RE: [CAD_CAM_EDM_DRO] Re:low cost stepper driver

Dave,

The same thing that makes the spark plug ignite the gasoline in a car is
what blows up the stepper drivers when you unplug the motor.

A simple non-CD ignition with breaker points allows current to flow
through the primary winding of the ignition coil creating a magnetic
field. When the points open the current stops and the magnetic field
collapses. That 12V potential across the coil now turns into almost 200V
for a very short time. The secondary of the ignition coil steps that up
from 200 to 20,000 or more. Capacitive Discharge ignitions first charge a
capacitor up to about 400V and the resultant spike goes to well over
45,000V.

You cannot guarantee that when you unplug a powered stepper motor that the
connector or switch won't have the contacts bounce on/off for a few
micro-seconds or even milli-seconds. During that time the stepper motor
windings behave just like the ignition coil and the resultant high voltage
spike will kill the drives.

That's why mechanical relays have a reverse diode across their windings.
Stepper motors have current going both ways and the drivers are internally
(or externally) protected while they are connected.

John Dammeyer

> Subject: [CAD_CAM_EDM_DRO] Re:low cost stepper driver
>
>
> I have a simple machine that is made of MDF. it is not
> designed to cut metal, just wood and plastics and such.
>
> 3 amp motors are more than enough.
>
> the machine cost under $300 to build (motors were free)
>
> it does not call for $100 each drivers.
>
> an hour to make a simple circuit board that will run the
> drivers properly is worth the tradeoff. not sure why there
> is an issue of disconnecting motors with the power on.
>
> it would be nice to make another one less than $100 in
> drivers and power supply and motors.
>
> $33 and axis is not impossible.
>
> Dave
>
>