Driving a Peizo Element

If you know the capacitance of the piezo, you can select an inductor that approximately resonates with that capacitance at the operating frequency. This increase the voltage across the piezo, considerably.

The resonance formula is 2*pi*f=1/sqrt(L*C).

Forgot something. Piezo elements don't like to have DC across them. But if you connect the piezo in parallel with the inductor, and turn the transistor off when the piezo is to be silent, you will have very little DC across it.

Thanks heaps John, that's really helpful and enough info for me to get some sample parts on order.

Best regards

Adrian

On Wed, 01 Nov 2006 03:38:07 GMT, "AJ" wrote:

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From your description, I assume this is your circuit configuration:

View in Courier.

+5V | [1000R] | +-----+ | | C | SQIN>---[R]---B [PIEZO] E | | | +-----+ | GND The first problem I see is that while the piezo is rated to deliver an SPL of 75dB minimum at a distance of 30cm with a 10VPP 3kHz square wave across it, your circuit can only deliver a 5VPP signal to drive the piezo. Also, the 1000 ohms will only allow, at best, 5mA of current into the transducer, so you won't get much of an amplitude out of it while it's charging.

A second problem may be that you're not driving the transducer with

3kHz. If you're not, you won't be driving it at its resonant frequency and its output level will not reach 75dB.

In order to get the 10VPP square wave across the transducer you could drive it with a full bridge made from two 555 timers.

Each timer has a totem pole output which can source or sink 100mA, which ought to be plenty for your piezo. Unfortunately there's no mention of what its impedance is in the data sheet, but with 20nF of capacitance that's a reactance of about 2600 ohms at 3kHz. Again, unfortunately, that's with a sinusoidal drive, and you're going to be driving it with a square wave, so there'll be some fairly high peak currents at the edges.

If you wanted to you could but an inductor in series with the transducer and tune out the capacitive and inductive reactances, and that would leave only the real part of the impedance to drive, but with a 3kHz and 2600 ohms that's a pretty good sized choke.

In any case, the LTSPCICE circuit file attached will show you a driver that you can use to get the 10VPP square wave drive or, with the inductor in there, whatever current the resistive part of the transucer's impedance needs.

Version 4 SHEET 1 1144 788 WIRE -640 -288 -768 -288 WIRE -560 -288 -640 -288 WIRE -192 -288 -560 -288 WIRE -48 -288 -192 -288 WIRE 368 -288 -48 -288 WIRE 512 -288 368 -288 WIRE 976 -288 512 -288 WIRE 1056 -288 976 -288 WIRE -48 -256 -48 -288 WIRE 368 -256 368 -288 WIRE -192 -240 -192 -288 WIRE 512 -240 512 -288 WIRE -192 -128 -192 -176 WIRE -48 -128 -48 -176 WIRE -48 -128 -192 -128 WIRE 368 -128 368 -176 WIRE 512 -128 512 -176 WIRE 512 -128 368 -128 WIRE -640 -80 -640 -288 WIRE 1056 -80 1056 -288 WIRE -192 -16 -192 -128 WIRE 512 -16 512 -128 WIRE 976 48 976 -288 WIRE 976 48 640 48 WIRE -560 144 -560 -288 WIRE -512 144 -560 144 WIRE -144 144 -288 144 WIRE 704 144 464 144 WIRE 976 144 976 48 WIRE 976 144 928 144 WIRE -640 208 -640 0 WIRE -512 208 -640 208 WIRE -64 208 368 -128 WIRE -64 208 -288 208 WIRE 384 208 -48 -128 WIRE 704 208 384 208 WIRE 1056 208 1056 0 WIRE 1056 208 928 208 WIRE -640 272 -640 208 WIRE -512 272 -640 272 WIRE -240 272 -288 272 WIRE -192 272 -192 48 WIRE -192 272 -240 272 WIRE -48 272 -192 272 WIRE 96 272 16 272 WIRE 272 272 176 272 WIRE 512 272 512 48 WIRE 512 272 352 272 WIRE 544 272 512 272 WIRE 704 272 544 272 WIRE 1056 272 1056 208 WIRE 1056 272 928 272 WIRE -64 336 -288 336 WIRE 640 336 640 48 WIRE 704 336 640 336 WIRE -64 448 -64 336 WIRE -64 448 -944 448 WIRE 544 512 544 272 WIRE 1056 512 1056 272 WIRE -944 528 -944 448 WIRE -768 528 -768 -288 WIRE -640 528 -640 272 WIRE -240 528 -240 272 WIRE -944 672 -944 608 WIRE -768 672 -768 608 WIRE -768 672 -944 672 WIRE -640 672 -640 592 WIRE -640 672 -768 672 WIRE -240 672 -240 592 WIRE -240 672 -640 672 WIRE -144 672 -144 144 WIRE -144 672 -240 672 WIRE 464 672 464 144 WIRE 464 672 -144 672 WIRE 544 672 544 576 WIRE 544 672 464 672 WIRE 1056 672 1056 576 WIRE 1056 672 544 672 WIRE -944 768 -944 672 FLAG -944 768 0 SYMBOL Misc\\\\NE555 816 240 R0 SYMATTR InstName U1 SYMBOL Misc\\\\NE555 -400 240 M0 SYMATTR InstName U2 SYMBOL voltage -768 512 R0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 WINDOW 3 24 104 Invisible 0 SYMATTR Value PULSE(0 5 0 1e-3) SYMATTR InstName V1 SYMBOL res -656 -96 R0 SYMATTR InstName R7 SYMATTR Value 2.4k SYMBOL cap -656 528 R0 SYMATTR InstName C1 SYMATTR Value .05e-6 SYMBOL res -64 -272 R0 SYMATTR InstName R1 SYMATTR Value 1k SYMBOL res 352 -272 R0 SYMATTR InstName R2 SYMATTR Value 1k SYMBOL cap -176 48 R180 WINDOW 0 24 64 Left 0 WINDOW 3 24 8 Left 0 SYMATTR InstName C2 SYMATTR Value 1e-8 SYMBOL cap 528 48 R180 WINDOW 0 24 64 Left 0 WINDOW 3 24 8 Left 0 SYMATTR InstName C4 SYMATTR Value 1e-8 SYMBOL diode -176 -176 R180 WINDOW 0 24 72 Left 0 WINDOW 3 24 0 Left 0 SYMATTR InstName D1 SYMATTR Value 1N4148 SYMBOL diode 528 -176 R180 WINDOW 0 24 72 Left 0 WINDOW 3 24 0 Left 0 SYMATTR InstName D2 SYMATTR Value 1N4148 SYMBOL cap 16 256 R90 WINDOW 0 0 32 VBottom 0 WINDOW 3 32 32 VTop 0 SYMATTR InstName C5 SYMATTR Value 20e-9 SYMBOL diode -224 592 R180 WINDOW 0 24 72 Left 0 WINDOW 3 24 0 Left 0 SYMATTR InstName D3 SYMATTR Value 1N4148 SYMBOL diode 560 576 R180 WINDOW 0 24 72 Left 0 WINDOW 3 24 0 Left 0 SYMATTR InstName D4 SYMATTR Value 1N4148 SYMBOL res 1040 -96 R0 SYMATTR InstName R3 SYMATTR Value 2.4k SYMBOL ind 256 288 R270 WINDOW 0 32 56 VTop 0 WINDOW 3 5 56 VBottom 0 SYMATTR InstName L1 SYMATTR Value 110e-3 SYMBOL res 192 256 R90 WINDOW 0 0 56 VBottom 0 WINDOW 3 32 56 VTop 0 SYMATTR InstName R5 SYMATTR Value 100 SYMBOL voltage -944 512 R0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 WINDOW 3 24 104 Invisible 0 SYMATTR Value PULSE(0 5 .005 1e-6 1e-6 .005 .01 2) SYMATTR InstName V2 SYMBOL cap 1040 512 R0 SYMATTR InstName C3 SYMATTR Value .05e-6 TEXT -706 714 Left 0 !.tran 0 .02 0

-- John Fields

Thanks heaps John, I will give that a try over the next couple of days and see how it goes, from what I could tell I would need a huge choke if I was to use one instead of a resistor in my original circuit.

Best regards,

AJ