Author Topic: Zero Crossing implementation ideas  (Read 599 times)

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Offline generic_usernameTopic starter

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Zero Crossing implementation ideas
« on: October 22, 2019, 11:05:01 am »
Hello everyone,

I have a sensor which is disturbed by my mains earth frequency lucky me I have a sync Pin to sync the sensor with 50/60 Hz So I tried that to sync the sensor IC with a zero crossing circuit on a rather crude setup and it worked way better than anticipated. But on my actual circuit I only have
 1 AC line to switch a Triac available
https://www.onsemi.com/pub/Collateral/FODM3083-D.pdf
at page 8.
So the ZC circuit I used can't be implemented, and I doubt I can get L and N of mains earth on my PCB due to various resons.


One Solution would be if I could have a zero crossing triac driver which would have an output for when the ZC is- I couldn't find any so far that would have this ability (and I doubt I will find one)


I have found a workaround for this probelm, but I'm not really happy with it.

I probed my analog signal going to my sensor IC and noticed that the AC line noise is really huge (around 1V PP ) when the sensor IC is disconected/ turned off. So I used this signal and amplified and filterd it and to my surprise I coud actually do a zero crossing with that signal. I only can use this signal to get the sync started with a µC and then disconnect my circuit and connect the measurement IC but that would be feasable...I think.
So I have this solution which worked in a quick and dirty test but I doubt this will really work all the time.


So I'm a bit of a loss right now how to make that work other than this method?
Any Ideas would be welcome.

cheers.




I always need 3 attempts to plug in a USB connector
 

Offline Ian.M

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Re: Zero Crossing implementation ideas
« Reply #1 on: October 22, 2019, 11:52:32 am »
If your board 0V is grounded via its PSU, tap the AC line via a class Y capacitor (so it doesn't fail any DC Hi-Pot insulation tests) and a chain of high value series resistors to keep the current under 75uA RMS (10% max permitted leakage current for most applications),  Zener clamp it and feed a comparator input to get a line frequency reference you can PLL your sampling to.  You need a resistor chain rather than one resistor so it can withstand >1KV fast transients on the mains supply. The capacitor impedance at your line frequency should be several orders of magnitude below that of the resistor chain for negligible phase shift.

If your board 0V is floating, consider a conventional optoisolated zero crossing circuit across the TRIAC + an isolated current sensor in series with it.  One or the other should always give a result if the load remains connected.  You may need to compensate for any phase shift of the current.
 

Offline generic_usernameTopic starter

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Re: Zero Crossing implementation ideas
« Reply #2 on: October 22, 2019, 12:36:55 pm »
Thanks a lot

the class Y cap sadly won't be possible as I have a really tight height restriction, but the current Sensor may be worth a shot   :-+
I always need 3 attempts to plug in a USB connector
 


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