Driving piezo sounder from a microprocessor
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From: markp on 8 Aug 2010 17:25 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.
From: Jan Panteltje on 8 Aug 2010 17:45 On a sunny day (Sun, 8 Aug 2010 22:25:13 +0100) it happened "markp" <map.nospam(a)f2s.com> wrote in <8c8llhFg2hU1(a)mid.individual.net>: >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. I avoided all those problems by using a sounder with build in circuit, driven directly from an I/O pin, it is LOUD.
From: Phil Hobbs on 8 Aug 2010 17:50 On 8/8/2010 5:25 PM, markp wrote: > 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. > > 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
From: markp on 8 Aug 2010 18:40 "Phil Hobbs" <pcdhSpamMeSenseless(a)electrooptical.net> wrote in message news:0P-dnaC8maAqu8LRnZ2dnUVZ_vqdnZ2d(a)supernews.com... > On 8/8/2010 5:25 PM, markp wrote: >> 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. >> >> > > 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.
From: petrus bitbyter on 9 Aug 2010 07:11
"markp" <map.nospam(a)f2s.com> schreef in bericht news:8c8llhFg2hU1(a)mid.individual.net... > 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 |