Opto-isolated zero crossing detector
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From: markp on 15 Jul 2010 04:49 "Paul Keinanen" <keinanen(a)sci.fi> wrote in message news:ji7t365771fv3q45d97dcil7n0ar9hgndr(a)4ax.com... > On Thu, 15 Jul 2010 01:53:54 +0100, "markp" <map.nospam(a)f2s.com> > wrote: > >>I have an sinusoidal AC signal between 30V and 160V AC and between 200 and >>600Hz in frequency, and I need an opto-isolated zero crossing detector. >>The >>AC is actually from a transformer output but I don't want to add any more >>windings to it. I do however have a centre tap on the AC output. Does >>anyone >>have any suggestions? > > Use two optoisolators with the photodiodes in antiparallel connection. > When neither phototransistor conducts, you are close to zero. > Thanks, John suggested something similar. I only really need a square wave output though, I can use the edges to detect zero crossing. Mark.
From: John Larkin on 15 Jul 2010 09:51 On Thu, 15 Jul 2010 09:45:38 +0100, "markp" <map.nospam(a)f2s.com> wrote: > >"John Larkin" <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote in message >news:o8os36lqahidlmr0khv46pc7k92lg0r9r3(a)4ax.com... >> On Thu, 15 Jul 2010 01:53:54 +0100, "markp" <map.nospam(a)f2s.com> >> wrote: >> >>>Hi All, >>> >>>I have an sinusoidal AC signal between 30V and 160V AC and between 200 and >>>600Hz in frequency, and I need an opto-isolated zero crossing detector. >>>The >>>AC is actually from a transformer output but I don't want to add any more >>>windings to it. I do however have a centre tap on the AC output. Does >>>anyone >>>have any suggestions? >>> >>>Thanks! >>> >>>Mark. >>> >> >> Ooh, you could use a variant of one of my favorite trick circuits, the >> totem-pole optoisolator: >> >> ftp://jjlarkin.lmi.net/ZCD.JPG >> >> >> John > >Interesting, the idea of back-to-back optocouplers is nice. I've realised I >only need a square wave output because i can use the edges to detect zero >crossing, so a diode follwed by a constant current source (2 PNPs) and then >the optocoupler diode might do. > >Mark. > The Supertex depletion-mode mosfets are interesting as current limiting devices here, back-to-back LND150s maybe. But resistors would be more reliable if, say, really big transients might be possible. One nice thing about the push-pull opto thing is that it doesn't need a lot of current into the optos, so you can use big current-limiting resistors, or the LNDs, off the transformer, as long as you don't get extreme and add too much time delay. You could also do a cheap opamp-based diffamp to sense the transformer voltage, if you don't need true isolation. That devolves to four resistors (two dividers down to logic ground) feeding a comparator. John
From: Joerg on 15 Jul 2010 10:29 markp wrote: > "Joerg" <invalid(a)invalid.invalid> wrote in message > news:8a74q8FbegU1(a)mid.individual.net... >> markp wrote: >>> Hi All, >>> >>> I have an sinusoidal AC signal between 30V and 160V AC and between 200 >>> and >>> 600Hz in frequency, and I need an opto-isolated zero crossing detector. >>> The >>> AC is actually from a transformer output but I don't want to add any more >>> windings to it. I do however have a centre tap on the AC output. Does >>> anyone >>> have any suggestions? >>> >> Why opto? Can't you use a modem transformer that gets a divided-down >> signal at the input and has the proper isolation specs? Then use a >> regular zero-crosser of your liking on the others side where things >> don't have to run isolated. >> >> From a size point of few such a signal transformer and an optocoupler >> aren't grossly different. A little, maybe. >> >> -- >> Regards, Joerg > > It's an option, but may be a little more expensive. I'm sure it can be done > with just an optocoupler. > Sure it can be. But the accuracy suffers greatly. You can only send so many milliamps into an optocoupler and then it signals "out" well before the real zero-crossing happens. -- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM.
From: markp on 15 Jul 2010 11:07 >>> >>>>Hi All, >>>> >>>>I have an sinusoidal AC signal between 30V and 160V AC and between 200 >>>>and >>>>600Hz in frequency, and I need an opto-isolated zero crossing detector. >>>>The >>>>AC is actually from a transformer output but I don't want to add any >>>>more >>>>windings to it. I do however have a centre tap on the AC output. Does >>>>anyone >>>>have any suggestions? >>>> >>>>Thanks! >>>> >>>>Mark. >>>> >>> >>> Ooh, you could use a variant of one of my favorite trick circuits, the >>> totem-pole optoisolator: >>> >>> ftp://jjlarkin.lmi.net/ZCD.JPG >>> >>> >>> John >> >>Interesting, the idea of back-to-back optocouplers is nice. I've realised >>I >>only need a square wave output because i can use the edges to detect zero >>crossing, so a diode follwed by a constant current source (2 PNPs) and >>then >>the optocoupler diode might do. >> >>Mark. >> > > The Supertex depletion-mode mosfets are interesting as current > limiting devices here, back-to-back LND150s maybe. But resistors would > be more reliable if, say, really big transients might be possible. > > One nice thing about the push-pull opto thing is that it doesn't need > a lot of current into the optos, so you can use big current-limiting > resistors, or the LNDs, off the transformer, as long as you don't get > extreme and add too much time delay. > > You could also do a cheap opamp-based diffamp to sense the transformer > voltage, if you don't need true isolation. That devolves to four > resistors (two dividers down to logic ground) feeding a comparator. > > John > I do really need isolation. The reality is the output is a centre tapped winding on a transformer, and I need to be able to tie either the centre tap, or one of the outputs, to ground depending on application. The LND150 looks interesting, they can used as constant current sources at high voltage with a single resistor it seems. My concern with just resistors is this has to work from 30VAC (possibly less) to 160VAC, and to get the gain needed to switch quickly within a few volts would mean lowish value resistors, and then at high voltages would dissipate power. This is why I thought a constant current device would be better. Having said that, replacing your resistors in your zero crossing circuit with LND150s (using the intrinsic diodes to conduct when the polarity is reversed) and using your push-pull approach with schmidtt buffer to reduce noise might do the job! Thanks for suggesting that. Mark
From: Joerg on 15 Jul 2010 11:10
markp wrote: > "John Larkin" <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote in message > news:o8os36lqahidlmr0khv46pc7k92lg0r9r3(a)4ax.com... >> On Thu, 15 Jul 2010 01:53:54 +0100, "markp" <map.nospam(a)f2s.com> >> wrote: >> >>> Hi All, >>> >>> I have an sinusoidal AC signal between 30V and 160V AC and between 200 and >>> 600Hz in frequency, and I need an opto-isolated zero crossing detector. >>> The >>> AC is actually from a transformer output but I don't want to add any more >>> windings to it. I do however have a centre tap on the AC output. Does >>> anyone >>> have any suggestions? >>> >>> Thanks! >>> >>> Mark. >>> >> Ooh, you could use a variant of one of my favorite trick circuits, the >> totem-pole optoisolator: >> >> ftp://jjlarkin.lmi.net/ZCD.JPG >> >> >> John > > Interesting, the idea of back-to-back optocouplers is nice. I've realised I > only need a square wave output because i can use the edges to detect zero > crossing, so a diode follwed by a constant current source (2 PNPs) and then > the optocoupler diode might do. > You can also buy optocouplers that have those back to back diode inside the same device. -- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM. |