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We have a Chinese made ultrasonic transducer, 120kHz approximately, 25mm x 25mm dia. It has three wires - red, green and white. The body is PVDF We have determined that the red and white are used to drive the crystal, but cannot understand what the green is for. The sparse data sheet had no explanation. The green wire seems to have no connection to anything that we can determine. The only thing I can think of is some kind of internal EMI shield

Does anybody know why it exists?

enter image description here

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    \$\begingroup\$ Potted, likely only the manufacturer would know. Suggest either sacrificing one (soaking in conformal remover might soften the potting compound) or ask the manufacturer (perhaps through google translate.) \$\endgroup\$
    – rdtsc
    Oct 1 at 11:50
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    \$\begingroup\$ It could be a feedback lead for output power control. As already commented only the manufacturer can say for sure \$\endgroup\$ Oct 12 at 10:10
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    \$\begingroup\$ Check with a VNA the response between each pair. It may also be that you have a dual element piezo with a common GND and two signals. \$\endgroup\$
    – Kelu124
    Nov 1 at 21:05
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Unless the green wire was the EMI shield, it might be feedbak. Below is a random image picked from Google search.

If your device is the same structure, I would drive two lines with audible frequency. The largest response (loud) set are Main & Ground, and the rest is Feedback.

enter image description here

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