Detecting 240V with a PIC (Part2)

[Melanie]

There are advantages and disadvantages to both methods, and both methods are equally valid in their use.

The Capacitor... doesn't generate heat, but costs 20x the price of a Resistor. You need to use an X2 grade because if it fails, it will take out the protection Diode and the Opto.

The Resistor... is far smaller and a lot cheaper, but it's entire raison d'entre is to dump heat into the planet, so it may not be the most appropriate component to use if for example you're planning to use a sealed unvented box. At 240V, you're going to have to deal with around 0.2W being wasted. Now, you have two dozen opto channels monitoring two dozen mains Fuses, and you suddenly have a fair amount of heat being generated.

[The Master]

I think i had better stick with the capacitor then. For a start ive actually got the capacitor to work with an LED while the resistor only blew up. The capacitor circuit does use a resistor too and the website i got the circuit from says that resistor acts like a kind of fuse. The whole circuit will be inside an IEC plug so yes, it will be a small enclosed space. I was also thinking of putting some kind of glue or silicon stuff over the whole thing but ill decide on that later.

The capacitor i was using costs 9p. If i buy 100 of them then its down to 6.8p. I guess thats not so bad. Its not like i plan to sell thousands of them where every penny counts. I was hoping to have another capacitor anyway to keep the opto on during the zero crossover. Compared to the rest of the circuit 9p is nothing.

[Melanie]

> the website i got the circuit from says that resistor acts like a kind of fuse

Well, if you know your X2's then you'll realise that it has a Fuseable Resistor inside it already... that's the bit that makes it fail-safe.