Opto-isolated zero crossing detector
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From: markp on 16 Jul 2010 09:00 "markp" <map.nospam(a)f2s.com> wrote in message news:8ab1p0Fa8cU1(a)mid.individual.net... > "John Devereux" <john(a)devereux.me.uk> wrote in message > news:877hkvod3t.fsf(a)devereux.me.uk... >> "markp" <map.nospam(a)f2s.com> writes: >> >>> "John Devereux" <john(a)devereux.me.uk> wrote in message >>> news:87bpa7ofcd.fsf(a)devereux.me.uk... >>>> "markp" <map.nospam(a)f2s.com> writes: >>>> >>>>> "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? >> >> [...] >> >>>>>> >>>>>> 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 impedance only 2kohms. >>> >>> Yes, but the variation in input impedance of the opamp inputs (and hence >>> input biasing and offset currents) will negate the balancing of the >>> 500:1 >>> resistor dividers. To get accuracy from this circuit the impedance of >>> the >>> opamp has to be orders of magnitude greater than 1M. >> >> Hi Mark, >> >> Don't think so - it just needs to be orders of magnitude greater than >> 2k, which is a lot easier :) As a worst case imagine there is a 1M >> resistor across one of the 2k. It only makes a ~0.2% difference to the >> ratio. >> >> -- >> >> John Devereux > > OK. Are you sure that input offset currents are not going to cause any > problem? If the common mode causes the inputs of the opamp to both be > above 2.5V then to get positive input bias and offset currents this has to > come from the 1M resistors. Is that not going to perturb the accuracy? > > Mark. Actually forget that. Since I'm strapping one of the outputs or the centre tap to ground, the point of accuracy will be when both inputs are at ground. As long as I don't strap these to any other potential and my logic ground is close to the strap potential it looks like it would be OK. Mark.
From: John Devereux on 16 Jul 2010 10:09 "markp" <map.nospam(a)f2s.com> writes: > "markp" <map.nospam(a)f2s.com> wrote in message > news:8ab1p0Fa8cU1(a)mid.individual.net... >> "John Devereux" <john(a)devereux.me.uk> wrote in message >> news:877hkvod3t.fsf(a)devereux.me.uk... >>> "markp" <map.nospam(a)f2s.com> writes: >>> >>>> "John Devereux" <john(a)devereux.me.uk> wrote in message >>>> news:87bpa7ofcd.fsf(a)devereux.me.uk... >>>>> "markp" <map.nospam(a)f2s.com> writes: >>>>> >>>>>> "John Larkin" <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote in >>>>>> message >>>>>> news:ah7v369s5g6m9s08rv2of08h1imffm5vvv(a)4ax.com... [...] >>>>>>> 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 impedance only 2kohms. >>>> >>>> Yes, but the variation in input impedance of the opamp inputs (and hence >>>> input biasing and offset currents) will negate the balancing of the >>>> 500:1 >>>> resistor dividers. To get accuracy from this circuit the impedance of >>>> the >>>> opamp has to be orders of magnitude greater than 1M. >>> >>> Hi Mark, >>> >>> Don't think so - it just needs to be orders of magnitude greater than >>> 2k, which is a lot easier :) As a worst case imagine there is a 1M >>> resistor across one of the 2k. It only makes a ~0.2% difference to the >>> ratio. >>> >>> -- >>> >>> John Devereux >> >> OK. Are you sure that input offset currents are not going to cause any >> problem? If the common mode causes the inputs of the opamp to both be >> above 2.5V then to get positive input bias and offset currents this has to >> come from the 1M resistors. Is that not going to perturb the >> accuracy? It is true that the 1M resistors need to be matched to 1% or 0.1%, as JL has shown. So if you think you could get <<100Mohm across one of them, due to surface contamination perhaps, then it is no good for you. But I was addressing the impedance of the *amplifier* and its nodes, which is what you were worrying about above. >> >> Mark. > > Actually forget that. Since I'm strapping one of the outputs or the centre > tap to ground, the point of accuracy will be when both inputs are at ground. > As long as I don't strap these to any other potential and my logic ground is > close to the strap potential it looks like it would be OK. > > Mark. > > -- John Devereux
From: John Larkin on 16 Jul 2010 10:12 On Fri, 16 Jul 2010 12:33:06 +0100, "markp" <map.nospam(a)f2s.com> wrote: >"John Devereux" <john(a)devereux.me.uk> wrote in message >news:87bpa7ofcd.fsf(a)devereux.me.uk... >> "markp" <map.nospam(a)f2s.com> writes: >> >>> "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 impedance only 2kohms. > >Yes, but the variation in input impedance of the opamp inputs (and hence >input biasing and offset currents) will negate the balancing of the 500:1 >resistor dividers. To get accuracy from this circuit the impedance of the >opamp has to be orders of magnitude greater than 1M. > >> >>> 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. >>> >> >> John Devereux >Mark. > Use a CMOS opamp or comparator. Since you're apparently going to ground one end of the transformer or the center tap, this just has to work. All three cases poke nice signals into the comparator, even if the voltage dividers use 5% resistors. John
From: John Larkin on 16 Jul 2010 11:01 On Fri, 16 Jul 2010 15:09:42 +0100, John Devereux <john(a)devereux.me.uk> wrote: >"markp" <map.nospam(a)f2s.com> writes: > >> "markp" <map.nospam(a)f2s.com> wrote in message >> news:8ab1p0Fa8cU1(a)mid.individual.net... >>> "John Devereux" <john(a)devereux.me.uk> wrote in message >>> news:877hkvod3t.fsf(a)devereux.me.uk... >>>> "markp" <map.nospam(a)f2s.com> writes: >>>> >>>>> "John Devereux" <john(a)devereux.me.uk> wrote in message >>>>> news:87bpa7ofcd.fsf(a)devereux.me.uk... >>>>>> "markp" <map.nospam(a)f2s.com> writes: >>>>>> >>>>>>> "John Larkin" <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote in >>>>>>> message >>>>>>> news:ah7v369s5g6m9s08rv2of08h1imffm5vvv(a)4ax.com... >[...] >>>>>>>> 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 impedance only 2kohms. >>>>> >>>>> Yes, but the variation in input impedance of the opamp inputs (and hence >>>>> input biasing and offset currents) will negate the balancing of the >>>>> 500:1 >>>>> resistor dividers. To get accuracy from this circuit the impedance of >>>>> the >>>>> opamp has to be orders of magnitude greater than 1M. >>>> >>>> Hi Mark, >>>> >>>> Don't think so - it just needs to be orders of magnitude greater than >>>> 2k, which is a lot easier :) As a worst case imagine there is a 1M >>>> resistor across one of the 2k. It only makes a ~0.2% difference to the >>>> ratio. >>>> >>>> -- >>>> >>>> John Devereux >>> >>> OK. Are you sure that input offset currents are not going to cause any >>> problem? If the common mode causes the inputs of the opamp to both be >>> above 2.5V then to get positive input bias and offset currents this has to >>> come from the 1M resistors. Is that not going to perturb the >>> accuracy? > >It is true that the 1M resistors need to be matched to 1% or 0.1%, as JL >has shown. If, as he says, he will always ground one end of the transformer or the ct, 20% resistors would work fine. John
From: quiettechblue on 17 Jul 2010 13:25
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. > Let's see, current limiting resistor, into bridge with 5V zener and opto. Maybe a resistor across the zener. |