Re: [CAD_CAM_EDM_DRO] Re: EDM machine

dave engvall wrote:

> After you get all the servo and stepper boards in production this
> might make a really good project. <hint>
>
> I have some nice igbt's sitting here but not sure I know enought to
> use them.

IGBT's are not good candidates for EDM as it is a shorted-load process.
IGBT's
like relatively slow di/dt slopes, which is not what you'll get with
EDM. Maybe a little
series inductance will make them work. The problem is if the collector
current rises
too fast, only part of the transistor turns on, and the current
'hogging' in that area
damages the transistor. It then explodes! I know, as I went through
about $400
of them getting one project working. Where you don't expect shorted
outputs,
or series inductors are going to be in the circuit so the overcurrent
logic can
shut it off in time, IGBT's are GREAT, especially above 500 Volts.

For the low voltage of EDM, I think low voltage MOSFETs will work best.
They parallel beautifully, withstand shorts like no other transistor
can, and
the low voltage ones are limited by the package leads, not the
transistor.
(Power FETs have a negative gain thermal coefficient, they conduct less
current
when hot, therefore they are self-stabilizing when paralleled, even when
shorted.)
They also will shut of when shorted and conducting several times rated
current, not all IGBT's and most bipolar transistors will NOT SHUT OFF!
This is called secondary breakdown, and is why all sorts of extreme
measures
are needed to protect other transistor types.

Jon