Piezo and Coils (and magnets?)

[rave154]

Hi all,

can someone explain thie following to me,

see attatchment,

the sig gen is putting out 2.58V measured as AC on the meter, even though its outputted as a 50% duty square wave @ 8.5kH...

with the piezo in the circuit, and the sig gen at 8.5kH (i swept to find best output) ,after a FWBR, Vout = 11.88 VDC

however, without the piezo in the circuit, Vout = 3.14 VDC

Thanks.

David. D

[wrtner]

You might like to have a look at this:
Joel McClain & Norman Wootan: Magnetic Resonance Amplifier ~ Collected Papers
Paul-R

[gyula]

IF you have a look at the equivalent circuit of a piezo, you can see a series RLC circuit and this is shunted by a Cp parallel capacitance.
eFunda: Equivalent Circuit of Piezo Materials

Now you have inserted the piezo in series with a coil that has another magnetically coupled coil (i.e. you bifilar coil), this can be considered as a current transformer. It monitors the AC current flowing in the piezo, driven by the generator.
From the impedance curve, you can see the piezo has a definite frequency where its impedance is the lowest, called the series resonant frequency:
eFunda: Electrical Impedance of Piezo Materials

Of course at this resonant frequency the current taken from the generator is the highest (because the loading effect from the piezo is the highest at that frequency, it is at 8.5kHz in your case, data sheets on piezos include the series resonant frequency).
The highest current will manifest as the highest voltage across the coupled coil's output and you find it while sweeping the generator: actually you found the series resonant frequency of your piezo.

Like in any other RLC circuit, a figure of merit or Q quality factor can be defined as XL/R, XL is the inductive reactance of the 'motional' inductance, L, of the piezo. Thus the voltage "amplification" is simply the ratio of this inductive reactance to the series R + your generator inner impedance (the series R can also be found in data sheet of the piezo).

So you have found this ratio by measuring the voltage from the coupled coil and compared it to the generator output.

Energy-wise you surely know there is no any gain... (when you load the output voltage, your load will reflect back in series with the piezo and will increase the series impedance, hence lowering the current taken from the generator)

rgds, Gyula

[rave154]

WRTNER,

thats where i got the idea from ;-) heheh

using the same circuit as above, but sweeping through the duty cycles i found that a duty cycle of 45% gives the best output ( voltage wise anyways ) of 12.25VDC

the coil is my Bedini Bi filar comprising 24 & 28 guage, about 270 grams of wire in total, so not a large coil by any means.

Using a Do measure current flow OUT of the sig gen gives 12.7 mA ( i am using a sound card Sig Gen btw )

voltage after FWBR and across a 100 ohm resistor is 0.218 VDC

[rave154]

Thanks Gyula, i figured pretty much that was what was happening as i noticed a similar thing happening when i hit harmonics of the 8.5kH...

i did try it with a bunch of NEO's in the core of the coil as per wooten....but i could see no effect at all, of course my setup is clumsy compared to theirs...but.....i saw nothing ( this shouldnt stop anybody else having a crack at it though )

i wonder if it would be worth trying to rig up the couple of ultra sonic transducers that i have floating around, in a similar way??? just to 'see' if anything happens.....i have no experience of ultra sonic trans though unfortunately :-(

[ashtweth]

More reading here guys
http://www.panaceauniversity.org/Mag...0Amplifier.pdf

Ash