Urgent! ! ! Kneel down and ask the great god to answer why the last resistor of the main circuit of this ultrasonic power supply is connected! ! I'll give you all the points. . .

The design of this diagram is really complicated, and the general idea should be to prevent interference (frequency bands other than working ultrasound):

At the initial stage of rectification, cf is a filter capacitor, generally 104, which has a large capacity.

Then use triode to stabilize the voltage.

Then the H-bridge symmetrically drives the transformer.

R2 R4, R3 R5 and the corresponding capacitors form series resonance, which should be the corresponding working frequency bands.

There is no drawing on the ground, so it can only be estimated that this D2 D3 D4 is to prevent reverse interference.

The obvious function of inductor is to filter and prevent the interference of working part on front-end voltage. It is estimated that this power supply also supplies power to other parts.

S and Sa are paired. One is to increase the current in parallel, and the other is to improve the stability. An inductor is connected in series between them, and their control positions are different.