On Thu, 30 Nov 2006 21:05:58 +1300, you wrote:

>I need an extra-loud sound from a small piezo disc and thought
>I'd give the inductor method a go. Usually a 1k resistor across the
>piezo as a non-capacitive load for the transistor produces a loud
>enough beep. This mail from a while back says how
>
>======
>
>Wed, 16 Jun 2004 09:12:57, from Matt Pobursky
>
>1. Find the capacitance of the piezo device, almost always listed in
>the spec sheet.
>2. Calculate the device's capacitive reactance at the frequency you will
>be driving the device.
>3. Calculate an inductor value for the equivalent inductive reactance.
>4. Place an inductor of said value in parallel with the piezo device.
>5. Drive the whole parallel piezo/inductor circuit with a FET/transistor
>rated ~10x higher voltage than your voltage source.
>
>======
>
>My piezo, 27mm diaphragm, is around 15nF and resonates at 4.6kHz
>
>Capacitive reactance Xc = 1/(2*pi f C), f = 4600, C = 15nF
>
>Xc = 1/(6.28 * 4600 * 15*10^-9)
>= 1/0.00043332
>= 2308 ohms
>
>Inductive reactance XL = 2*pi f L, XL = 2308, f = 4600, 2*pi = 6.28
>
>L = XL/(2*pi f)
>= 2308/(6.28 * 4600)
>= 0.080H
>
>80mH seems rather large. I was expecting perhaps a couple of
>hundred uH (no evidence to presume that, apart from physical
>size). Are my units and the method correct ?


This is definitely in the ballpark. Piezos are very complex loads - far from a simple capacitance,
so it is usually easiest to do it by trial and error & measurement rather than calculating. 
In practice the value is not especially critical unless you're trying to squeeze the last drop of
efficiency, and the value will often be simply 'the biggest in the range' or whatever form factor
inductor you use. Even 1mH gives a decent voltage boost. 
Epcos do a nice range of axial leaded parts which are ideal for this :

100uH : 
http://uk.farnell.com/jsp/endecaSearch/partDetail.jsp?SKU=518300

If you need small, look at inductors aimed at EL backlight inverters  - these will be in the right
inductance range - 10s of mH.

Note that if the piezo gets disconnected, the voltage can easily hit the hundreds, which can kill
teh transistor. I usually use an MPSA42 which can take 300V - needs fairly strong base drive (2K2
from 5V) as gain is low.  Standard NPNs are usually OK as long as the piezo stays connected. 

Another good option if space is limited is a flyback converter + H-bridge - using two SOT-23 dual
transistors for the H-bridge saves space - dual digital-transistors save even more - component count
ends up as 2xDTR, 1 transistor, 1 diode, & inductor. Only snag is you need 2 PIC pins to dribe the
H-bridge and 1 for the flyback PWM. Drive the flyback switching transistor from a PWM peripheral if
available at, say 100KHz ( can easily be bit-bashed if no PWM available) , the inductor will be in
the 100uH area, and you can control volume by adjusting the PWM ratio. A bit more complex but very
cheap & small. This solution works especially well at low supply voltages. 




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