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Driving piezo sounder from a microprocessor

M

markp

Jan 1, 1970
0
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.
 
M

markp

Jan 1, 1970
0
Phil Hobbs said:
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

--
Dr Philip C D Hobbs
Principal
ElectroOptical Innovations
55 Orchard Rd
Briarcliff Manor NY 10510
845-480-2058

email: hobbs at electrooptical dot net
http://electrooptical.net

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.
 
P

petrus bitbyter

Jan 1, 1970
0
markp said:
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.

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
 
M

markp

Jan 1, 1970
0
petrus bitbyter said:
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

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):
www.murata.com/catalog/p15e6.pdf

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.
 
P

petrus bitbyter

Jan 1, 1970
0
markp said:
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):
www.murata.com/catalog/p15e6.pdf

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.

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
 
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