Re: Gecko burned out (not about tinned leads)

The body diodes in FETs work fine for recirculation
currents but cannot stop damage due to arcs because
circuit inductance and the slow speed of the body
diodes allow the FET to be subjected to damaging
voltages. See message number 2194 from Mariss in
the Geckodrive group addressing this issue.

http://finance.groups.yahoo.com/group/geckodrive/messages/2194

Regards,
Steve Stallings

--- In CAD_CAM_EDM_DRO@yahoogroups.com, Major A <mandras76@y...>
wrote:

>
> 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