At 09:26 PM 2/4/2000 -0500, you wrote: >Contrary to what Wagner says, I believe the best solution is to make the >PWM frequency so high that the current in the coil changes very little >during a single on or off time. This would make the circuit look like a >'continuous' mode switching power supply. Continuous mode means that the >inductor current never reaches zero. If the current in the coil reaches >zero, then the relay begins to release. When the current rises again, >the relay will pull back in, causing a buzzing at the PWM frequency. (By >the way, AC relays don't buzz at power line frequency because they are >built with a 'shaded pole' that maintains magnetic pull during the off >part of the phase. DC relays will buzz with AC or rectified AC drive.) One note of experience I'd add to this - be sure to add a high-speed "catch" diode back-biased across the solenoid coil. In a project quite some time back, I had several large contactors to throw - I managed each through a "hard" on cycle when first turning on the coil - basically just leave the FET full-on for a time, and then after a bit switch over to PWMing. The reason for this is the power to throw is often (always?) higher than the power to hold it down. The reason for the diode, though, was to give current through the inductor somewhere to go - obviously if you're switching on and off, there will be interruptions, and if the current doesn't have a convenient path to flow through when the FET is off, the open side of the coil (the one into the transistor) will fly up and *poof* your FET just became a resistive lump of silicon in a pretty black package. Oh yeah, and make sure the diode is >fast<. One of the FR80X series or something with similar switching times would be my call. ND Holmes -------------------------------------------------------------------------- Nathan D. Holmes maverick@drgw.net, ndholmes@iastate.edu 122 Shepard #3 Box 328 Gilbert, IA 50105 Iowa State University - EE http://www.drgw.net/~maverick PH: 515-663-9368 -------------------------------------------------------------------------