I am making an ultrasonic transducer cleaning circuit. Please guide me on how to make it as I am not an expert in electronics but have some skill in it. It has to be a few hundred watts in power, say 500 watts.
Here is my idea: As a power source I am using a variac rated at 2000 watts. It is connected through a diode as a half wave rectifier to the emitter of a power transistor (2n3055) and controlled by a signal generator connected to the base. It passes through the transistor to a transformer which steps up the voltage from say 10v to 100v, and then is led back to the other terminal of the variac. Up to here there is no problem, but the problem arises when I try to add transistors, since one is not enough for 500 watts. Each is rated for 100w, and I can use only 50 watts in my understanding of that.
So how do I connect transistors to increase the total power?

 

Welcome to AAC!
1) A Variac provides no isolation from mains voltage, so the circuit you are proposing, so far as I understand it, would be 'live' and extremely dangerous!
2) A transformer needs an AC input, not a half-wave rectified DC one as you propose.
3) What transducer are you going to use (make/model/specification)?
4) The 2N3055 is totally unsuited to this application. It would get excessively hot and has an insufficient voltage rating.

 

It would not work.
An ultrasonic transducer requires an AC power source operating at ultrasonic frequency.

 

Welcome to AAC!
1) A Variac provides no isolation from mains voltage, so the circuit you are proposing, so far as I understand it, would be 'live' and extremely dangerous!
2) A transformer needs an AC input, not a half-wave rectified DC one as you propose.
3) What transducer are you going to use (make/model/specification)?
4) The 2N3055 is totally unsuited to this application. It would get excessively hot and has an insufficient voltage rating.

For 2) it still works because I hear a sound depending on the frequency i set the signal generator to. For example, if I set the SG to 1,000 hz i hear a 1,000hz noise. 10,000hz I hear 10,000 hz. So I can tell it is ok
For 3) https://smile.amazon.com/gp/product/B00L24JE4G/ref=ppx_yo_dt_b_asin_title_o01_s00?ie=UTF8&psc=1
For 4) Then what is a good transistor. I have looked for something stronger but cannot find.

 

It would not work.
An ultrasonic transducer requires an AC power source operating at ultrasonic frequency.

It is operating at Ultrasonic frequency because of the signal generator I have. It can go up to 15Mhz. and I hear tones coming when I set it at audible frequencies.

 

it still works because I hear a sound depending on the frequency i set the signal generator to.

The huge DC current through the transformer winding when driving the transducer at 500W would saturate the transformer core, so the transformer would no longer work as you want. What transformer do you plan to use? Is it rated for ultrasonic frequencies?

Then what is a good transistor.

One or more high-power N-channel MOSFETS with extremely low Rds(on) ratings.
The ultrasonic transducer in the link is rated for 60W, not 500W.

 

Jameco Valuepro P-8658 Power Transformer, 12V at 4A, 117VAC, Wire Leads, 2.4" W x 4" L x 2.7" H: Electronic Power Transformers: AmazonSmile: Industrial & Scientific
That is my transformer. Do you think I should connect several in parallel in order to increase the power rating?

 

That is my transformer.

It's a transformer designed for mains frequency, not an ultrasonic frequency.

 

It's a transformer designed for mains frequency, not an ultrasonic frequency.

What is the problem with using it for ultrasonic- At audible frequencies it works and I hear noise?

 

Coil impedances and core losses will be much higher at the higher frequency.
This article may interest you.

 

You need a transformer tuned for the high frequency, like in a switchedmode psu.