Eisermann, Phil [Ridg/CO] wrote :

> 110V input throught two 10K resistors in series an H11AA814A
> optocoupler. collector of optocoupler pulled to 5V via a 22k resistor.

Or use the "weak pullup" and save a resistor...

> collector fed into GP2 a 12F629. the PIC is running on
> interal clock. GP is configured as an external interrupt,
> so it should be a ST. I expect the pulse length to vary between
> devices due to variations in current transfer ratio of the opto.
> To get a better reading of the actual zero crossing point, I would
> measure the typical pulse length before the application does
> anything. Half of the pulse length should get me pretty close
> to the true zero crossing point.

So you'd detect the start of the zero-crossing, and then wait for
half of the the former calculated zero-crossing width ? Seem OK.

Note that you can flip the INTEDG on each interrupt to capture
*both* edges using interrupts.

>
> The application will remain in service maybe 10 to 20
> minutes before it is powered down. would the oscillator frequency
> remain stable enough during this point,

Stable enough for what ?
Are you thinking on "free wheeling" during this 10-20 min period
without further zero-crossing detection ?

> or am I making this too complicated? It is
> for phase control, and I will always be switching "very soon"
> after the zero-crossing point;

What are you switching ? And what is "very soon" ?

For a Triac, you must at least wait until there is enough current
to hold the Triac conducting.

> I do not need a large control range.

What is "a large range" ? Or rather, what is "not a large range" ?

Jan-Erik.

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