Wow, was Gecko burned out ( about tinned leads and other things)

>
> 2) The damage to Keith's drive resulted from a loose wire that
> touched an adjacent winding phase connector. This conclusion is

based

> on destruction of the phase A and D half-bridge MOSFETs. I'm

guessing

> B came loose and touched C or visa versa.
>
> 3) Protection circuits. This is a philosophically interesting

subject.

>

Wow, these threads do spin off in strange directions.

The speculation (stated fact) was that a loose wire not touching
anything else would cause failure of the FETS and I was questioning
that premise. The reverse kickback from the inductive load would
have to generate a high voltage spike across the other FET pair. I
do know that if there is a way, it WILL happen, but I see the
probability to be down the list as a possible failure mode. As
stated the cause ending up being a direct short.

I do disagree with the other post that stated the diodes protect the
FETS against overvoltage. They give the reverse voltage a path to a
lower impedence but don't clamp a positve going overvoltage spike
and will also fail in reverse breakdown like any diode. I've
destroyed too many helpless FETS (blown their little caps off!)that
did not have external snubbers to limit the DV/DT of the waveform.

I agree about the wire. I worked as a design engineer for a power
supply company and the big linear supplies we made had dozens of
crimp connectors. Our industrial customers specs called for no
tinning of leads and a perfect crimp on every connection. They
would send a supply back for rework for one strand of one wire not
being in the barrel of the crimp.

The decision to include protection or not actually is yours as an
economic one. The cost of fixing or replacing a drive in the field
or of losing a good customer has a dollar value. If the failure
rate for any reason starts to become excessive you make the decision
to change the design. If the failure remains low at one or two per
thousand drives shipped you keep making and shipping them the way
you do.

We could easily design a car that would not ever let you get hurt.
Few would be willing to pay twice the price and drive something that
looked like it's made out of silly putty.

Auh Oh, I'm going into one of my rant modes....gotta go Bye!