If you are designing a light dimmer that needs to use phase control for
dimming you need to use a non zero cross opto. It allows for random
triggering of the TRIAC or whatever is attched to the opto (you might be
using 2 SCR's for dimming).  Zero cross optos of course trigger on the zero
cross only

-----Original Message-----
From: pic microcontroller discussion list
[mailto:PICLIST@MITVMA.MIT.EDU]On Behalf Of michael brown
Sent: Saturday, September 13, 2003 5:52 PM
To: PICLIST@MITVMA.MIT.EDU
Subject: [PIC:] MOC30xx How do I pick one?


Hello all,

What is the difference between the MOC30xx's?  I've noticed that some
have zero-crossing circuitry built in, and some don't.  Since the
zero-crossing circuitry doesn't appear to connect to any of the external
pins of the MOC30xx chips, what purpose does it serve.  I've looked at
the datasheets, but I don't see the purpose of the zero cross circuitry.

Is it so that when I tell the MOC to turn on, it waits until the next
zero-cross before actually turning on it's internal TRIAC, or is there
some other reason?

I was poking around on the net and thinking that building some type of
light dimmer might be fun and educational.  I'm far from being expert
with analog devices, but I think I have a basic understanding of a
TRIAC.  Since the circuits I've seen already use a TRIAC (in combination
with a MOC30xx) to switch the high voltage, why would the MOC need to
know anything about zero-crossing?

michael brown (confused)

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