i in there. But for the purposes you mentioned so far it doesn't yet matter. Also note that it is for a single frequency component (a sine-wave). So if you put a square wave through it, it will be a sum of the currents for all the sine waves that are required to produce the square wave. \$\endgroup\$
    – DKNguyen
    May 14, 2022 at 19:22
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    \$\begingroup\$ Oh I guess the unit is not listed. How sloppy. That should be pF. It's definitely not nF, let alone uF or F. \$\endgroup\$
    – DKNguyen
    May 14, 2022 at 19:45
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    \$\begingroup\$ You just do the same thing with 1st, 3rd, 5th, 7th, 9th, harmonics and add up the currents. Square waves are odd sinusoidal harmonics infinite. Look up a square wave. I would stop at 5th. \$\endgroup\$
    – DKNguyen
    May 14, 2022 at 20:05

1 Answer 1

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The capacitance given at 1 kHz (far from resonant frequency) is just an electrical capacitance. It is used to calculate matching network to transducer (usually inductor, connected in series). In simplest case, electrical resonant frequency of this matching inductor, combined with transducer capacitance, should be a couple kHz higher than working frequency (mechanical resonance).

Ultrasonic transducers work at resonant frequency (40 kHz for this one), at which, due to mechanical resonance, their impedance changes greatly. Best solution, in the absence of information, is to measure its impedance while powered from any suitable generator, at low voltage.

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