🚀 go-pugleaf

In article <1992Aug21.222100.15735@sics.se> boortz@sics.se (Kent Boortz) writes:
>
>Hi,
>
>I will interrupt a microcontroller at 220V AC zero crossing time to reset
>an internal timer. Any ideas of a simple but reliable zero crossing detection
>circuit? The triggering has to be isolated with a optocoupler. The 220V
>will beside driving a halogen lamp be transformed down to 9V AC to drive
>the processor etc. Could I detect the zero crossing point on the 9V side
>or will there be a phase shift because of the induction in the transformer
>and the load?

You can just run that 220V AC straight to an opto-coupler through a BIG
resistor.  (220K comes to mind...)  If your opto-coupler only transfers
in one direction (i.e., there's only one LED inside instead of 2 back to back),
you should make sure that that LED can handle 220*1.5V reverse voltage.  It
probably can't, so you'll need to use an extra diode.

I've used some cheap NEC optocouplers (sold by DigiKey, page 77 in their
catalog) to do this.  (I used the 2502 and 2506 models -- they have Darlington
outputs so that you only _need_ 1ma of that 220V, and the less 220V I need,
the happier I am!)  The output is rather distorted (they aren't perfectly
linear).  What you want to do, though, is run this through a series
capacitor so that you get a differentiator.  Now, feed those pulses to a
comparator, and you'll all set!

As for the second question... yes, you could take the zero crossing from the
low voltage side.  And yes, it _could_ be out of phase depending on your
transformer and the load.  It may be best to look at the voltage vs. current
on an o-scope and see if you care...

I assume that you want to do more than just get a timer reset pulse though,
right?  If you use that timer to, say, control pulses to a TRIAC or something,
the low voltage side will most definitely look different that the primary.
(And if you _are_ firing TRIACs, remember to put some filtering into your
system so that we can all still listen to our radios without too much 
static. :-) )

						---Joel Kolstad