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Topic: AVR zero-cross detection on 240 "household" ac (Read 1 time) previous topic - next topic

qwerdy

Jun 03, 2012, 03:24 pm Last Edit: Jun 03, 2012, 03:26 pm by qwerdy Reason: 1
Hello, I need a little help on this topic.

I know you can detect it this way:


But will the internal clamping diodes protect correctly when using 240 volt ac, and there's no common wire to use on ground?

Or will i need to use a rectifier (full / half wave) ?

This is how the 240 household ac looks like:

Runaway Pancake

Don't do that at home - or anywhere else.

There's an example of a zero-cross detector as part of a project --
http://arduino.cc/playground/Code/ACPhaseControl

My only quibble is the AC isn't shown fused.  Some would argue two resistors on the input, one on each end.
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qwerdy

I know the best is to use an optocoupler (I'm currently waiting on my order for a MOC3022 and a H11AA1)

But if i theoretically dropped the optoisolators, would the above circuit need any changes in my case?

Runaway Pancake

Let's say the internal clamping diodes are perfect and tough as nails, there would be a HIGH all through one alternation and a LOW all through the other alternation.
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qwerdy

oh, ok. So i guess it's pretty straight forward then.

Guess i was thrown off by the fact that in every other example of this, they used it with 110 and a common (neutral?) wire.

But won't the alternation causing LOW hurt the circuit in my case, as the polarity change?

Runaway Pancake

The point that I was making was that if this scheme was valid, given perfect and tough-as-nails clamping diodes (which is definitely NOT the case), it would still not "zero-detect": Given a 50 Hz cycle, the period of which is 20msec, there'd be a "high" for 10msec and a "low" for another 10msec - it would "positive alternation detect". 

In any event, I do not countenance your proposed circuit.  Just wait for the components you've ordered.
"Hello, I must be going..."
"You gotta fight -- for your right -- to party!"
Don't react - Read.
"Who is like unto the beast? who is able to make war with him?"

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