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I want to regulate the input current into an ultrasound transducer (40 kHz, 50W). The signal is being generated by PCB. I read that regular potentiometers are usually employed up to a power of 1 Watt. I have found potentiometers that could be used, but they seem a bit bulky and expensive (ca. 80 EUR). Is there an alternative to a standard potentiometer to regulate the current up to 50W?

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    \$\begingroup\$ Usually one feeds the signal to a low cost potentiometer, then amplifies it as needed. 1 watt is about the limit for potentiometers that have reasonable cost. \$\endgroup\$ – Sparky256 Feb 6 '18 at 8:10
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    \$\begingroup\$ Say what now? Seems like this is an X-Y problem. \$\endgroup\$ – Trevor_G Feb 13 '18 at 18:31
  • \$\begingroup\$ If one has 30 dB power gain you can use a 0.5W pot. \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Feb 14 '18 at 0:18
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Don't try to scale down the 50 W signal coming out of the driver. Instead, control the driver circuit to produce a lower amplitude signal.

This is the same concept as a volume control for audio. There isn't some big fat pot between the power amp output and each speaker. Instead, the volume control adjusts the amplitude of the low power signal going into the amplifier.

Show the power driver, and how it is controlled. From that we can probably recommend ways to cause it to make less power. Without those details, there is little to say.

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You could use a MOSFET as a Voltage Controlled Resistor, as long as you use a large heat sink and perhaps a fan.

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Depending on the impedance of the ultrasonic transducer, I have in the past used a signal generator and an audio power amplifier. Some of them do go beyond 40kHz

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  • \$\begingroup\$ Thanks for the answer. Do you have information on the specs/data sheet of the amplifier? I have a signal generator able to generate well above 40 kHz signals. Is it safe to assume that the limiting factor (regarding the frequency) is the audio power amplifier in this case? \$\endgroup\$ – Keine Feb 14 '18 at 7:13
  • \$\begingroup\$ @Keine You can google for an audio amp with 40kHz (or above) bandwidth. The limiting factor is the impedance of the transducer. Audio amps are rated with loads between 4 and 8 Ohms. Also, in case you are thinking of it, do NOT use an audio tweeter. Their power ratingas are not what they seem. \$\endgroup\$ – Dirk Bruere Feb 14 '18 at 8:37
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Piezo devices are dielectric resonators and as such the current is proportion to voltage and frequency.

If you want to regulate the power, the most convenient way is to have a voltage controlled Vcc to the driver which can be done with a low power pot to an adjustable SMPS or LDO in the feedback ratio.

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