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Driving piezo sounder from a microprocessor

Hi All,

I'm intending to drive a piezo sounder using antiphase PWM outputs from a micro (ATtiny88). The sounder is actually mounted on another PCB which is on the underside of the lid of the box, with about 20cm of cable from the micro on the motherboard. I'm concerned about what protection I need for the micro

My first thoughts were to use a pair of schottky diode clamps close to the PWM outputs, which raises a question:

Can I assume these schottky diodes will start to conduct before the substrate diodes, or should I put a small resistor between the micro and the clamps?

Also since the sounder is essentially resonating, should I put a series resistor in line with each drive to allow the voltage at the sounder to rise above Vcc and below ground (and hence get louder sound output)?

Finally, do I need to worry about EMC, and would it be worth putting a filter on each signal on the lid PCB (maybe a ferrite/capacitor or just a capacitor)?

Thanks! Mark.

Reply to
markp
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On a sunny day (Sun, 8 Aug 2010 22:25:13 +0100) it happened "markp" wrote in :

I avoided all those problems by using a sounder with build in circuit, driven directly from an I/O pin, it is LOUD.

Reply to
Jan Panteltje

Generally you're OK using an appropriately-sized Schottky for that job. It'll have a forward voltage something like 200 mV less than the protection diode, and with fancy modern protection networks the delta will sometimes be much larger.

That way the Schottky will be taking very roughly exp(8)~3000 times more current than the protection diode.

Cheers

Phil Hobbs

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Dr Philip C D Hobbs
Principal
ElectroOptical Innovations
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Reply to
Phil Hobbs

Thanks Phil, I've seen these protection Schottkies placed next to processor IO pins before without a resistor, but the datasheets are somewhat vague on the substrate diodes' characteristics.

Mark.

Reply to
markp

"markp" schreef in bericht news: snipped-for-privacy@mid.individual.net...

Don't know the specs of the internal diodes. I'd consider them a kind of emergency exit and put a set of Schottkys next to the I/O-pin.

A small resistor will not harm but can't say you really need it. Depends too much on the specs of the piezo. Same for the value of that resistor.

A capacitor will smooth the edges of PWM-pulses and will suppres some of the hf. Using shielded wire will reduce unwanted radiation as well. A ferrite will do the same.

If you need all those components I guess you'd better go for a piezo with build in elecronics. Will be cheaper and saving space as well.

petrus bitbyter

Reply to
petrus bitbyter

Unfortunately I'm interfacing to another board with a single PWM output (in fact the next board will have an anti-phase signal as well, both going to ground when off.

Anyway, I think I'm going to put a 74LVC2G86 dual xor on the lid PCB and configure one of them as an inverter so that it is completely buffered from the processor.

I found a Murata document that uses AC coupling (fig 11, page 8):

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I assume this relies on the bleed resistance (i.e. the parasitic resistor in parallel with the piezo capacitance) to discharge the piezo capacitance so no DC is across the piezo element when off. If the piezo element was purely capacitive you'd expect a large DC voltage, assuming that the AC coupling caps are much larger than the piezo cap. Another benefit might be that the AC couplers act, to a certain degree, as hf filters using the substrate diodes of the xor gates.

Any comments on this configuration? Also, anyone got a piezo sounder model for LTspice?

Mark.

Reply to
markp

"markp" schreef in bericht news: snipped-for-privacy@mid.individual.net...

The "bleed resistor" may run into tens of MegaOhms (depending on the type) so you'd place a - let's say -

1M resistor parallel to the sounder.

As yo want to protect in- and outputs of both micro and XOR you may need Schottky diodes near them. Alternatively two serial zeners instead of the parallel resistor may work as well. See fig. 7 of your Murata document.

Be aware that using the circuit of fig. 11 doubles the peak to peak voltage on the sounder.

IMHO you should not rely on the substrate diodes for the required function. As I said before, consider them emergency exits.

As for the hf filter, you have a capacity already. You can add not too big a serial resistor to the output of the micro.

petrus bitbyter

Reply to
petrus bitbyter

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