the safest way is to use an opto-coupler.

I have vast experience in this field (dimmers in professional lighting field)
and if you only knew what comes down those AC lines...... you'd use an opto
too.
Nicest way of doing it is to put the INT pin via a zener and res after
step-down trafo, separated by a diode from the main storage caps.

Microchip say you can do it with just a resistor, and that it will be ok (
reasoning is: high impedance resistance.. a few megs, capacitance of pin for
slew limit, maximum current through pin is ok and clamps to PSU rail, due to
certain circumstances use 2 diodes to strap the pin to v+ and 0V ). I tend to
agree with them. The diodes internally vary from port type to port type so
BEWARE.

This is also documented in the Microchip AppNotes.

The safest way of doing it is  via an opto-coupler, which needs a reverse
diode accross the internal LED, and suitable current limit resistor so as not
to get too much delay on the sync relative to the 0 point on the wave.

Regards,
John

On Monday 04 November 2002 05:21 pm, you wrote:
> Roman,
>
> > The zero cross detect, the power supply is a capacitve reactance type.
>
> You have no issues with these?
>
> > I know. It's an expression of speech. :o)
>
> Yes, I know. But you came across as if I was trying to shove this upon
> the list, and that is not what I was trying to do.
>
> > Yes opinions vary, we have been discussing this
> > for months (years?) now on the list with wide
> > varying opinions.
>
> Sorry. I did not realise it was an old topic. I must have been napping.
> I will try to search my archives, then the PIClist.
>
> > The PIC doesn't have a zener.
>
> I was referring to the Zener in Paul Benford's circuit. Isn't it
> holding VDD at 5.6 volts relative to "0 volts" connected to VSS? But
> then this would also mean that the neutral can swing from zero to
> (339.5 - 5.6) as referenced to zero volts right? And that is what you
> are getting at. You do not trust the internal clamp diodes to clamp
> this off - even when it appears in many Microchip application notes and
> those of third parties?
>
> >  It has clamp diodes,
> > very different story. :o) In the circuit you posted
> > (yes I know it's not your circuit, sorry if I gave
> > that impression)
>
> No. I just don't want to be seen to be putting up someone else's work
> without giving them the credit - whether it's good or bad! :-)
>
> > the circuit is live to AC mains
> > active, and the Xc supply and zero cross detect
> > are driven from the neutral. This has EXACTLY the
> > same effect as ac mains is, well, ac. :o)
>
> Yes. Agree.
>
> > The PIC pin is connected to the AC mains full potential
> > via just one series resistor.
>
> Yes.
>
> > My reasons for an external zener;
> > * much larger silicon and testable
> > * can be physically away from PIC
> > * WILL clamp 5.1v, even fast spikes
>
> OK, thanks.
>
> > And a capacitor across the zener will give some
> > spike protection for negative spikes and reduce
> > zener surge current for all spikes + and -.
> > These two parts cost a few cents and go some way
> > to preventing the AC mains killing the PIC. :o)
>
> OK, Agreed. I don't care about the PIC as much as I do as making a safe
> product and one that will pass standards.
>
> > I've changed the topic as I know lots of people
> > have their own ideas and may want to add...
>
> OK
>
> > Russell has posted a long and detailed post on
> > the topic recently.
>
> What was the subject? I can not find it on my machine at home. Will
> check in the office tomorrow.
>
> Regards,
>
> Sean

--
Thank-you for your time.

John Ward

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