Opto-isolated zero crossing detector
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From: Joerg on 15 Jul 2010 11:12 markp wrote: > "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. > But keep in mind that the "zero" indication will be quite wide, you won't be able to nail that to fractions of a degree unless you watch when it comes back and calculate the middle via a uC timer or something. -- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM.
From: John Larkin on 15 Jul 2010 11:36 On Thu, 15 Jul 2010 16:07:37 +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 >> > >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. Two voltage dividers would give pretty good common-mode rejection, probably good enough that you could ground the transformer anywhere, or float it, and not see much difference at the comparator output. Use 1% or even 0.1% resistors. > >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 > > The LND150s run pretty consistantly around 1.4 mA Idss. Since each one in a series pair dissipates power half the time, the SOT23s would be fine. There would be a pleasing symmetry if each of the resistors in my sketch were replaced by one fet. John
From: krw on 15 Jul 2010 18:52 On Thu, 15 Jul 2010 09:48:31 +0100, "markp" <map.nospam(a)f2s.com> wrote: > ><krw(a)att.bizzzzzzzzzzzz> wrote in message >news:0ups36de7ngmg9bcb7u49u5nqol0m8bk12(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? >>> >> How close to "zero" does it have to detect? The output has to be isolated >> from both the primary and secondary? >> > >The output from the transformer is essentially floating, but can have either >output connection, or the centre tap, strapped to earth. So the feedback has >to be isolated from the output side and produce a signal referenced to the >secondary input drive ground. How close to zero?
From: John Larkin on 15 Jul 2010 19:49 On Thu, 15 Jul 2010 09:49:35 +0100, "markp" <map.nospam(a)f2s.com> wrote: > >"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. > This should work. ftp://jjlarkin.lmi.net/ZCD_2.JPG You don't need galvanic isolation, you just need a little common-mode rejection. John
From: markp on 16 Jul 2010 06:37
"John Larkin" <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote in message news:ah7v369s5g6m9s08rv2of08h1imffm5vvv(a)4ax.com... > On Thu, 15 Jul 2010 09:49:35 +0100, "markp" <map.nospam(a)f2s.com> > wrote: > >> >>"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. >> > > This should work. > > ftp://jjlarkin.lmi.net/ZCD_2.JPG > > You don't need galvanic isolation, you just need a little common-mode > rejection. > > John > Thanks for this. I think this circuit might work, but I worry about the opamp/comparitor (which I think might have to be an instrumentation type) being very high impedance and the possibility of noise picked up in that area from elsewhere. I'd rather have true isolation to be honest. The original back-to-back optoisolator variant looks good, possibly with FET current limiters in place of the resistors and I may use an opamp configured as a schmidtt buffer with a fixed mid-point reference rather than a gate. In any case I'll try to do some LTSpice simulation if I can get the models or create them. Mark. |