While the idea of direct triac drive is appealing- and yes, I've tried it once- in practice it is safer and more treliable to drive an optoSSR or optoTRIAC with built-in zero-crossing features. General notes: In motor drives, use an MOV or Transorber (something FAST) to absorb inductive spikes. On driving 110/220 to a PIC pin, two+ resistors in SERIES is much better than a single resistor, unless the resistors and their creepage path is plastic coated. I like two 1M 1/4w rather than a single 2M 1/2w. A small cap at the PIC pin will be needed to prevent lightning or static from damaging the PIC (100pF seems OK). --Bob At 09:13 AM 12/10/2003, you wrote: >I actually did a gated zero cross design, out of necessity. It was in a >system that generated lots and lots of noise, so false hits were common. I >would wait for a zero cross, then time the next zero cross, and if it was >"half of 60Hz" later, then it was a valid zero crossing. We were firing a >relay that had to open, switch, and bounce all within a half cycle. To >get it to work it had to be energized a few hundred microseconds before a >zero crossing. There was a long list of reasons why Triacs could not be >used, too involved for this discussion. > >You are also going to fight noise. Get ready for: > >Ground planes under your PIC >decoupling caps on your MCLR line >Get rid of those 1n4004 freewheeling diodes on your coils and replace them >with a faster, lower Vf schottky. >You may even need to use a RESISTOR as a freewheeling diode. It actually >does a good job believe it or not. >0.1uf+10ohm snubber across various coils, power inputs, and so on >Separate digital and power grounds joined at one point >ferrites in your power supply leg (not in your ground leg) >AND >wierd problems that happen once in a blue moon and can't be replicated >easily but give you ulcers at midnight. > >The system I was working on used all the above tricks, plus some software >tricks as well. It was designed to fail safe in various ways, designed so >the unit would shut off in a safe way if it never detected a zero cross >due to hardware failures, had some feedback mechanisms in some of the I/O >to determine if they were working properly. WDT of course was there, and >if a reboot was due to a WDT it would shut down safely. You can even add >a sanity check periodically - set several registers to a value on startup, >later if those registers have changed, your PIC is not sane and should be >shut off. Finally, in unprogrammed memory locations the default is to >execute instructions that eventually loop around to zero. If these can be >set to "GOTO $" or some other opcode that is harmless and triggers a >watchdog, then if the PIC is out in the weed patch executing those >instructions it will shut down safely. > >Agreed that 2meg resistors are out due to leakage hazards. Having been >nearly shocked to death by a handheld tool, I am all for safety systems >that work every time, not most of the time. > > >-- Lawrence Lile > > > > > >"Eisermann, Phil [Ridg/CO]" >Sent by: pic microcontroller discussion list >12/10/2003 08:34 AM >Please respond to pic microcontroller discussion list > > > To: PICLIST@MITVMA.MIT.EDU > cc: > Subject: Re: [PIC]: zero crossing idea, comments? > > >llile@saltonusa.com wrote: > > I would recommend sensing zero crossings every time: > > > > 1. You;ve already spent the money to sense them, it costs no parts > > to sense them again. > > > >yes, that is my intent. > > > 2. Your first measurement might be a noise hit, why be wrong twice? > > > >I would average the readings over several cycles. say 32 or 64 > > > 3. You can get fancy and gate your measurements after the first, if > > the zero crossing is not at the right time, it is spurious. this is > > probably overkill. > >That's a good idea. Will have to see if it becomes necessary. > > > > > 4. You really really can sense zero crossings with a 2meg resistor to > > the PIC pin. Unless you have regulatory requirements for isolation > > (being Ridge Tool CO I assume you do) this is very simple. But the > > questions is, would the 2meg resistor provide enough isolation? Or > > two 1meg resistors, for redundant safety? I've done this and had > > success with it. Mchip has an app note on the technique. resistors > > must be rated for 120V service, 0805's don't have enough creepage > > distance. I used 1/4 watt resistors to maintain plenty of creepage. > > > >yes, PICREF-4. I started on that path, figuring that would make a >good starting point. However, this project actually has two PICs in >it, so it is easier in this case to put the phase control on the >isolated side. And we do have mandatory regulatory requirements. > >regardless of whether Mchip has published this app note, it's out >of spec. You're relying on the protection diodes to clamp the >voltage, and that means they are conducting. I agree if you limit >the current enough, it will probably work most of the time. But >i don't get paid to design products that probably work most of >the time :) > > > > > BTW, where is Ridge Tool Co Located? > >We are worldwide, but headquartered in Elyria, OH. About 20 min >west of Cleveland. > >Thanks for the feedback everyone. > >Phil Eisermann >Electronics Engineer >The Ridge Tool Company >(440)329-4680 > >-- >http://www.piclist.com hint: The PICList is archived three different >ways. See http://www.piclist.com/#archives for details. > > > >-- >http://www.piclist.com hint: The PICList is archived three different >ways. See http://www.piclist.com/#archives for details. -------------- Bob Axtell PIC Hardware & Firmware Dev Tucson, AZ 1-520-219-2363 "I lose some on each sale but make it up in volume." -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details.