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Need help getting 2.4MHz piezo to oscillate at its resonate frequency
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MatCat:
The Piezo is supposed to be roughly 2 ohms at resonate frequency.
T3sl4co1l:

--- Quote from: MatCat on October 28, 2018, 10:08:22 pm ---Another circuit that is often used in actual products is this one, though I don't really fully understand it:



--- End quote ---

And you mentioned elsewhere (in chat) the expected bandwidth is about 4%, i.e., a Q of 25, and this was the datasheet: http://www.dongiltech.co.kr/pdf/DUNU-Series.pdf

Referring to this circuit, it seems to oscillate better (in SPICE) with L1 small (~5n), C1 much larger (~2.2nF), and... C2 controls output power, roughly speaking?

I'm finding better results with the large inductor in the collector circuit, as a usual common-emitter Pierce circuit has it.

I don't like the E-B diode, it supplies excess base current making the V-I dependency hard to control.  But it's also necessary to prevent squegging, as given.

You're definitely not going to run it at 50V.  2 ohms and 25W implies 7V RMS.  Probably 10-12V supply will do.

These values seem to work nicely:



Tim
MatCat:

--- Quote from: T3sl4co1l on November 08, 2018, 11:13:11 am ---
--- Quote from: MatCat on October 28, 2018, 10:08:22 pm ---Another circuit that is often used in actual products is this one, though I don't really fully understand it:



--- End quote ---

And you mentioned elsewhere (in chat) the expected bandwidth is about 4%, i.e., a Q of 25, and this was the datasheet: http://www.dongiltech.co.kr/pdf/DUNU-Series.pdf

Referring to this circuit, it seems to oscillate better (in SPICE) with L1 small (~5n), C1 much larger (~2.2nF), and... C2 controls output power, roughly speaking?

I'm finding better results with the large inductor in the collector circuit, as a usual common-emitter Pierce circuit has it.

I don't like the E-B diode, it supplies excess base current making the V-I dependency hard to control.  But it's also necessary to prevent squegging, as given.

You're definitely not going to run it at 50V.  2 ohms and 25W implies 7V RMS.  Probably 10-12V supply will do.

These values seem to work nicely:



Tim

--- End quote ---
Interesting, I will have to try your version of the circuit out.  This specific piezo is specified to run at 24V.  How much current is that 1 ohm resistor seeing? 
T3sl4co1l:
Not much.



That comes to about... uhhh... 0.39A RMS.

These also show reasonable efficiency, consistent with a class C amp/osc.  YMMV with a real BU406 or whatever.  Probably I'd recommend MJE15012 or another switching or fast-ish amp type if I were designing this.  Nudging R and C values back and forth has pretty strong impacts on power output, distortion and efficiency.  Probably piezo load has a strong effect as well.

Tim
leeatljs:
The cheap way of driving these 'atomising' piezo's is to drive them from a fairly accurate oscillator.  The correct way is to drive them from a waveform which is in phase with the current through the element.
An ultrasonic cleaner works this way, but at a lower frequency than you want.
'greatScott' has a video wich you might find interesing

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