Re: [CAD_CAM_EDM_DRO] Stepper Motor Driver

Hi Jon,

If I decide to us the better circuit with the opto-isolation and the FET's
IRF630's (needing 10volts to trigger it full on) could you spoon feed me
with a diagram corrected with the addition of the fast diodes you suggest? I
can send you a diagram.

I also notice the motor direction CW and CCW are two pins whereas most
drivers use only one pin, how can this be altered.

Regards,

Terry

| juan gelt wrote:
|
| >the ir series are hexfets - very special fets.
| >i have the international rectifier designers handbook
| >here, so i'll quote it:
| >"The integral body-drain diode is a real circuit
| >element, and it's current handling capability is
| >typically as high as that of the transistor itself."
| >
| Yes, but the POLARITY of the pulse will be wrong to make the
| diode conduct. The drain voltage will go very positive when
| the other transistor turns on, due to the center-tapped transformer
| action of the motor winding.
|
| The best circuit puts a diode with the anode to each FET drain and a
| power resistor connected to both diode cathodes and back to the
| motor power supply + terminal. This allows the inductive spike to
| be dissipated safely, at a voltage acceptable by the transistor,
| by selecting the resistor value. The transistor has to be able to
| handle twice the DC supply voltage at an absolute minimum.
|
| >i admit i have never scoped it, so i don't know first
| >hand just how fast it is. i can say i've never had a
| >hexfet powerfet fail on me from driving steppers with
| >it.
| >
| The body diodes on the standard hexfets are quite slow. On my
| PWM servo amplifiers, I saw them fail to conduct with as much as -12
| V across the FET for over 250 nS. That was enough to pop the
| driver chips. I put in a separate MUR820 fast diode, and it cut
| the undershoot to less than 4 V, which the driver chip could
| accept.
|
| Jon