Here is the ultrasonic generator circuit I'm trying to understand:

Ultrasonic generator circuit

It is powered by USB and it produces 100V at 100kHz on the piezoelectric speaker BQ1.

Two mutually inverted PWM signals go from the micro-controller to the transformer T1.


1) Is this the simplest, cheapest circuit of the ultrasonic generator with similar output?

2) Why do we need C1-C2 capacitors?

3) Center tapped transformer T1 is used here, but is it ok to replace T1 with a transformer without center tap and to use single PWM signal as input?


4) Why C1?

5) If I'm powering the circuit from a separate power supply which is able to deliver several amps do I need R1, C1, C2?

  • \$\begingroup\$ R2/R1=10 and R1 limits current. T1 must be symmetrical centre tap to eliminate DC flux and N=step-up >20, Caps provide low ESR for low ripple V on supply due to high ripple current. \$\endgroup\$ Mar 28 '17 at 21:05

Yes, this is the simplest way to do it.

C1 and C2 are needed for supplying current to the transformer as it switches, as its supply is decoupled from the main +5V by a 100Ω resistor. This is done to avoid ripple coming from the transducer circuit leak into the main +5V.

The center-tapped transformer is needed to keep it simple, as you cannot deliver a DC component to the transformer core. Which you would if you used only one transistor. If you don't have a center-tapped transformer, you need a H-Bridge of four transistors, and correct triggering.

About the added questions:

C1 ist needed because C2 is a 220µF electrolytic cap and thus, its ability to deliver sharp spikes of inrush current into the transformer is fairly limited. The internal inductance of such caps is rather high. C1 is a low-inductance type cap which does the job until C2 can deliver.

If you use an external power supply for the transducer circuit, you have to leave out R1 (only connect GND), and you may leave out C2, but not C1. That is because the cable to your power supply has an inductance, too.

  • \$\begingroup\$ "you cannot deliver a DC component to the transformer core". Can I add a capacitor in series with the transformer to stop DC current? \$\endgroup\$
    – A-student
    Mar 29 '17 at 7:47
  • \$\begingroup\$ No, because your circuit with only one transistor can only deliver DC. Actually, there is a way to do this with two transistors and a bipolar electrolytic capacitor and a non-middle-tapped primary winding, but then, one of the transistors had to be a high-side switch, which adds another round of complication for its control. If you want to do that, you better use a +5V single-supply power op-amp instead of transistors. \$\endgroup\$
    – Janka
    Mar 29 '17 at 13:25

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