2 ohm piezo needs 200V

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szzuk

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I have a piezo that has a resonant impedance of 2 ohms. I have an inductor in series that is matched for LC resonance. The piezo needs 200v, so how can i practically get the volts to 200? I have a drive board that supplies 14 amps and 50v. I currently get about 40v at 14A on the piezo at the resonant frequency. The capacitance of the piezo is 1.3uF and it resonates at 40KHz. Thanks, szzuk.
 
I would use parallel MOSFETs and a good 200v power supply.
200 volts across a 2 ohm load would require 20,000 watts.
What are you building?
 
I'm building an ultrasonic atomiser circuit. The part is only 20mm across so 20Kw of power would be a bit over the top.
 
BeerBelly said:
I would use parallel MOSFETs and a good 200v power supply.
200 volts across a 2 ohm load would require 20,000 watts.
What are you building?
Click to expand...
Only if voltage and current are in phase and 2ohms is the resistance. But in this case 2ohms is the effective impedance at 40khz, so the actual power will be much much less.
 
kubeek said:
Only if voltage and current are in phase and 2ohms is the resistance. But in this case 2ohms is the effective impedance at 40khz, so the actual power will be much much less.
Click to expand...

Yes 2 ohms is the impedance of the piezo, with a 10uH inductor in series the circuit impedance drops and is just resistive. I'm not sure what exactly that resistance is but i think its around 0.5 ohms.
 
A simple thought, could you put the piezo/inductor in series acrross another inductor, connect the + end to a lower supply volatge and drive the low side with a high voltage fet, using the mentioned inductor as a flyback to give you the higher voltage.
This is the sort of idea you see in pieze sounders, only without the resonant inductor.
 
That sounds interesting! I think a piezo sounder circuit would work. I will see if i can find one.
 
The circuit looks simple but the electronic theory is not going to sink in for a while. I can read an amplifier circuit but have never made a FET circuit. So V+ would be the 40KHz at 50V from the power board? I'll have to return to this on Saturday, thank you Dr Pepper.

View attachment 68809
 
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The simple trransistor circuit uses a 3rd contact on the piezo (on the right side of the schematic) that feeds positive feedback to the base of the transistor so it oscillates.
Most piezo beepers have this circuit inside.
 
Dont think so, that looks like a self oscillating circuit as mentioned.
Is your piezo 3 terminal then?
If you want to drive it from an ac source then its probably just a case of leave the 3rd feedback terminal open and use it as a 2 terminal device with the inductor accross it.
Note that putting an inductor across the piezo will change things for the drive circuit, peak current and voltage will be higher as the load is now reactive, doesnt mean to say it wont work.
One thing about driving the piezo from a fixed freq is you'll have to tune the piezo to the drive circuit, one of the handy things of a self oscillator is that the whole circuit tunes itself, even to the acoustical load.
Do you have a schema of the drive circuit?
 
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I've attached the circuit i normally use. I use a signal generator as the input to an amplifier so i can tune easily. It is basically a high power high frequency AC source. Typically i use piezos with a low capacitance the ordinary sort with C~1-4nF, and have a 1:20 transformer on the output of the amp so i can get ~1000V. This new piezo has C~1.3uF and needs something else. My piezo has 2 electrodes so no feedback electrode. Thanks, Ant.
 
The plot thickens.
An inductor would give you an increase in voltage, but using what you just described might lead to problems, voltage spikes from a transformer/inductor/piezo setup could be way more than a standard speaker and blow up your amp.
If you can get 1000v from your tranny then you wont really need any devices to increase it further, I seem to have lost the plot here.
You'd use a inductor to get resonance if you were driving the piezo from a square wave provided by a fet or something, a stepped up clean sine wave from a siggen/amp ought to be more than enough.
 
I can't get the piezo ringing with a transformer, i have a few but none do anything. The piezo isnt a sounder, imagine 80 sounders all stacked very tightly and having 80x the capacitance and you're closer to the mark. So you haven't lost the plot! With piezos if you double the number of layers you more or less have to double the current too, so do you see my problem? As i have 80 layers i need theoretically 80x the current. So i need a sensible work around. Or perhaps i've made some kind of other error with what i think is going on. Thanks Ant.
 
Further up you mentioned 1.3uF, at ultrasonic frequencies thats gonna be a low impedance, 3 ohms at 40 kc, you'd need a heck of a transformer for that kind of load at that voltage, and then theres the inductance of the transformer, if its designed for the job then it might be ok, if not then the trans inductance in conjunction with 1.3uF could be way off resonance.

I think maybe you need a high voltage supply in conjunction with a chopper transistor and inductance to drive those, that said it sounds like a specialist thing.

If you can get to it with an iron try soldering up just one of the cells, at least you'd get some idea of what they are like, I dont know if the resonance of one will be the same as many, probably not a million miles out.
 
Last month i did split one of these piezos into thirds, it had a capacitance of around 300nF and 26 layers, at resonance with a series inductor i was getting around 100V at 14A on the piezo. I could split the layers again and it'd be a bit easier to get 200V. It'd be better to not have to split it though. The resonance was the same irrespective of layer number, more or less. So you're spot on with both of them comments.

What is a chopper transistor? Is it specialist beyond my 2 years electronic experience?

Regards, Ant.
 
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BeerBelly said:
I found this circuit which might be adapted to your needs or at least give you some ideas.

https://www.mindchallenger.com/inductionheater/inductionlevitation4.html
Click to expand...

Yes that does look specialist, it'd take me 6 months what with errors, adaptations and part time here and there.

Maybe its better to split the piezo, i have a 20A amplifier i could use so i'd just need to buy a 20A mains transformer and i can squeeze more out of something i already know how to do.
 
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