Re: Delagrange stepper driver board

--- In CAD_CAM_EDM_DRO@yahoogroups.com, R Rogers <rogersmach@y...>
wrote:

> It says that now. They also claim 50 volts, or did then. 12 volts

and 6 amps fried it. The hexfets arent or weren't even on a heatsink.
A 10 amp rating should mean about a 12 amp failure conservatively.
Would anyone expect to get 50 volts and 12 amps through a hexfet out
in mid air and nothing to sink on?

>
> Ron
>

The key is the LR approach. Kinda like dragging around chains behind
your car to limit the top speed. You have to match the resistors to
the load and half the power is used to heat the room (through the
resistors). The whole thing is an exercise in frustration because to
limit the current, you need higher value resistors, so if you apply
more voltage, to keep current in check, you have to increase the size
in wattage and the ohmic value of the series resistor. That in turn
lowers the applied coil voltage so you have to raise the raw DC which
means you have to raise the resistor.....well, you get the picture.
You can never get to the optimum operating point of the stepper and
the higher voltages to get higher speeds.

Do yourself a favor and don't scrimp on the motor drivers. The
inital pain of the higher price will wear off quickly and you will
not have to worry about that part of the equation.

Chopper drives and even better, the microstepping drives have clear
advantages over the previous types.

The power dissipation in a FET is directly porpotional to the ON
resistance (I^2 * R). Some of the new low voltage FETS have really
low on resistances and are almost perfect switches. It requires that
the gate drive be properly designed to keep turn-on/turn off as close
to zero as possible to prevent power being dissipated during that
transition time. The FET actually is a variable resitor and turning
it on/off slowly (in the millisec range) will cause the R value to
move through a linear part and power dissipation goes WAY up.
Consider if you have 20A times .01 ohm = (400 * .01) or only 4W. Now
increase that to 10 ohms and you get 4000W !