On 2/3/07, Jinx <joecolquitt@clear.net.nz> wrote:
> Hi Vasile
>
> > Joe, I'll go further with the question. What the hack need this
> > circuit an enable input as long the driving signal could be tied to
> > ground by command ?
>
> What I'm doing is experimenting with the circuit to find out what's
> going to be useful in different applications
>
> > Only a series inductance with the piezo, computed on the resonant
> > frequency of the piezo and one diode to protect the CE of the signal
> > driver transistor against inverse voltage is required here. The rest
> > is unuseless.
>
> Now, you say series, and I've seen that mentioned a couple of times
> elsewhere, but parallel seems more common. For example Murata
> suggest a few 10s of mH in parallel with their elements. You'll see in
> the attached schematic how the voltage has been boosted by the
> inductors, in this case small transformers/chokes. This sound bomb
> is very loud (105dB), and took two towels to muffle so I could take
> some readings


It could be a serial resonance oscillating circuit either a parallel
resonant circuit, or a transformer. You agree the loud is proportional
with the energy absorbed into the piezoelectric dish, and with the
amount of absorbed energy which is transformed in mechanical vibration
of the piezo (with a proportionality factor k, which unfortunately is
much lower than 1)

The serial resonance is known as current resonance (the equivalent
resistance of series LC at resonance is zero) while the parallel
resonance is known as voltage resonance (the equivalent parallel
resistance of LC at resonance is infinite). The parallel resonance
could look better with higher supply voltage but is not a general
rule.

The loudest piezos I've seen where driven with a transformer and where
sinking quite large current (from 300mA to 1A at 12V)


>
> Also attached is a little logic problem. It's the footprint of one of the
> transformer-like inductors, showing the 0V and piezo connections
> in-circuit. For the life of me I can't figure out how it might be wound
> and internally configured. I thought it might be a small impedance-
> matching audio transformer, but there's no isolation. There seem to
> be basic windings with 45 ohm DC resistance, perhaps some centre-
> tapping, but beyond that I don't know. Anyone have a clue ?

Imagine an autotransformer (probably called self transformer in
english). There is one coil between A and B and another coil between B
and C.  AB and BC are series connected. Piezo is connected between A
and C. C is connected to ground and B is connected to the colector of
the driving transistor.
This is doubling  the voltage on the piezo even there is no resonance.
Changing the frequency will change also the amplitude of the signal as
long one frequency is equal with the resonant frequency of the
transformer-piezo assembly.
You may get interesting chirps based only on the frequency change
arround the resonant value.


>
> Using this method
>
> http://www.thekeeser.com/Electronics%20info/measure_an_unknown_inductor.htm
>
> I get 37mH at 4580Hz from A or B to 0V pin, with C=33nF, the
> approximate capacitance of the piezo, so I guess no matter what
> that 'transformer' actually is, it's the effective inductance that's
> important, and 37mH is definitely in the suggested range
>
> > A piezo may sound loudly if it's mounted into a resonant chamber.
> > Such chamber could increase the noise from 50dB to more than
> > 90dB measured at 1 m in front of the piezo keeping the same driving
> > voltage
>
> That's quite true. SPL is very dependent on mounting and enclosure.
> I'd say, from my tinkering, that there's more to gain from driving with
> the correct frequency and mounting them properly than from simply
> increasing the voltage
>
> > All piezo sirens used in alarm systems are based on this tehnique.
> > But most of them run at 12V because the resonance is producing a
> > higher voltage on piezo. BTW, standard piezo are almost dying at 50V
> > ac across them. You may hear the effect, it's a very high frequency
> > wistle over the driving frequency range.
>
> The small ones I have do sound being over-driven. However, note
> that the sound bomb does have a fairly square-ish 50V as the output
> and the tone is penetratingly clear
>
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