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[SOLVED] Favorite low phase shift 240Vrms isolated zero cross detector

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Don't you need at least one extra diode anyway since the optos surely can't block mains? So rectifier would be one more......
No because two opto's are used with the leds connected in inverse parallel.

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Hi,

To be true I didn'd go through each word in this thread.
What I am missing: There is a requirement for "low phase shift" ...but I don't see any specification.
And I can't find the information what the signal is used for...

Without a value you may discuss a long time and never get to an end.

****
Zero cross.
I'm designing high precision phase control for the industry. And I use zero cross circuits to align the firing angle oir the SCRs.
I found out, that the "phase shift" is a minor problem for a precise regulation.

There are other, like
* timing jitter caused by overtones / distortion of the input (mains) signal
* multiple zerocross trasitions
* voltage fluctuations of mains, causing output signal fluctuations (even with precise phase)

Klaus

Hello Klaus thank you for your reply. For a specification I could try a transition <=100uS either side of zero cross in the most economical least cost manner. I have a circuit that works reasonably well BUT the power dissipation (~1.5W) is a bit of a problem on a compact circuit board with adjacent components, mostly because I laid out the PCB before I realized how low a resistance I would need to close the deadband gap.
As for the application lets just say the wider the deadband the more distortion occurs at zero cross as the resulting signals are used to drive an unfolder.
A lot of filtering and noise rejection is done in software BUT the timing accuracy of the basic signals is still important.
The picture (if I can load it) shows 240V grid and a software representation of zero-cross)


ZX_blank red GRID yel 40K_15nF pha.png
 

I found out, that the "phase shift" is a minor problem for a precise regulation...

Very correct.

A sine wave has a large slope near zero crossing (largest actually). The voltage rises very fast and unless you are trying to get close to 99% (i.e., within 2 degrees or so), it may not be a serious problem.
 

It depends on the application. I've seen zero crossing detection used as a means of synchronizing multiple systems that are separated by large distances where absolute phase accuracy matters.



Ok so here is my best shot for you. I think you're going to continue to have problems relying on the opto turn-on for the threshold. This adds a bunch of components but with careful selection they can all be very small and cheap. Because with the zener outside the rectifier all the rest sees less than 15V. The comparator too is 'looking' at the clamped 15V version of the line so its 100k resistors can be any package.

Besides improving accuracy this reduces power because you need only enough current for the opto and no extra (so 1mA is fine).

This shows one additional trick which is to AC couple the comparator to the opto so it sends only a short pulse at the crossing. This lets you increase the peak opto current for sharper turn-on while reducing the overall power consumption. You can see this is running with 200k resistors.

You can see the red (opto output) negative going edges align well to the line positive going zero crossings.
Opto Zero Cross With CMP.JPG
 

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Hi asdf44 thanks for your hard work, could you perhaps run that sim again but no more than +-500uS either side of zero cross so I can see the timing in more detail ?
Unfortunately I should have mentioned I also need a small amount of lead (that probably knocks out the current regulators) because I need to do some tidying up before zero cross actually happens!! I had tried predicting it with a PLL but in practice the frequency wander (grid) is enough to spoil that scheme in practice.

Here is my current circuit and simulation result (actual scope shots are similar)
Basic_ZCD copy.png
Basic_ZCD copy2.png
 

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Here is another screenshot with a slightly modified circuit.
-Comparator replaced with fets for cost
-Two optos shown
-Optional RC AC coupling network gives the diodes a 'kick', reduces steady state losses
-Could combine fets into dual package
-Can get all 5 diodes in a single TVS diode array such as (Wurth 824015)

With the package optimizations mentioned above this seems like a decent solution. I didn't study the sim too closely, the timing looks good, almost too good considering it has no leading C's.

Finally I'll mention that another option would be to use a small pulse transformer and push short pulses through it every zero crossing. Basically the circuit stays the same but the LED's get replaced with coils in a single transformer (and the DC coupling resistors get removed - AC only).

Opto Zero Cross With CMP2.JPG
 
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Ohh that is very neat! I like the decoupling of the switching point from anything to do with the opto's. Many thanks I shall certainly experiment, for now I just put up with the hot resistors until I get some more parts.

In my case I use the opto signals directly to interlock some hardware in case of software/dsp failure so pulses are not good and the long turn off time of this circuit would be a problem, however for anybody wanting accurate zero-cross pulses this would be very good indeed as the turn off time could be almost half a cycle and not matter.
 
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