Re: L298 goes boom

A step motor's winding with current applied is simply an inductor
sucking up power and generating a magnetic field. If the current flow
is abruptly interrupted, such as when being physically disconnected,
the magnetic field will collapse, the speed of collapse is determined
by the size of the inductance of the winding, resistance of the
winding and other factors. The collapsing magnetic field induces a
reverse voltage into the windings. This Voltage can be several
hundred volts under certain conditions, jumping the broken connection
and frying the drivers.
The fact that one can sometimes get away with unplugging a powered up
motor, indicates that the driver design engineer had the foresight to
include the protection circuitry for such events. or... as mentioned
above, the winding resistance and inductance are such that only low
back EMF is generated.
Regardless... Do it enough times, and you will let the smoke out.
In actuality a step motor's current is turned on or off for each
step, and can be turned on and off thousands of times per second,
(even at rest) if a "chopper" drive is used. The drive uses special
circuitry to handle this.

Tom M.