here are some rough calculations. I'm making this up as I'm typing. I remember something about 7A steady state. Let's say the initial current is 70A at full voltage. I have been taught, and have used 7:1 as an inrush current ratio for tungsten for the past 20 years without any problems. If the FETs were 1/2 on with cold bulbs (it won't happen like this, I'm just trying to pick something to see where it leads), that would be 35A x 60V = 2100 watts. If this happened for 20mS, that would be 42W averaged over 1 second. So, I get abou 30 watts/sec avg. Hmm. I think this is saying that this very crude model doesn't provide conclusive evidence on way or the other, but that a much more realistic model needs to be properly worked thru to prove this is OK before implementing it. That fact that it is close enough to require more detailed calculations makes me nervous about this method again. As long as the ramping starts below 15% and the ramp is long enough, this scenario should never happen. With large and more costly loads, I like longer ramps, say as much as 5 sec full-scale. This gives the tungsten time to heat a bit more and the heat sink time to actually do something since the resistance increases significantly along the way up the ramp. I haven't followed this thought from start to finish, nor have I done the analysis, but I do a fair bunch of AC power controllers for tungsten, and when I use a 5 second soft start with a 7:1 inrush, I have gotten into the practice of using devices at/near their rated power level without inrush failure. Besides, with a 1 or 2 second soft-start, most non-geeks wouldn't even notice it, and the 'wow-factor' would be mostly lost. I have found that the most common cause for failure occurs when the output to the controlling devices is left to do spurious things during power-up or reset, etc.. Chris ******************************************************************** Olin Lathrop, embedded systems consultant in Littleton Massachusetts (978) 742-9014, olin@embedinc.com, http://www.embedinc.com -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details. -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details.