I still have an important question...
Do you know why I'm stupid??????????

Because I forgot to put the 10K resistor at the base of the transistors!!! I forgot and connected it directly!!!!
Now the hole circuit is draining 50mA at max... This means the circuit is consuming 450mW at max, and I feel this isn't enough for the device to be powerful, especially that the 450mW power isn't the output power but the total power...

What to do to increase the output power? use a darlington pair?

I've uploaded this video for you to see
http://www.youtube.com/watch?v=9Pb1Aas0IIk

 

There wasn't any detail in that video so I'm assuming that your were measuring battery current. If so, it's one of the finest examples of thermal runaway ever recorded!

Can't do it now but we have much to discuss. You need to lower your power consumption, not increase it. 9V batteries are not work horses!

 

If the piezo can survive a higher signal voltage which causes it to flex more (and maybe will break it) then you can use a bridged amplifier to give it double the signal voltage which causes double the current which calculates at 4 times as much power.

 

If you need a darlingtons' worth of amplification!

What is your output voltage? What do you want to boost it to?

 

CDRIVE, in the video description on youtube I wrote that I'm just changing the frequency, you can see me changing it with the potentiometer in the video.
the output voltage is above 20V rms a little but don't know how much, when the multimeter is one the 20V measuring range and I test it, the DMM gives values 18..19.. then OL (over 20V), when I put it to 200V range, it gives 0V !! I think this is because the signal's frequency is high. I'll measure the output voltage using the oscilloscope and measure the current Ic.

 

A DMM accurately measures the voltage of 50Hz and 60Hz. It can barely measure 1kHz or more. Your frequency is much too high to be measured by a DMM.

A piezo transducer has many resonant frequencies where it has a strong output. If your frequency is not at a resonant frequency of the piezo then it will have a weak output.

 

Hi
Before discussing anything about amplification there are two things I should say. First of all, I had simply did the same as in the commercial unit except the addition of a potentiometer to be able to try and change the frequency of the piezo. In the original they used two transistors at the output, they are SS8050 which have Ic=1.5A.
Now the circuit I've built as I said before is consuming less than 50mA, so there is some problem... for sure the original unit doesn't need two 1.5A BJTs if it drains 50mA only.

The second point is what Audioguru said:
"A piezo transducer has many resonant frequencies where it has a strong output. If your frequency is not at a resonant frequency of the piezo then it will have a weak output."
Do you think this would be the reason why the circuit isn't consuming except 50mA or even less? personally no I don't think so, but he may be true, but first off all lets solve the first problem.

Regards,
Hazim

 

If a piezo transducer is not operated at its resonant frequency and its enclosure also resonates (and amplifies) at its resonant frequency then it is almost useless.
A piezo transducer does not use much power anyway.

The 1.5A transistors were used maybe because they were cheap or because they saturate well.

 

A piezo transducer does not use much power anyway. The 1.5A transistors were used maybe because they were cheap or because they saturate well.

Possibly, it's because of this...

The DC resistance of the inductor is 1.5Ω only

I'm curious if he'd be better off with a shorter 'ON' period in his duty cycle?

 

I'm curious if he'd be better off with a shorter 'ON' period in his duty cycle?

The duty cycle is about 50% the same as the commercial unit. Is this what you are asking about? I didn't understood you.

The 1.5A transistors were used maybe because they were cheap or because they saturate well.

I don't think so because there are two parallel transistors that means they share the current/power dissipation, right?

 

there are two parallel transistors that means they share the current/power dissipation, right?

If the tweeter is a 4 ohm coil and magnet one then it uses a lot of current.
But a piezo tweeter uses a low current.

A high power piezo ultrasonic transducer used in a fog machine or cleaner uses a high current. But they vibrate a fluid, not air.

 

It's a piezo... So why do you think they used two parallel ones? remember that the piezo is connected in parallel with a 1mH inductor (I don't know its power rating but it's in the size of say a 220uF 25V capacitor, that black one with ferrite....)
So the first terminal of the inductor (and piezo) is connected to Vcc and the second to the transistors' collector. The emitters are connected to ground.

 

A piezo is a capacitor. It will resonate with the parallel coils and produce a fairly high AC voltage and current when the transistors start the resonance with a short pulse of high current into the tuned circuit. You would need to tune the inductance somehow to match the resonant frequency of the piezo. The piezo and transistors must be rated for the fairly high voltage.

 

I can't get a piezo with specific specifications, the best source in Lebanon is this one: http://www.ekt2.com/ekt/prodList.asp, try searching for "piezo"... I don't think they even know the specifications of the piezos they sell.

 

Without spec's, a piezo transducer or piezo tweeter might not produce any ultasonic frequencies.

 

A piezo is a capacitor. It will resonate with the parallel coils.......

Actually, it appears to be modeled more like a series resonant circuit,

http://www.efunda.com/materials/piezo/electronics/elec_equiv_circuit.cfm

which is similar to a Quartz Crystal model.

http://www.maxim-ic.com/app-notes/index.mvp/id/2127

Since there's no DC path, one must be provided. Hence, the coil across the Piezo,... but I've used a resistor in place of a coil for moderately loud drive circuits. Furthermore, I don't believe the driver coil forms a resonant element of the circuit.

 

A piezo tweeter has many physical resonant frequencies.
A few ultrasonic transducers do not have physical resonance, its response is fairly flat from 35kHz to 45kHz. Many peak sharply at 40kHz.

Here is the frequency response of a Murata piezo speaker:

 

Nice chart AG. I saved it!