On 29 Nov 2003 at 16:16, Jinx wrote: Date sent: Sat, 29 Nov 2003 16:16:00 +1300 Send reply to: pic microcontroller discussion list From: Jinx Subject: Re: [PIC:] 220V Lamps with PIC- best switch To: PICLIST@MITVMA.MIT.EDU > > But actually, the Motorola Thyristor Device Data book discusses > > phase control using zero-crossing opto isolators. By switching > > complete half-cycles, EMI is eliminated whereas a PWM solution > > using those PVI optos would introduce EMI. My original comment: Actually, there's still going to be harmonic content in a half wave, but not as much as in a square wave. Complete cycles would be better. If you skip every other cycle, you effectively have a 50% DC. DC is usually used in the context of a square wave, but in this case the square wave can be thought of as the on/off time or switch cycle/block cycle much as duty cycle is used to characterize RF power amplifier duty. When controlling AC phase, delay angle and conduction angle are the terms used. > > As for EMI ; > > As I understand it, phase control requires that the triac must be > turned on after the z-c point so as to maintain its conduction. But only by a little. The point of z-c switching is to do it at the z-c point because as little as 10V across the TRIAC at turn on time into a load of a few 100 W will cause EMI and you've just pissed in the wind. Once a TRIAC is on, the gate voltage can be removed and the TRIAC will remain on until the current drops below I sub h, the holding current - maybe around 100uA. > It > then turns off when current through it falls below a certain value > at the next z-c point. So you're using the trailing half of the cycle. > If this is true then is it possible to turn the triac on at the leading > z-c point (-> no EMI) and somehow force it to stop conducting at > some point in the cycle while there's V and I across it ? And if so, > is there any EMI generated ? Yes. Any fast edge will generate harmonics. Switching full cycles reduces or eliminates EMI. Using the trailing half of the cycle was practical because a DIAC (now we have SIDACs) could be used to delay triggering and I know of no simple way of doing otherwise and there's no point to it anyway. BRs, Mike -- 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