Wouter van Ooijen wrote: > Anyway: more circuit attached. I assume you set only one of RC0 or RC3 high depending on direction, then put the PWM signal on RC1? I didn't notice this before, but what are C7 and C8 supposed to accomplish? These will slow down switching of the top FETs, which I guess is OK if you are reversing direction infrequently. Still, I don't see any advantage from them. A possible issue is the gate voltage of the lower FETs when they are supposed to be off. Aren't the ULN2003 darlington drivers? If so, figure at least 1V when on. Then add in the voltage from the divider formed by R119 and R112, which is another 580mV. The lower FET gates will therefore be at 1.6V when they are supposed to be off. I haven't looked up IRFR024, so I don't know if this is a problem or not. These kinds of FETs are often poorly specified in this region. They may have substantially more leakage at 1.6V than at 0V. Unless they were specified for some maximum current or minimum resistance at 1.6V or higher, I would be little nervous although probably has nothing to do with your problem. Do you realize that the top zeners D18 and D15 will never come on given 24V supply? R123 and R124 form a divider so that there can not be more than 12V accross R123 even if IC3-6 would drive all the way to ground. You will have to leave considerable dead time when switching direction. The limiting factor is not the FETs, but the way they are driven. R123 and C7 have a time constant of 5uS, and the effective gate capacitance could make this considerably larger. Then you want to wait probably 3 time constants. Just from quick calculations without looking up the details, it looks like you need a dead time of at least 100uS on switching direction. Making this too short could cause damage eventually. > I assumed Schottky's to be better (= faster) at surpressing the spikes > than the body diodes. Yes, they would be. Hopefully you have at least some capacitance accross the motor right by the motor. You don't want too much else there will be switching transient currents as the cap is charged and discharged each PWM pulse, but you want some just to slow down the voltage edges. With a reasonable cap by the motor and another on the board for additional protection, the body diodes should be able to react fast enough. They are usually about the speed of the FET. > No, it is meant to detect a stalled motor being started. What I meant is that just an ordinary stopped motor will likely look stalled in the first 20mS. Unless you've got an exceptionally agile motor, it's probably not going to pick up significant speed from a dead stop in the first 1/50 second. >> All the >> resulting generator power ends up being dumped into those >> diodes. > > Is that true? I would expect all the power to be transferred to the > power lines. Yeah, I said that wrong. I meant to say all the *current* will go thru those diodes. Most of the power will be dumped back onto the supply as you say. ****************************************************************** Embed Inc, Littleton Massachusetts, (978) 742-9014. #1 PIC consultant in 2004 program year. http://www.embedinc.com/products -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist