> Van: FScalini > Aan: PICLIST@MITVMA.MIT.EDU > Onderwerp: [OT]Re: Completely stupid question - flame away!! > Datum: zondag 10 mei 1998 22:53 > > I've seen zero crossing detection of AC currents come up frequently on this > list lately, especially with respect to dimming applications. What is the > practicality of zero crossing detection? > [Cut] The AC current is mostly switched by a Triac. This device will, when switched, stay that way until there is no voltage left over the Kathode & Anode. then it switches off and waits for a new (trigger) signal (on the Gate). Now for the trigger signal. If you switch a DC current you will have to take care of the effects the change in current has on a coil or alike. The same applies when you switch an AC current. But the AC current will be zero 100 times a second (50Hz means 100 passes thru Zero, 50 downward & 50 upward). If you switch your current when it is Zero, it will go up (or down) gradualy (along the sine-wave) and thus _not_ generate to much problems with coils and the like. That is the practicality of zero crossing detection. (and using it ofcourse :-) To prove my point : take a coil (a lot of loops and some resistance). Take a 4.5 volt battery. take the wires of the coil in your left & right hand. Connect them to you battery and dis-connect them (Touch-and-go). The (tiny) shock you will experience is the effect I mentioned. this effect is called 'self-induction' and can destroy electronic equipment. Now think of 220 Volts and a unknown (for example a transformer) coil. There could be a lot of self-induction ! Greetz, Rudy Wieser