On Thu, 8 Nov 2001 14:31:35 -0500 Spehro Pefhany writes: > The idea with reverse phase control is to turn the IGBT "on" at the > beginning of > the cycle, and let the natural increase of the sine wave take place, > then > turn the IGBT "off" when the filament has gottent enough energy this > cycle. > Because you don't get the high di/dt turn-on there is less filament > singing > and it may be easier to reduce EMI with a filter. > I can see that perhaps since the lamp is a bit warmer at this time (how much does lamp resistance really vary through a half cycle? Is temperature relatively constant due to thermal mass, or does it follow the voltage?), the resistance would be a bit higher causing the di/dt to be lower just because the I is lower. But, again, how much does resistance vary through a cycle? If the rise or fall time is the same (and assuming the current at that particular point in the cycle is the same whether using forward or reverse phase control), it seems that it would make no difference as far as EMI is concerned whether the current slope at the switching time is positive or negative. It seems it'd purely be a matter of how big is it (how many amps are we changing), and how fast are we doing it. If we are limiting di/dt because of slow turn off, it seems that it makes little difference whether that is done at turn on or turn off. So, the question, it seems, is WHY would reverse phase control be better than forward phase control? The only reason I can think of is varying lamp resistance through the cycle, but I wonder how much that actually varies. Thanks for the comments! Harold FCC Rules Online at http://hallikainen.com/FccRules Lighting control for theatre and television at http://www.dovesystems.com ________________________________________________________________ GET INTERNET ACCESS FROM JUNO! Juno offers FREE or PREMIUM Internet access for less! Join Juno today! For your FREE software, visit: http://dl.www.juno.com/get/web/. -- 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