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I am trying to understand how my humidifiers' circuit driving ultrasonic atomizer membrane works. Here is how the board looks: enter image description here

I couldn't find the specifications for the atomizer membrane, but I think the model (as written on its back) is JYC-9201.

Here is the schematic of the circuit I created (only the atomizer driver part): enter image description here

SW2 is on/off humifier button, enabling/disabling all the functionality of the humidifer. SW3 disables everything except RED led, indicating not sufficient amount of water in the tank.

The humifier's atomizer is enabled if and only if both SW2 and SW3 are conducting. When the atomizer is not enabled, CH2-4 are at the GND level, CH1 is always at +34V.

When the atomizer is enabled, with oscilloscope I found that all the CH2-4 are at equal voltage level at any time, but oscillating:

enter image description here

CH1 is at stable +34V.

Here is an example driver circuit I found for a piezoelectric membrane: link. I can understand how it works. AC for the membrane SP1 is generated by charging (when the mosfet is not conducting) and discharing (when it is conducting) the capacitor periodically.

In my case, since CH3 and CH4 are at equal voltage level, it means that the voltage across the capacitor C5 is 0V. Does it mean that the capacitor is never charged? In that case, how can current flow through the membrane, without charging the capacitor? Even if the capacitor C5 could charge, how would it discharge? I see the only discharge path - back through the membrane (which is desired, producing AC) and then through the +34V, which, I think, is not possible, because the current won't flow from lower voltage (at the capacitor, never seen voltage higher +34V there) to higher voltage.

How does this circuit produces AC, and regulates frequency of alternations?

EDIT:

When the atomizer is off: enter image description here

When the atomizer is on: enter image description here enter image description here enter image description here enter image description here Oscillogram in Rigol's WFM file: link

It seems like CH3 is at higher voltage than I previously thought. The voltage of CH3 is about 26.4V higher than CH2 and CH4. CH2 and CH4 are at equal voltage level at any time. The phase of all CH2-4 seems to be the same at any time.

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Your 3 tracks CH2-CH3 may have a similar voltage, but it is hard to see if there is a phase shift between them.

A better plot, containing voltage and time scales would be helpful.

I was unable to establish oscillation in the simulator until I placed the load (C5 and the piezo) on the other side of C4. Please verify your schematic.

The inductor values are just estimated from the photo.

schematic

simulate this circuit – Schematic created using CircuitLab

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  • \$\begingroup\$ If you model the piezo as a crystal equivalent circuit, you may find it works better in the given position; this is a standard tuned-base Pierce oscillator circuit. \$\endgroup\$ Commented Nov 4, 2024 at 11:19
  • \$\begingroup\$ I edited the post and added more plots and oscillogram (in rigol's wfm file). I verified the circuit, and I confirm that my schematic is correct. C5 should be directly connected to R1, and C5 is not directly connected to C2. \$\endgroup\$ Commented Nov 4, 2024 at 11:22

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