| ??? 05/05/10 14:04 Modified: 05/05/10 14:15 Read: times |
#175707 - Then take a better opto! Responding to: ???'s previous message |
Arun said:
The first reason i avoid optos for ZCD is the coarseness, in this case its 25V which is roughly 10% of 240VAC. It's only 7% of the peak voltage, corresponding to 4% phase angle. Again, take a better opto, like the IL755-2, with a current transfer ratio of >1000% at 1mA, if you want to go closer to the zerro crossing. Arun said:
Second,heating in the current limit resistors, for which one has to go for double the actual required wattage - increased cost and board space Entirely irrelevant, if you take into account what the triac's heat sink will dissipate. And the board space is wasted by the heat sink and the choke, not by two resistors. Arun said:
third, aging of the led in the opto. Entirely irrelevant, if you plan only 8 steps of brightness. The actual phase angle shift of zero crossing detector is irrelevant, because you can correct this very easily in the code. Just measure the length of the high pulse, by periodically polling the corresponding port pin for a few periods after power on and take the half of this pulse length. By this you will automatically take into account even the aging of opto. What you plan with the NPN or MOSFET isn't much better either. If you take a NPN with a current gain of 100 and want to switch a 10k pull-up, then about 5V will drop across the 1M base resistor. If you take a MOSFET, then you will introduce an additional phase shift. Also, the turn-on voltage is higher than that of a NPN. There's another reason, why all too high impedant loading isn't a good idea. Assume a stray capacitance of about 0.3pF across the 1M resistor and a gate source capacitance of 30pF. Then, this forms a capacitive voltage divider, which makes the MOSFET turn-on during brief overvoltages and spikes on mains. A 200V voltage peak can be enough to erroneously turn-on a BS170... Kai Klaas |
