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### Topic: Zero crossing detector (Read 2084 times)previous topic - next topic

#### nesh_reds

##### Apr 23, 2013, 03:06 pm
I am doing a gate controller circuit for SCR. I will be needing a zero crossing detector from the main to detect the sine so I could synchronize with my arduino to fire the scr at the perfect time. But so far the i have tried using basic zero crossing circuits like comparator comparing the sine wave with a ground but to no avail. Any other suggestions?

#### Boardburner2

#1
##### Apr 23, 2013, 07:44 pm
http://www.dextrel.net/diyzerocrosser.htm

#### KeithRB

#2
##### Apr 23, 2013, 08:06 pm
There is a simple one using an XOR gate in this old issue of byte here:
http://www.strikequick.com/BYTE/BYTE%20Vol%2000-07%201976-03%20Cassette%20Interfaces.pdf

It is the bit-boffer on page 20.

#### runaway_pancake

#3
##### Apr 23, 2013, 08:34 pm
H11AA1

http://www.jameco.com/Jameco/Products/ProdDS/18825.pdf
"Who is like unto the beast? who is able to make war with him?"
When all else fails, check your wiring!

#### DVDdoug

#4
##### Apr 23, 2013, 09:10 pm
I assume the input to your comparator is isolated from the AC?  i.e. From the secondary of a transformer?

Quote
But so far the i have tried using basic zero crossing circuits like comparator comparing the sine wave with a ground but to no avail.

That's how I did it... Is the comparator "firing"? (Is the output changing state?)   If the comparator is running from a single supply, it might not work near ground.    If the comparator is running from positive & negative supplies, it should work fine and put-out a square wave.

If you have a single supply, you can set the reference voltage higher, and calculate when the actual zero--crossing occurs.

Actually, it's probably better if you set the comparator reference to around 45 degrees (half the peak AC voltage).      The actual-exact zero-crossing is difficult to detect.  If your detection circuit is detecting small voltages, a little noise on the AC line near the true zero-crossing can give you a false-trigger.   The peak is even more difficult, since the AC voltage vaires a bit.   Somewhere in-between is more ideal.

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