From: Jeroen Belleman
Subject: Re: Need real loud anti-dog barking ultrasonic device.
Date: Fri, 13 Dec 2002 16:57:06 +0100
References: <email@example.com> <3DF73938.2C99A7B4@cern.ch> <3DF88597.60AD3446@cern.ch>
X-Mailer: Mozilla 4.7 [en] (X11; I; Linux 2.2.12-20 i686)
Winfield Hill wrote:
> Hmm, looking at the design, I realize that two of the transformer
> windings aren't required, a simple center-tap unit will suffice.
> The resonating inductor dramatically steps up the voltage to the
> piezo, so the transformer is just a current-drive output device.
> . ___
> . ______|| ||_________________
> . | ||___|| |
> . | Vbatt #
> . | (5 to 9V) # L
> . | ,--|<|--+-----o #
> . | | | |
> . | ========|=======|====== |
> . +--####----|-------+--####-----+
> . | | |
> . | +---|(-- gnd |
> . +---|>|--+-+-+----+------|<|---+
> . | | | | |
> . | | R R TC4426 |
> . D /| | | |\ D
> . G--+-O< |--+ +--| >O-+--G
> . S | \| | | |/ | S
> . | | | \ / | |
> . gnd | gnd \/ | gnd
> . | C /\ C |
> . '-/\/-||-' '-||-/\/-'
Now, there I do not agree. Leaving out these windings will
halve the applied voltage, and thus reduce the power by
a factor four. The series L is chosen to resonate with the
piezo capacity, leaving basically only the piezo resistance,
which is of the order of 10 Ohms, at least on the one I
measured. Most of the time, either one of the FETs is ON,
so the ends of the bridge present a low source impedance,
i.e., a voltage source, not a current source.
As for testing, you could vary the frequency somewhat,
looking for a maximum in current drawn from the battery.
The duty-cycle should remain close to 50%, or the
transformer will saturate.
Come to that, the transformer I'm using is nothing special.
I stacked a few ferrite rings to get to about 50mm^2
effective cross section, and put on four windings of ten
turns each. That yields 600uH for one section. The peak
magnetization lies in the vicinity of 120mT, which is
perfectly OK for ferrite. The magnetization current is
about 100mA, negligable compared to the total current
of around 5A.