Opto-isolated zero crossing detector
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From: John Devereux on 16 Jul 2010 07:06 "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. > 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
From: markp on 16 Jul 2010 07:33 "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.
From: Jan Panteltje on 16 Jul 2010 07:47 On a sunny day (Fri, 16 Jul 2010 12:33:06 +0100) it happened "markp" <map.nospam(a)f2s.com> wrote in <8aatssFibmU1(a)mid.individual.net>: >> 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. Bollucks.
From: John Devereux on 16 Jul 2010 07:55 "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
From: markp on 16 Jul 2010 08:39
"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. |