Dale Botkin wrote... >On Fri, 22 Feb 2002, Olin Lathrop wrote: > >> Not if the PWM frequency was much higher than the line frequency. If = so, >> you could just ignore line sync and output asynchronous PWM which will >> average nicely each 1/2 line cycle. 1.2KHz would be 10 PWM periods in= each >> 1/2 line cycle, which should be good enough. The FET transition time = should >> still be a negligeable percentage at that frequency if driven right. = Note >> that the lowest hardware PWM frequency a 20MHz 16xxx PIC can produce = is >> 1.22KHz. Frankly I like this much better than low pass filtering the = gate >> to turn on the FETs slowly. I guess it can be OK if done just right, = but it >> I don't like the power those FETs are dissipating even for a short = time. > >I can understand your position... I was more concerned with what I >suspect would be horrible wideband RF hash, but of course it would only >last a second or so once a day. Guess even I could live with that. I have a distinct distaste for having potential problems with my designs pointed out in public, because it makes me say, "Dave, you doofus, you should have done your homework before suggesting that!!" In this case I didn't do my homework beforehand, and Olin's concern about the instantaneous power dissipation in the MOSFETs is a very realistic one. It's an issue that should be addressed before proceeding with any sort of "ramped gate drive" soft-start scheme such as the one I proposed. So now I'll go and do my homework and figure out how much stress these MOSFETs might be subjected to by this soft-start arrangement. I downloaded the datasheet for the 2SK2614's you mentioned, and note they're rated for a typical Rds(on) of 32 milliohms; are these what you're planning to use? Also, what's the current rating of the bulbs in your yard lights? I recall you saying the total current drain for all the bulbs together is 7 amps, but what about each individual bulb? =20 This is important because the filament size is going to determine how fast the bulb heats up and gets past the "inrush current" period during turn-on. It's the inrush current that's of concern here, and how fast it decays as the bulbs heat up and reach incandescence. A single bulb which draws 7 amperes steady-state will have a thick, massive filament that will probably take several scores of milliseconds to come up to temperature, whereas smaller bulbs, with light, thin filaments, will reach operating temperature much more quickly. =46rankly, I don't think we have a problem, here; but I'll do some calculation and some modeling to make sure we don't once I get some data from you. Dave -- http://www.piclist.com hint: The list server can filter out subtopics (like ads or off topics) for you. See http://www.piclist.com/#topics