Opto-isolated zero crossing detector
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From: Paul Keinanen on 15 Jul 2010 01:41 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.
From: markp on 15 Jul 2010 04:40 >> >>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. >> > > Maybe a bridge rectifier, resistor or FET current limiter, and an > optocoupler. Depends a bit on how close to the zero you want to go > (should the pulses vary in width with voltage, for example, and at > what voltage threshold should the output go "true"). > > Yes, that sounds like an option. In fact I only need a square wave output, I can use the edges to detect zero crossing. So I'm thinking now of a diode with a constant current source (standard two back-to-back PNP transistors circuit) then the optocoupler diode. The constant current source will allow more gain at lower voltage so getting closer to zero when switching, and would limit the current at higher voltages. Mark.
From: markp on 15 Jul 2010 04:42 "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. Mark.
From: markp on 15 Jul 2010 04:45 "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.
From: markp on 15 Jul 2010 04:48
<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. Mark. |