Author Topic: Need help getting 2.4MHz piezo to oscillate at its resonate frequency  (Read 2281 times)

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Offline MatCat

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I am trying to create a 2.4MHz oscillator which would output a square wave good enough to drive a fet or power transistor, I found the following circuit in my googling which does oscillate (I changed the values to get close to my 2.4MHz), I am running it with a 12V supply and getting a 4.2V peak to peak  at my desired frequency, however its riding on an 8V DC bias!  I tried going through a cap to block the DC which does give me the desired waveform but of course half of it then dips below ground, I tried using a diode to block the negative going transition but it doesn't appear to work at all!

Original Circuit
 

Output of oscillator
                         

Output through cap and diode
« Last Edit: November 07, 2018, 07:36:27 pm by MatCat »
 

Offline T3sl4co1l

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Re: Need help with DC Bias in oscillator circuit
« Reply #1 on: October 26, 2018, 12:33:19 am »
You need a DC restorer.

What are you actually using the 2.4MHz for?

Tim
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Offline MatCat

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Re: Need help with DC Bias in oscillator circuit
« Reply #2 on: October 26, 2018, 03:33:57 am »
You need a DC restorer.

What are you actually using the 2.4MHz for?

Tim
I want to use it to drive a power transistor to drive a piezo (humidifier)
 

Offline spec

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Re: Need help with DC Bias in oscillator circuit
« Reply #3 on: October 26, 2018, 07:30:43 am »
+ MatCat

Your best bet, by far, is to use an integrating oscillator made from a Schmidt trigger.

The Schmidt trigger could be a Schmidt trigger logic chip (74HC14 or 74HC132), or a Schmidt trigger made from a comparator chip

The output from the above circuits would drive the gate of a power MOSFET nicely.
« Last Edit: October 26, 2018, 07:55:36 am by spec »
 

Offline IanMacdonald

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Re: Need help with DC Bias in oscillator circuit
« Reply #4 on: October 26, 2018, 12:11:15 pm »
Look up the Hartley and Colpitts oscillators. These are two well tested arrangements that work reliably.

In the circuit you show I think the 10k will be far too low, and will be near-saturating the transistor. Try maybe 47k.
 

Offline T3sl4co1l

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Re: Need help with DC Bias in oscillator circuit
« Reply #5 on: October 26, 2018, 06:17:21 pm »
Are you sure that's not a typo for 24kHz?  I know piezos can run that fast, but I don't know that nebulizers run that high, or need to...

How much power, what voltage and current?

Tim
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Online t1d

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Re: Need help with DC Bias in oscillator circuit
« Reply #6 on: October 28, 2018, 12:50:38 am »
I want to use it to drive a power transistor to drive a piezo (humidifier)
Just as another DIY information and idea source, I have noticed that there a lot of DIY vapor cigarette circuits showing up. They might give you some ideas. See Google circuit images. But, p-l-e-a-s-e, do not smoke... Anything!
 

Offline MatCat

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Re: Need help with DC Bias in oscillator circuit
« Reply #7 on: October 28, 2018, 10:05:01 pm »
No its not a typo, typically they run from 1.6MHz to 2.7Mhz, 24V at 650mA, You have to get to that frequency to break up the water molecules small enough to become a vapor, the higher the frequency the smaller the molecule clusters will be.
« Last Edit: October 28, 2018, 10:06:41 pm by MatCat »
 

Offline MatCat

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Re: Need help with DC Bias in oscillator circuit
« Reply #8 on: October 28, 2018, 10:08:22 pm »
Another circuit that is often used in actual products is this one, though I don't really fully understand it:

 

Offline T3sl4co1l

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Re: Need help with DC Bias in oscillator circuit
« Reply #9 on: October 28, 2018, 11:12:04 pm »
Right, you don't want an open loop oscillator and driver, you want feedback so it tracks resonance.

Load changes, frequency shifts, phase goes bonkers, driver explodes.  You need a PLL to close the loop and do it that way.

One of those self-oscillating circuits will work well enough with an audio output transistor, one of the kind with fT peak ~50MHz, I would think.

More or less, scale the inductor and capacitor values proportionally by frequency (L and C down with rising F) and by impedance (L up, C down with rising Z), where Z is determined by the parameters of the piezo, and to a lesser extent, the desired V/I supply requirements.

Tim
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Offline MatCat

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Re: Need help with DC Bias in oscillator circuit
« Reply #10 on: November 07, 2018, 07:35:24 pm »

Ok so I have been digging around and found that schematic on a patent which I am trying to get working. 

I tried this circuit with the following components:
R1: 1K (Though I tried a 10K pot from 10k to the point of it blowing up)
C1: 1.2nF
L1: 8.2uH
TD: 20mm Piezo with resonate frequency of 2.4MHz ~100KHz
Q1: BU406
C2: I tried a few different values, 33nF, 1.2nF, 100nF, 10nF
L2: 100nH
C3: 4.7uF

Also I am not using the bridge rectifier, I am feeding it 24V (rated voltage for the Piezo) from a bench power supply.

As listed above with a 1k for R1 the circuit draws 23mA but does not oscillate.  Can someone help me to figure out proper values to force oscillation?  The full patent is located at https://patents.google.com/patent/US3989042
« Last Edit: November 07, 2018, 07:37:35 pm by MatCat »
 

Offline brybot

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Re: Need help getting 2.4MHz piezo to oscillate at its resonate frequency
« Reply #11 on: November 07, 2018, 07:57:28 pm »
To fix your original circuit, if you choose to do so, I'd add some resistance to the base of your BJT, or drop the load resistor. Right now your transistor is turning on too hard for it's load.
 

Offline MatCat

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Re: Need help getting 2.4MHz piezo to oscillate at its resonate frequency
« Reply #12 on: November 07, 2018, 08:23:26 pm »
I've given up on the original circuit as its better to have the circuit work as the resonate frequency instead of trying to carefully find it and tune it seperately.  I just did some math based on RC and I changed out C1 for a 560pF and c2 with a 47nF, now instead of 23mA draw the circuit is drawing 1.1A and not oscillating, think R1 needs go up ;)  BUT still no oscillation
 

Offline MatCat

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Re: Need help getting 2.4MHz piezo to oscillate at its resonate frequency
« Reply #13 on: November 07, 2018, 09:45:53 pm »
man I dunno why this is so difficult, I have been scowering the web for a week on this now, there are 3 or 4 threads here on EEVBlog of others trying to get this to work and no one has ever succeeded, what magic are these $5 atomizers on ebay / amazon doing that no one on the forums can figure it out? :)  I did buy one on Amazon to take apart but the damn thing was completely potted in PLASTIC and I was unable to get it apart without destroying the circuit beyond the ability to even reverse engineer it.
 

Offline T3sl4co1l

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Re: Need help getting 2.4MHz piezo to oscillate at its resonate frequency
« Reply #14 on: November 07, 2018, 10:07:18 pm »
*Shrug*, if you can make an impedance plot, we could simulate it.  Or if you want to set something up I can take a more in-depth look, send me one to play with or something.

Tim
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Offline MatCat

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Re: Need help getting 2.4MHz piezo to oscillate at its resonate frequency
« Reply #15 on: November 07, 2018, 10:32:55 pm »
The Piezo is supposed to be roughly 2 ohms at resonate frequency.
 

Offline T3sl4co1l

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Re: Need help with DC Bias in oscillator circuit
« Reply #16 on: November 08, 2018, 11:13:11 am »
Another circuit that is often used in actual products is this one, though I don't really fully understand it:



And you mentioned elsewhere (in chat) the expected bandwidth is about 4%, i.e., a Q of 25, and this was the datasheet: http://www.dongiltech.co.kr/pdf/DUNU-Series.pdf

Referring to this circuit, it seems to oscillate better (in SPICE) with L1 small (~5n), C1 much larger (~2.2nF), and... C2 controls output power, roughly speaking?

I'm finding better results with the large inductor in the collector circuit, as a usual common-emitter Pierce circuit has it.

I don't like the E-B diode, it supplies excess base current making the V-I dependency hard to control.  But it's also necessary to prevent squegging, as given.

You're definitely not going to run it at 50V.  2 ohms and 25W implies 7V RMS.  Probably 10-12V supply will do.

These values seem to work nicely:



Tim
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Offline MatCat

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Re: Need help with DC Bias in oscillator circuit
« Reply #17 on: November 08, 2018, 07:24:15 pm »
Another circuit that is often used in actual products is this one, though I don't really fully understand it:



And you mentioned elsewhere (in chat) the expected bandwidth is about 4%, i.e., a Q of 25, and this was the datasheet: http://www.dongiltech.co.kr/pdf/DUNU-Series.pdf

Referring to this circuit, it seems to oscillate better (in SPICE) with L1 small (~5n), C1 much larger (~2.2nF), and... C2 controls output power, roughly speaking?

I'm finding better results with the large inductor in the collector circuit, as a usual common-emitter Pierce circuit has it.

I don't like the E-B diode, it supplies excess base current making the V-I dependency hard to control.  But it's also necessary to prevent squegging, as given.

You're definitely not going to run it at 50V.  2 ohms and 25W implies 7V RMS.  Probably 10-12V supply will do.

These values seem to work nicely:



Tim
Interesting, I will have to try your version of the circuit out.  This specific piezo is specified to run at 24V.  How much current is that 1 ohm resistor seeing? 
 

Offline T3sl4co1l

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Re: Need help getting 2.4MHz piezo to oscillate at its resonate frequency
« Reply #18 on: November 09, 2018, 01:37:28 pm »
Not much.



That comes to about... uhhh... 0.39A RMS.

These also show reasonable efficiency, consistent with a class C amp/osc.  YMMV with a real BU406 or whatever.  Probably I'd recommend MJE15012 or another switching or fast-ish amp type if I were designing this.  Nudging R and C values back and forth has pretty strong impacts on power output, distortion and efficiency.  Probably piezo load has a strong effect as well.

Tim
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Offline leeatljs

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Re: Need help getting 2.4MHz piezo to oscillate at its resonate frequency
« Reply #19 on: November 09, 2018, 02:07:01 pm »
The cheap way of driving these 'atomising' piezo's is to drive them from a fairly accurate oscillator.  The correct way is to drive them from a waveform which is in phase with the current through the element.
An ultrasonic cleaner works this way, but at a lower frequency than you want.
'greatScott' has a video wich you might find interesing

 

Offline MatCat

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Re: Need help getting 2.4MHz piezo to oscillate at its resonate frequency
« Reply #20 on: November 09, 2018, 08:04:19 pm »
His video was certainly interesting, but didn't help :)  The reason I am  even trying to build this circuit vs just use one of those drop in ones is that this needs to go in a water tank with pretty small dimensions and the tall height of the existing modules is too much as it will require the tank to basically be 1/3rd full for the atomizer to work, vs having the fiarly flat plug in disks that would allow it to be only a few mm.  Also a 555 isn't going to get to 2.4MHz.
 

Offline MatCat

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Re: Need help getting 2.4MHz piezo to oscillate at its resonate frequency
« Reply #21 on: November 13, 2018, 11:25:07 pm »


Ok we are getting somewhere!  I built up the circuit you simulated only difference being I  used a 5.6nH for L1 since 5nH is not standard, I am getting 2 oscillations, a 205KHz and a 22MHz, not sure what I should tweak here, perhaps that 10uH should be 100uH?
 

Offline MatCat

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Re: Need help getting 2.4MHz piezo to oscillate at its resonate frequency
« Reply #22 on: November 15, 2018, 05:41:37 am »
I have a cheap DDS coming to me that can do up to the required freqiuences, I am going to experiment with this piezo and see what I can figure out.  What would be a good way to measure the resonate resistance?
 

Offline T3sl4co1l

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Re: Need help getting 2.4MHz piezo to oscillate at its resonate frequency
« Reply #23 on: November 15, 2018, 02:06:14 pm »
Set up an impedance divider, the source (50 ohms) into the transducer.  Measure the gain, and phase if possible, of this network.  (Gain is easy enough, the output should be constant voltage and constant resistance, at all frequencies -- check this by sweeping the output with the generator unloaded, and with a 50 ohm load.  It should be flat, and the 50 ohm load should reduce its amplitude by half.)

Phase, you won't be able to measure the "top" of the impedance divider (because that's internal to the generator), but you probably have a trigger output to reference that.

Write down frequency, amplitude and phase in a spreadsheet.

Measure at least a reasonable density of points around the main intended resonance, but also sweep the whole range (say 10kHz to whatever max is, 10MHz say?) to see if there are other modes too.

Also repeat the measurements for different load conditions.

To convert that into a network, a rational approximation method is used to fit the curve.  This isn't trivial, but there is an algorithm to do it.  Then you can put the network into SPICE.

Tim
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Offline MatCat

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Re: Need help getting 2.4MHz piezo to oscillate at its resonate frequency
« Reply #24 on: November 28, 2018, 10:06:47 pm »
I found some data on the 2.4MHz 20mm  elements, capacitance of 1500pF +/- 20%, Resonate Resistance: < 3Ω, could you redo the values and simulation for this info?
 


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