Re: [CAD_CAM_EDM_DRO] Re: Gecko burned out (not about tinned leads)

> A loose wire can kill a driver that does not
> have protection from the high voltages produced
> by interrupting the current through an inductor.
> If you break the circuit while current is
> flowing through the stepper motor coil, it is
> possible to produce several thousand volts
> of inductive "kick". If it is not clamped,
> it is enough to eat the MOSFETS alive. The
> Gecko products are engineered to operate well
> when used properly, but they do not have the
> extra circuitry to protect against this type
> of abuse/fault.

Do I detect a major misconception here?

The FETs that drive the motor's windings can be regarded as on/off
switches. When they switch off, the idea is that no more current will
flow through the winding. Unfortunately, the motor windings have a
self-inductance, which means that every change in current is
accompanied by a large voltage across the winding. When the FET turns
off, this effect would cause a high voltage to drop across the FET,
which would indeed fail instantly. Now all such drivers have, however,
a diode connected across the FET which clamps this voltage to <1V, and
through which the current that is no longer flowing through the FET
can pass and decay gradually.

If you disconnect the wire from the drive during operation, while
current is flowing, the large voltage will be present indeed because
there is no protective diode across the failed contact between the
motor and the drive. This voltage can indeed cause sparks to appear at
the terminal, but one thing it cannot do is harm the FET because the
FET is still protected by the diode. The only way you could harm the
FET is by applying a large voltage of the opposite polarity, but a
motor winding cannot cause that to happen.

By pulling the wires, you CANNOT harm a drive, however cheap it may
be.

Andras