exactly! hence the desire for a high drive voltage, with current limited by "chopping", i.e. turning the power source on and off and providing a path for the back emf induced current until it drops bellow 8 amps, and then turning the power back on until it just exceeds 8 amps, typically at 1khz to 150khz or better. this is the only way to get high step rates out of a stepper motor. this system is more drastic than most, i.e. smaller systems may use 5V motors with a 24V supply, but it's the same idea. the current limit is incorporated directly into the drive electronics for each coil, and usually has to supply either polarity so a full bridge is usually used. for smaller motors, up to 2.5A at least there are single chip solutions that drive both coils and handle the current limit, and even provide some microstepping (smaller steps produced by providing current to roughly both windings in sin and cos magnitude proportions at some number of intermediary electrical degrees of rotation, easily done by varying the reference of the current limit circuitry). it isn't pretty, but it works very nicely. there are simpler methods that don't work as well, such as using a higher voltage and a series resistor, or applying a higher voltage for a specific time period and then switching to the "rated" voltage for normal current. i've seen old 8" floppy drives that used the series resistor approach for instance on the head motor. William Chops Westfield wrote: ------- > The "kick" is that when you attempt to move the rotor, the current > through the winding is no longer limitted by the steady state > resistance, > but by the inductance; for a step applied voltage, you a nice > exponential > multiplied by V. To get the 8A you want through the coil for max > torque, > thoughout the pulse, you have to apply a much higher voltage and either > make it a complex waveform v(t) or separately current-limit via chopping ------- -- "We cannot simply suspend or restrict civil liberties until the War of Terror is over, because the War on Terror is unlikely ever to be truly over," Judge Gerald Tjoflat wrote for the three-member court. "September 11, 2001, already a day of immeasurable tragedy, cannot be the day liberty perished in this country." _______________________________________________ http://www.piclist.com View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist