Interrupts question

[Charles Linquis]

I'm in agreement that a .47uF cap works well, but you still have to have some sort of resistor in series (1K or so), otherwise really short risetimes will blow the LEDs. I use resistors here because they are smaller than a 400V cap.

RE: Transformers - I needed to measure the "quality" of the AC line. I thought I would use a cheap transformer. The transformer would power both my PIC and provide the low-voltage isolated signal to the PICs A/D.
I realized that the peaks would be chopped off by the bridge rectifier as the voltage rose a diode drop higher than the main storage capacitor, so I put a 150 ohm resistor in series with the diode to "soften the blow". The
PIC didn't take much current, anyway, so the extra drop of the resistor wasn't a problem.
But I was mistaken! Even with the current limit, the waveform coming from the transformer was horribly distorted. I wound up using a (TAMURA brand) transformer that was about 6X as big (in current ability) than I needed, and one that had TWO separate secondaries. One secondary ran the PIC, and I used the other to measure the line. The "unloaded" secondary fed a bridge rectifier that had only a 680 ohm resistor as a load. That worked.

[mister_e]

Good point and tips there. I thought about the dual winding for my application but I needed it really small to fit in an existing installation (kind of reverse engineering job), none of the dual secondary winding tranformer I found locally were small enough, the transformerless solution was REALLY not an option and everything was for...yesterday (yeah it happen ) ... so in the end I added a TLP2098 (if my memory serves me well) i had in stock, problem solved.

If I had a toroidal one on hand, I would have try to wind a seconday on the top for testing purpose, maybe it would have work... or really not hahaha.

[Charles Linquis]

I couldn't use an opto for my particular application because I needed to know the peak voltages of the AC waveform, cycle-by-cycle.

[alec]

Hi All, and apologies for not getting back to you all sooner (damned work!!)
I hate to admit this, but a wire had gotten disconnected on my bread board - amazing how much better a PIC works when it has a five volt supply...
My zero crossing detector works great, and I'm getting a nice clean pulse train to my triac optocoupler. Now to go back to the code to make my kitchen heater controller - I'm making a sous vide controller for a crockpot..
Thanks again for all the help - before I tear hair out again I'll make sure to check my hardware first...
Alec

[alec]

BTW - I'm using a PS205-1 AC input photocoupler. Works great with a 10k 1 watt resistor in the input.
Alec