Opto-isolated zero crossing detector

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.

Reply to
markp
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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").

Best regards, Spehro Pefhany

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Reply to
Spehro Pefhany

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.

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Regards, Joerg

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Reply to
Joerg

Ooh, you could use a variant of one of my favorite trick circuits, the totem-pole optoisolator:

ftp://jjlarkin.lmi.net/ZCD.JPG

John

Reply to
John Larkin

How close to "zero" does it have to detect? The output has to be isolated from both the primary and secondary?

Reply to
krw

Use two optoisolators with the photodiodes in antiparallel connection. When neither phototransistor conducts, you are close to zero.

Reply to
Paul Keinanen

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.

Reply to
markp

It's an option, but may be a little more expensive. I'm sure it can be done with just an optocoupler.

Mark.

Reply to
markp

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.

Reply to
markp

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.

Reply to
markp

Thanks, John suggested something similar. I only really need a square wave output though, I can use the edges to detect zero crossing.

Mark.

Reply to
markp

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

Reply to
John Larkin

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.

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Regards, Joerg

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Reply to
Joerg

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

Reply to
markp

You can also buy optocouplers that have those back to back diode inside the same device.

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Regards, Joerg

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Reply to
Joerg

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.

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Regards, Joerg

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Reply to
Joerg

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 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

Reply to
John Larkin

How close to zero?

Reply to
krw

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

Reply to
John Larkin

Hey, Joerg, Does that look medical grade to you ?:-) Looks like another crap Larkin circuit to me :-) ...Jim Thompson

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| James E.Thompson, CTO                            |    mens     |
| Analog Innovations, Inc.                         |     et      |
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Jim Thompson

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