[SOLVED] Driving 135kHZ Ultrasonic transducer

[engineer1000]

I have been using a generator with the output from the transformer being connected to an inductor in series with the transducer making an LC resonant cct. The output from the generator is approx 250V unloaded.

When I attach the transducer to it and find a resonant frequency ( one being 119khz) the current goes up and the pk pk Voltage across the transducer also goes up to about 1.4kv ( the transdcuer is ringing)

The problem is I want to run the transducer loose, when I do this it gets very hot very quickly.

Now if I remove the inductor and connect the transducer directly to the output of the transformer I get a sort of 200V square wave.
To my supprise this is just enough to get some activity out of the transducer.
I don't understand how it is working as its not ringing can anyone help?

 

[chuckey]

The transducer is acting as a small capacitor with a resistor in series. It is the resistive bit that is actually coupling the energy to the air/water/WHY. In the first case the inductor and the capacitance were resonating in a high Q circuit - lots of sinewave amplitude. In the second arrangement the only resonating element is the leakage inductance of the transformer which is far too small to resonate at the operating frequency, rather its probably resonating at 20 X the operating frequency. The real problem is that while you have 200V, there is not much current due to the series capacitance. What is the voltage rating of the transducer?
Frank

 

[engineer1000]

Hi Chunkey,
Thanks for your help. The output from my generator when unloaded is about 260V. When the inductor and transducer are fitted the the pk pk voltage measured across the transducer I get about 1.4kv and the transducer gets very hot. I need to run this transducer loose so the heating is a big problem.
If I increase the voltage from say 200v to 500v will I get better activity from the transducer?

 

[FvM]

When the inductor and transducer are fitted the the pk pk voltage measured across the transducer I get about 1.4kv and the transducer gets very hot. I need to run this transducer loose so the heating is a big problem.

You can expect, that the losses are primarly related to the actual ultrasonic "activity" of the transducer. There's little chance to get ultrasonic magnitude with less losses by circuit tricks.

Regarding high voltage, it should be noted that piezo transducers may be damaged by exceeding the depolarization voltage threshold and also broken due to too high vibration magnitudes, particularly tensile stress.

 

[engineer1000]

Would limiting the currrent to the transducer by adding a series resistor by any help?

 

[engineer1000]

Hi AGAIN,
Do you think it may be down to inpeadance matching.?