Post by thread:[PIC]: zero crossing idea, comments?

I have the following idea to detect zero-crossing on an AC line reasonably close. Looking for comments/feedback on any other ideas that might be implemented. 110V input throught two 10K resistors in series an H11AA814A optocoupler. collector of optocoupler pulled to 5V via a 22k resistor. collector fed into GP2 a 12F629. the PIC is running on interal clock. GP is configured as an external interrupt, so it should be a ST. I expect the pulse length to vary between devices due to variations in current transfer ratio of the opto. To get a better reading of the actual zero crossing point, I would measure the typical pulse length before the application does anything. Half of the pulse length should get me pretty close to the true zero crossing point. The application will remain in service maybe 10 to 20 minutes before it is powered down. would the oscillator frequency remain stable enough during this point, or am I making this too complicated? It is for phase control, and I will always be switching "very soon" after the zero-crossing point; I do not need a large control range. Thanks for any feedback Phil Eisermann Electronics Engineer The Ridge Tool Company (440)329-4680 -- http://www.piclist.com hint: The list server can filter out subtopics (like ads or off topics) for you. See http://www.piclist.com/#topics

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. 2. Your first measurement might be a noise hit, why be wrong twice? 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. 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. 5. It is easy to filter a 2meg resistor, a small ceramic cap in parallel makes a nice low pass filter that eliminates noise hits. Put the pole of the filter at about 300 Hz and it will kill off all that noise from your motor brushes without generating too much phase shift. BTW, where is Ridge Tool Co Located? -- Lawrence Lile "Eisermann, Phil [Ridg/CO]" <spam_OUTpeisermaTakeThisOuTRIDGID.COM> Sent by: pic microcontroller discussion list <.....PICLISTKILLspam@spam@MITVMA.MIT.EDU> 12/09/2003 04:03 PM Please respond to pic microcontroller discussion list To: PICLISTKILLspamMITVMA.MIT.EDU cc: Subject: [PIC]: zero crossing idea, comments? I have the following idea to detect zero-crossing on an AC line reasonably close. Looking for comments/feedback on any other ideas that might be implemented. 110V input throught two 10K resistors in series an H11AA814A optocoupler. collector of optocoupler pulled to 5V via a 22k resistor. collector fed into GP2 a 12F629. the PIC is running on interal clock. GP is configured as an external interrupt, so it should be a ST. I expect the pulse length to vary between devices due to variations in current transfer ratio of the opto. To get a better reading of the actual zero crossing point, I would measure the typical pulse length before the application does anything. Half of the pulse length should get me pretty close to the true zero crossing point. The application will remain in service maybe 10 to 20 minutes before it is powered down. would the oscillator frequency remain stable enough during this point, or am I making this too complicated? It is for phase control, and I will always be switching "very soon" after the zero-crossing point; I do not need a large control range. Thanks for any feedback Phil Eisermann Electronics Engineer The Ridge Tool Company (440)329-4680 -- http://www.piclist.com hint: The list server can filter out subtopics (like ads or off topics) for you. See http://www.piclist.com/#topics -- http://www.piclist.com hint: The list server can filter out subtopics (like ads or off topics) for you. See http://www.piclist.com/#topics

> 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. > > 5. It is easy to filter a 2meg resistor, a small ceramic cap > in parallel makes a nice low pass filter that eliminates > noise hits. Put the pole of the filter at about 300 Hz and > it will kill off all that noise from your motor brushes > without generating too much phase shift. The app note is here: http://www.microchip.com/download/appnote/devspec/16c5x/00521c.pdf Also note the following from the app note:: ---- The user needs to be aware that the circuit required to connect the T0CKI input to an AC power line is slightly different than the other I/O pins. Each of the I/O pins has two diodes for input protection whereas the T0CKI pin has only one protection diode connected to Vss (Figure 3). Therefore, it is necessary to connect a diode externally between the T0CKI pin and VDD in order to clamp the voltage on the T0CKI pin to VDD +0.6V (approximately). See Figure 4. It is also recommended that resistor R be at least 2M ohm. ---- Tom Deutschman Wizbang Designs, Inc. 1629 W 8TH AVE Spokane, WA 99204 Phone: (509) 251-4814 http://www.wizbangdesigns.com -- http://www.piclist.com hint: The list server can filter out subtopics (like ads or off topics) for you. See http://www.piclist.com/#topics

In a product I designed, I used the full-wave rectified transformer secondary driving a PIC pin through a current limit resistor. This worked great with the 16C74 and 18C452. However, when I moved to the 18F452, it stopped working. It apparently does not like even current limited over-voltage on a pin. I finally added a transistor and drove the base of it with the current limited full wave rectified AC. The collector then pulls down the PIC pin. Harold {Quote hidden}>> 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. >> >> 5. It is easy to filter a 2meg resistor, a small ceramic cap >> in parallel makes a nice low pass filter that eliminates >> noise hits. Put the pole of the filter at about 300 Hz and >> it will kill off all that noise from your motor brushes >> without generating too much phase shift. > > The app note is here: > > http://www.microchip.com/download/appnote/devspec/16c5x/00521c.pdf > > Also note the following from the app note:: > > ---- > The user needs to be aware that the circuit required to connect the > T0CKI input to an AC power line is slightly different than the other I/O > pins. Each of the I/O pins has two diodes for input protection whereas > the T0CKI pin has only one protection diode connected to Vss (Figure 3). > Therefore, it is necessary to connect a diode externally between the > T0CKI pin and VDD in order to clamp the voltage on the T0CKI pin to VDD > +0.6V (approximately). See Figure 4. It is also recommended that > resistor R be at least 2M ohm. > ---- > > > Tom Deutschman > > Wizbang Designs, Inc. > 1629 W 8TH AVE > Spokane, WA 99204 > > Phone: (509) 251-4814 > http://www.wizbangdesigns.com > > -- > http://www.piclist.com hint: The list server can filter out subtopics > (like ads or off topics) for you. See http://www.piclist.com/#topics > -- FCC Rules Online at http://www.hallikainen.com/FccRules/ -- http://www.piclist.com hint: The list server can filter out subtopics (like ads or off topics) for you. See http://www.piclist.com/#topics

> > In a product I designed, I used the full-wave rectified > transformer secondary driving a PIC pin through a current > limit resistor. This worked great with the 16C74 and 18C452. > However, when I moved to the 18F452, it stopped working. It > apparently does not like even current limited over-voltage on > a pin. I finally added a transistor and drove the base of it > with the current limited full wave rectified AC. The > collector then pulls down the PIC pin. > > Harold I have T0CKI connected through a 2.2M resistor to the AC side of a bridge (24VAC source) with a diode to VCC. Increments timer 0 at a 30Hz rate. Since I'm using the internal RC oscillator, I use this to keep real time. The first time I power up the product, it calibrates itself to the power line, storing a constant in EEPROM. The constant is used when AC power is lost and I need to keep reasonably accurate time until the AC is restored (running off of backup 9VDC battery). Not great over temperature but ok for this application. Schematic at http://www.wizbangdesigns.com/images/Timer2B.PDF Tom Deutschman -- http://www.piclist.com hint: The list server can filter out subtopics (like ads or off topics) for you. See http://www.piclist.com/#topics

The fact that you're using RA4 probably helps, since it's an open collector output and does not include a clamp diode to Vcc, so you get no on-chip clamp diode current (which seems to mess up the F chips but not the C chips). I was using RB0 and generating an interrupt on a negative edge, just prior to zero cross. I've now inserted the transistor and doing an interrupt on a positive edge, still just before zero cross. I use the capture compare register to figure out when to turn on a triac. I also do a 50/60 Hz check on power up, then use different tables to figure out what to use in the capture compare register. I'm detecting every zero cross since I'm using full wave rectification. Do you really have a 30 Hz line? Harold {Quote hidden}>> >> In a product I designed, I used the full-wave rectified >> transformer secondary driving a PIC pin through a current >> limit resistor. This worked great with the 16C74 and 18C452. >> However, when I moved to the 18F452, it stopped working. It >> apparently does not like even current limited over-voltage on >> a pin. I finally added a transistor and drove the base of it >> with the current limited full wave rectified AC. The >> collector then pulls down the PIC pin. >> >> Harold > > I have T0CKI connected through a 2.2M resistor to the AC side of a > bridge (24VAC source) with a diode to VCC. Increments timer 0 at a 30Hz > rate. Since I'm using the internal RC oscillator, I use this to keep > real time. The first time I power up the product, it calibrates itself > to the power line, storing a constant in EEPROM. The constant is used > when AC power is lost and I need to keep reasonably accurate time until > the AC is restored (running off of backup 9VDC battery). Not great over > temperature but ok for this application. > > Schematic at http://www.wizbangdesigns.com/images/Timer2B.PDF > > Tom Deutschman > FCC Rules Online at http://www.hallikainen.com/FccRules/ -- http://www.piclist.com hint: The list server can filter out subtopics (like ads or off topics) for you. See http://www.piclist.com/#topics

Eisermann, Phil [Ridg/CO] wrote : > 110V input throught two 10K resistors in series an H11AA814A > optocoupler. collector of optocoupler pulled to 5V via a 22k resistor. Or use the "weak pullup" and save a resistor... > collector fed into GP2 a 12F629. the PIC is running on > interal clock. GP is configured as an external interrupt, > so it should be a ST. I expect the pulse length to vary between > devices due to variations in current transfer ratio of the opto. > To get a better reading of the actual zero crossing point, I would > measure the typical pulse length before the application does > anything. Half of the pulse length should get me pretty close > to the true zero crossing point. So you'd detect the start of the zero-crossing, and then wait for half of the the former calculated zero-crossing width ? Seem OK. Note that you can flip the INTEDG on each interrupt to capture *both* edges using interrupts. > > The application will remain in service maybe 10 to 20 > minutes before it is powered down. would the oscillator frequency > remain stable enough during this point, Stable enough for what ? Are you thinking on "free wheeling" during this 10-20 min period without further zero-crossing detection ? > or am I making this too complicated? It is > for phase control, and I will always be switching "very soon" > after the zero-crossing point; What are you switching ? And what is "very soon" ? For a Triac, you must at least wait until there is enough current to hold the Triac conducting. > I do not need a large control range. What is "a large range" ? Or rather, what is "not a large range" ? Jan-Erik. -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details.

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. {Quote hidden}> > 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.

pic microcontroller discussion list wrote: > Eisermann, Phil [Ridg/CO] wrote : > >> collector of optocoupler pulled to 5V via a 22k >> resistor. > > Or use the "weak pullup" and save a resistor... > that's a thought. {Quote hidden}>> To get a better reading of the actual zero crossing point, I would >> measure the typical pulse length before the application does >> anything. Half of the pulse length should get me pretty close >> to the true zero crossing point. > > So you'd detect the start of the zero-crossing, and then wait for > half of the the former calculated zero-crossing width ? Seem OK. > > Note that you can flip the INTEDG on each interrupt to capture > *both* edges using interrupts. > that's what I had in mind, actually. I would set INT to trigger on the rising edge. when the interrupt occurs, start TMR0. Flip INTEDG to trigger on falling edge. the next interrupt stop TMR0. half of TMR0 should be pretty close to the actual zero crossing. The problem I'm trying to address is that the interrupt pulse generated by the opto will start before the actual zero crossing and end after the zero crossing. the above described technique should allow me to determine how long after the interrupt the true zero crossing occurs. I hope I explained that correctly. >> >> The application will remain in service maybe 10 to 20 >> minutes before it is powered down. would the oscillator frequency >> remain stable enough during this point, > > Stable enough for what ? > Are you thinking on "free wheeling" during this 10-20 min period > without further zero-crossing detection ? No, i would still detect the zero crossing. But I would not re-calculate the length of the pulse. I would think the internal oscillator would not drift enough in this application to deviate more than a few usec. > For a Triac, you must at least wait until there is enough current to > hold the Triac conducting. > >> I do not need a large control range. > > What is "a large range" ? Or rather, what is "not a large range" ? > Well, that still needs to be fine-tuned, but initial calculations indicate a conduction angle range between 120 and 180 degrees. That means worst-case I would be firing 60 degrees from zero crossing. 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.

Eisermann, Phil [Ridg/CO] wrote: > > Are you thinking on "free wheeling" during this 10-20 min period > > without further zero-crossing detection ? > > No, i would still detect the zero crossing. But I would not > re-calculate the length of the pulse. I would think the internal > oscillator would not drift enough in this application to deviate > more than a few usec. Well, "a few usec" is a very short time when talking about 50/60 Hz AC. After "a few usec" I'm not sure there are enough voltage/current to keep a Triac conducting. Anyway, we still don't know *what* you are triggering :-) And the intosc will more probably "jitter" in the sort run, and maybe drift (depanding on the temp, this should be mounted outside, right ?) in the longer run. But in 10-20 minutes, the temp drift will maybe not be that large. > Well, that still needs to be fine-tuned, but initial calculations > indicate a conduction angle range between 120 and 180 degrees. That > means worst-case I would be firing 60 degrees from zero crossing. 60 deg *before* z-c, not ? Or 120 deg *after* z-c ? The 120-180 deg range are the *last* 60 deg of one half-cycle ? Or did I missunderstand this ? Anyway, "a few usec" is still very little in this regard. Jan-Erik. -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details.

Jan-Erik wrote: > > Anyway, we still don't know *what* you > are triggering :-) > I'm using an alternistor to trigger a transformer. The transformer supplies the voltage to a 3-phase brushless motor. In normal mode of operation, the alternistor is on for the full 180 degrees. The phase angle is adjusted under certain conditions to reduce the motor voltage. Main purpose is to limit the current to prevent the motor from burning out. The motor draws 10A@36V under full load. Using linear control on the secondary side would require significant heat sinking. Yes, there are other (perhaps better) ways to do this. I am taking over an existing design, and many design decisions were based on product cost. Sound familiar? A buck on the secondary side was considered, but cost analysis showed it was cheaper to use phase control. >> Well, that still needs to be fine-tuned, but initial calculations >> indicate a conduction angle range between 120 and 180 degrees. That >> means worst-case I would be firing 60 degrees from zero crossing. > > 60 deg *before* z-c, not ? > Or 120 deg *after* z-c ? > The 120-180 deg range are the *last* 60 deg of one half-cycle ? > I mean 60 degrees *after* zero crossing. That would give me a conduction angle of 120 degrees. I will be firing anywhere from continuously (180 degrees) to 60 degrees after zero crossing. 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.

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]" <.....peisermaKILLspam.....RIDGID.COM> Sent by: pic microcontroller discussion list <EraseMEPICLISTspam_OUTTakeThisOuTMITVMA.MIT.EDU> 12/10/2003 08:34 AM Please respond to pic microcontroller discussion list To: PICLISTspam_OUTMITVMA.MIT.EDU cc: Subject: Re: [PIC]: zero crossing idea, comments? @spam@llileKILLspamsaltonusa.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. {Quote hidden}> > 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.

I've used that very successfully, but my design used two optos, anti parallel on the diode side and parallel on the transister side. This way I got 120 pulses. I didn't worry about noise - it wasn't something I ran into, and if I got a bad zero cross it didn't have more than a 1/2 cycle delay before getting a correct timing, and I've yet to see a lamp filament that can react that fast. There are other methods to zero cross. The only other isolated method I'm aware of is using the output of the transformer that's powering the PIC. I suspect a simple op-amp or at most comparator circuit would give you the exact zero crossing. For my application I calculated the time the LED would turn off in the opto, and found it reasonably close to later measurements. This way I knew when the zero cross would occur before it actually occured, and I ignored the trailing edge. This could be made automatic, as you suggest. If you'd like the same capability with knowledge beforehand, you can time it each time, and apply it to the next zero cross. Good luck! -Adam Eisermann, Phil [Ridg/CO] wrote: {Quote hidden}> I have the following idea to detect zero-crossing on an AC line >reasonably close. Looking for comments/feedback on any other ideas that >might be implemented. > > 110V input throught two 10K resistors in series an H11AA814A >optocoupler. collector of optocoupler pulled to 5V via a 22k resistor. >collector fed into GP2 a 12F629. the PIC is running on interal clock. GP is >configured as an external interrupt, so it should be a ST. I expect the >pulse length to vary between devices due to variations in current transfer >ratio of the opto. To get a better reading of the actual zero crossing >point, I would measure the typical pulse length before the application does >anything. Half of the pulse length should get me pretty close to the true >zero crossing point. > > The application will remain in service maybe 10 to 20 minutes before >it is powered down. would the oscillator frequency remain stable enough >during this point, or am I making this too complicated? It is for phase >control, and I will always be switching "very soon" after the zero-crossing >point; I do not need a large control range. > > Thanks for any feedback > >Phil Eisermann >Electronics Engineer >The Ridge Tool Company >(440)329-4680 > > >-- >http://www.piclist.com hint: The list server can filter out subtopics >(like ads or off topics) for you. See http://www.piclist.com/#topics > > > > > -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details.

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: {Quote hidden}>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]" <KILLspampeisermaKILLspamRIDGID.COM> >Sent by: pic microcontroller discussion list <RemoveMEPICLISTTakeThisOuTMITVMA.MIT.EDU> >12/10/2003 08:34 AM >Please respond to pic microcontroller discussion list > > > To: spamBeGonePICLISTspamBeGoneMITVMA.MIT.EDU > cc: > Subject: Re: [PIC]: zero crossing idea, comments? > > >TakeThisOuTllileEraseMEspam_OUTsaltonusa.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.