Do you want to make or buy it?
Since you are running at such a low frequency with such a high impedance, the current drawn is likely to be very small. Build a boost switched mode PSU to run your switching transistor/FETs off it.
Or you could drive a mains transformer in reverse and use the output to switch a transistor to revert the signal to a square wave.
Most piezo transducers do not work well below their resonant frequency of a few kHz, not 100Hz.
Piezo ceramic is made by static charge polarizing polycrystaline ceramic. It is the electrostatic equivalent of charging a magnet. Usual chargng field is about 4 kV per mm thickness of ceramic.
When electrodes are place across the polarized ceramic is will compress or stretch depending on applied voltage relative to polarized direction. As with most mechnical devices there will be a resonance point that will create a conservation of energy resulting in a build up in the physical excursion.
If the resonator is placed in a highly dampened environment (acoustical coupling) the the resonator will de-Q. You get less excursion but wider bandwidth. This is what most medical ultrasound imaging machines do. They operated at a loaded Q of 1 to 2 versus unloaded resonator Q of about 30.
You can make a low frequency ceramic piezo transducer. It is just a matter of size and mix of ceramic. Anyway, in your application creating water droplets it is likely pretty well dampened. Just about all ink jet printers use piezo transducer to 'spit' out droplets of ink.
As to audio amps, most will be okay with a high impedance load. Some without a minimum load may have high frequency oscillation. Put a 20 to 50 ohm resistor in parallel with output and you should be fine driving the ceramic transducer.
As a side note, if you apply excessive drive you can depolarize the ceramic causing the piezo properties to degrade or disappear. This will start to happen a about 200 volts per mm thickness of ceramic. (for most ceramic mixes).
Right now I can see the miniscus of the liquid (recall that I’m building a droplet generator to produce droplets on demand of a liquid) moving at the tip of the nozzle, but droplets are not being ejected at 20 V p-p.
Someone suggested using a low voltage square wave from my signal generator to turn a MOSFET switch on and off, allowing a higher voltage supply to drive my disk. Since I’m only seeking 100 Hz or less (to produce 100 Hz train of droplets), perhaps this is a good way to go. Will this unit do the trick?
**broken link removed**
So then I need a 50V DC supply to be switched?
Any suggestions? Ideally, I’d like a variable Voltage supply. Do inexpensive power supplies go as high as 50 volts?
Based on the input of RCinFLA, I should not exceed 200 V/mm of piezo disk thickness. That means 55 volts for my disks. If I kill a few in the process of figuring this out, no harm.
Hi there Pluk,
The piezo element will contain some capacitance that has to be driven. This usually requires a lower than usual impedance. Depending on your capacitance a higher than optimal impedance would cause distortion of the square wave. You could look up the effect of a low pass filter on a square wave to find out what kind of distortion to expect. Since im not familiar with your application i cant say how much degradation from a perfect square wave your app can stand, i guess you'll have to determine that. If it seems like it might be a problem, then you would have to go with a symmetrical driver rather than a single transistor driver. If you arent sure, i guess you can try it and see if you get the results you are expecting.
A regular transistor with a collector resistor constitutes a square wave 'amplifier', where the smaller input can drive a much larger output. The collector resistor has to be small enough to fully drive the capacitance. The trade off is the smaller the resistor the more waste power, but the larger the resistor the less current drive is available for that capacitance.
A symmetrical driver would have a transistor on top and on bottom (known as a half bridge). There is a delay introduced between turning one transistor off and the other on so that they both cant turn on at the same time even for a tiny fraction of the cycle. This kind of drive provides very low output impedance for the load with little waste.
Do you really need 100 volts?
You stated that you are using this to drive a liquid out of a nozzle. I was thinking of doing something like this too for an entirely different reason. What i wanted to ask you was are you using any kind of check valve or do you find you dont need one?
If you are using a check valve, how are you constructing it?
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