1.6Mhz atomizer

[amolina]

Hi everybody, I want to play a little bit with the 1.6Mhz atomizer from Prowave (https://es.farnell.com/prowave/m165d25/atomizador-ultras-nico-l-quido/dp/1007369). I would like to test its atomizing capability at first without implementing any difficult circuit. I was thinking using a basic circuit with just a mosfet amplyfing a square wave signal from a function generator, but it's not working. Do you know which is the reason?

Thanks!

 

[Externet]

Bienvenido.
You are missing a pull-down resistor across gate-to-ground to ensure the mosfet turns off at every cycle low ? The piezo is a capacitive load. It would not hurt to parallel a resistor to it, just in case.

 

[audioguru]

I think your function generator cannot supply enough output current at about 10V p-p to drive the very high capacitance of the Mosfet gate at such a high radio frequency. But if it did then 24V power supply might collapse.

 

[alec_t]

Welcome to ETO!
Since the transducer is, as Externet says, essentially a capacitor, your circuit will simply charge the capacitance up but not discharge it again: hence no oscillation. You need a push-pull drive for the transducer. Also, the transducer ideally should be driven at its natural resonant frequency, which will vary depending on its loading by water etc.

 

[large_ghostman]

Can your Mosfet switch at such high frequency? Seems fairly to me, I use one of those for humidity control. Its not the same one but I use those designed for reptiles that run from 24VAC, dont fall for the sterilizes water bit, I tried testing that and it dosnt work. Also dont stick your finger onto the disk when its on

 

[audioguru]

..... I use those designed for reptiles that run from 24VAC

Hee, hee. What kind of reptile runs from 24VAC? A snake?

Duh. The Mosfet has no load, no wonder it does not drive the piezo transducer that is a capacitor.

 

[large_ghostman]

Hee, hee. What kind of reptile runs from 24VAC? A snake?

Duh. The Mosfet has no load, no wonder it does not drive the piezo transducer that is a capacitor.

LOL yeah better than a 240V snake .

I was asking even if there was a load will the average Mosfet switch that fast? Also the ones I have dont seem to work well with a square wave, even though I cant see a logical reason why

 

[audioguru]

Wikipedia: "Because of their unipolar nature, the power MOSFET can switch at very high speed. Indeed, there is no need to remove minority carriers as with bipolar devices."
BUT they have a very high capacitance from gate to source and between drain to gate. The Miller Effect increases the apparent drain to gate capacitance which is negative feedback trying to cancel the switching.
Therefore a power Mosfet needs a high current push-pull driver to quickly charge and discharge the capacitances.

 

[large_ghostman]

So as they stand no they cant switch at high speed, I will have a look at drivers but seems a bit of a PITA.

I hope you dont believe everything wikipedia says

 

[audioguru]

I looked at Wikipedia because Fairchild talks about "the high cell density" of their Mosfets which I believed to be because a power Mosfet is made from thousands of tiny Mosfets in parallel creating the high power and high capacitances.
Many circuits use a complementary push-pull emitter followers driver like this:

 

[large_ghostman]

The down side surely is you are going to limit the switch speed to that of the BJT driver? So in many cases apart from high power you might as well go with just a BJT? Sort of negates the advantage of higher switching frequencies of the MOSFET if you have to drive with a BJT

 

[audioguru]

The BJT driver switches very fast. Ordinary little 2N4401 and 2N4403 transistors can supply hundreds of mA at very high frequencies so that power Mosfets are frequently used for low loss PWM switches.

 

[large_ghostman]

The BJT driver switches very fast. Ordinary little 2N4401 and 2N4403 transistors can supply hundreds of mA at very high frequencies so that power Mosfets are frequently used for low loss PWM switches.

Trust you to use posh fancy transistors while the rest of us non Canadians have to use the BC547 . Actually really useful info.

 

[audioguru]

European BC327 and BC337 transistor are almost the same as 2N4401 and 2N4403.