Driving a piezo with a step-up transformer

[ken_k]

I expect there are many in the forum that a much better at RF design than myself. 26kHz is quite a reasonable frequency to work with. To make calculations easier and more predictable it might pay to add a load resistor across the transducer, power can be controlled by PWM, Q and moving off frequency to reduce power. Some sort of gate drive I.C. will be required at 26kHz, I used a very simple NPN PNP emitter follower buffer at 1.1 MHz (cost way less than a chip).

BTW the 1.1MHz transducers were PZT ceramic transducers they had both a series and parallel resonant frequencies, both worked fine for driving.
Ken

[rick.curl]

It just dawned on me that you don't need a transformer, charge pump, or H-bridge. Use a CMOS HEX inverter like a CD4049. Parallel three gates and have them drive another set of three paralleled gates. Tie the Piezo across the second set of gates. If you supply it with 12 volts this will give 24 volts P-P across the piezo.

-Rick

[ken_k]

Rick you are correct See_Mos also posted a similar higher current drive in post number 38.

My submission is very much an overkill, I see the maximum drive for the device is 30Vpp.

BTW this is what the waveform looks like from a small single FET circuit. If the load resistor is changed to 100 ohms the output will still be over 28Vpp across the load.



   

I would be very interested to see a spice model of the air pump, what is it's actual impedance?
Not much information is given.

[TimB]

Hi All thanks so much for the feed back

I had an old board that had a Brushed motor controller. So I wrote code to reverse the direction at approx. 22khz ( I cahned it later to 26)

What I noticed was that I was generating 25v PP AC of my 12v input. Well that is what my scope said. This was using a mains powerbrick

The test on the piezo pump was disappointing. A little buzzing but no air. So I changed to a desktop dev power supply and reduced the voltage to 8v and my PP dropped to 7v?
After upping the the voltage to 18v the device came into life a little, but nothing like it should do.

I conclude the device is really sensitive to the drive circuit so If I continue with it I will use the Murata design. Its not as simple as I hoped.

What I have to state is what I need to achieve.  I have a flow of water around 0.4ml a second in a 2mm id pipe. I need to inject a micro bubble around 2mm long in the flow. It needs to be a whole bubble. From experience bubbles in small tubes are a Fing pain. They contract in volume with pressure, they change the nose shape with velocity. I have managed to develop what I call a bubble accumulator that joins little bubbles and makes them big.

Below are some images I have of the scope readings and what I'm looking at next. Using a venturi to suck a bubble into the feed line and use just a small solenoid valve to let the air in. Ideally I would use a solenoid and piston to "punch" a set volume bubble into the stream. But the volumes I'm working with mean its will be very hard.

I need to design a system I can produce in the 100's and be VERY reliable as they may run 24/7



 

[See_Mos]

Ken's post #39 reminded me of the time I had to repair our Branson ultrasonic tank which used the same resonant drive.

[shantanu@india]

Tim,
Another method of bubble formation is to lower the pressure below vapour pressure of water at that temperature ( vacuum pump?).
The dissolved air in the water shall form natural bubbles.
But very difficult to control.
Maybe injecting a miniscule amount of say CaCO3 to create a bubble of CO2?

[TimB]

Hi all

So as a final update, I came up with a system. It meets and exceeds my expectations first in simplicity, reliability, cost and a few others.

I now have an outlet pipe that is approx 200mm long and hangs so water when it exits it, is causing a small draw on the line. I have a tee in the line and that is connected to a small solenoid valve. All I need to do is open the valve for around 50ms and the draw on the line sucks in the bubble.

Thanks every one for their help.

Tim

[See_Mos]

Ah, the venturi effect.

Nice one Tim!