llile@saltonusa.com wrote:
> I would recommend sensing zero crossings every time:
>
> 1.  You;ve already spent the money to sense them, it costs no parts
> to sense them again.
>

yes, that is my intent.

> 2. Your first measurement might be a noise hit, why be wrong twice?
>

I would average the readings over several cycles. say 32 or 64

> 3. You can get fancy and gate your measurements after the first, if
> the zero crossing is not at the right time, it is spurious.  this is
> probably overkill.

That's a good idea. Will have to see if it becomes necessary.

>
> 4. You really really can sense zero crossings with a 2meg resistor to
> the PIC pin.  Unless you have regulatory requirements for isolation
> (being Ridge Tool CO I assume you do) this is very simple.   But the
> questions is, would the 2meg resistor provide enough isolation?  Or
> two 1meg resistors, for redundant safety? I've done this and had
> success with it.  Mchip has an app note on the technique. resistors
> must be rated for 120V service, 0805's don't have enough creepage
> distance.  I used 1/4 watt resistors to maintain plenty of creepage.
>

yes, PICREF-4. I started on that path, figuring that would make a
good starting point. However, this project actually has two PICs in
it, so it is easier in this case to put the phase control on the
isolated side. And we do have mandatory regulatory requirements.

regardless of whether Mchip has published this app note, it's out
of spec. You're relying on the protection diodes to clamp the
voltage, and that means they are conducting. I agree if you limit
the current enough, it will probably work most of the time. But
i don't get paid to design products that probably work most of
the time :)

>
> BTW, where is Ridge Tool Co Located?

We are worldwide, but headquartered in Elyria, OH. About 20 min
west of Cleveland.

Thanks for the feedback everyone.

Phil Eisermann
Electronics Engineer
The Ridge Tool Company
(440)329-4680

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