Hi, You need a diode across that inductor (motor)! Cathode (the band) to +24V, anode to the fet-motor junction. Tell us a little more about your motor. Regards, mink "Fansler, David" wrote: > I have an application I am working that uses a PWM on a 16C74B to control > the speed of a DC motor. The output of the PWM goes to the gate of a > IRLIZ44N hexfet with a 5v gate. The motor has one lead connected to 24v dc > and the other to the drain of the hexfet. The source of the hexfet is > connected to ground. The PWM is set up for 19kHz period. Ignoring the 2 > least significant digits, I am using the PWM in a 255 step resolution. > Looking at the junction of the motor and the hexfet, with the PWM running I > get a funky wave form from values 0-A0 hex. It looks sort of like the > beginning of a square wave (vertical rise, flat top) then it looks sort of > like a damped sine wave until the next vertical rise. As the period of on > time increases, the damped portion decreases until I get a clean vertical > drop, At which time the motor starts slowly turning (hex value of A0). As I > increase the period toward 255, the square wave at the motor/hexfet junction > grows smaller and the motor speeds up. One other piece to this puzzle - the > peak of the square wave at the motor/hexfet junction is about 60 volts. > > Three questions: > 1. Is the coil of the motor acting as an inductor causing the damped > ringing and the 60v? > 2. Would a different frequency be better? > 3. In that this motor is being run in both directions (switching motor > leads via relays) is it possible to get a better range of speed from the > motor (more that A0 - FF) > > Thanks, > David V. Fansler > Network Administrator > TriPath Imaging, Inc. (formerly AutoCyte, Inc.) > 336-222-9706 Ext. 261 > dfansler@autocyte.com > Now Showing! www.mindspring.com\~dfansler\ > > Updated December 16, 1999 > Ann's Cancer & David's Observatory