Antwort: Re: [sdiy] midi to mosfet - question

[Harry Bissell Jr]

--- Colin Hinz <asfi at eol.ca> wrote:
> On Wed, 4 Feb 2004, Harry Bissell Jr wrote:

> [donning my normally-inactive "robotics engineering"
> hat]
> 
> There's a downside of this approach, though. MOSFETs
> switch with lightning-fast speeds and require little
> current to do so, so it's quite likely that in
> (electrically)
> noisy environments you'll get brief spurious
> switching
> pulses -- and even if you're driving something with
> a
> really slow response time, you'll still be
> generating
> electrical noise and heat.
> 
> The cable connection should be part of an interface
> circuit which requires some current flow to energize
> the
> loads. The best way to do this is to use cheap 4N38
> (or similar)
> opto-couplers at the load end. If your solenoids are
> scattered
> all over the place, the really swank approach would
> be to have
> little integrated single-channel driver boards with
> MOSFET,
> interface, and opto-coupler all together. You may
> find that
> someone out there already makes such a thing, as
> it's a pretty
> basic piece of "circuit Lego" for robotics and
> control
> applications.

(donning his customary industrial engineer hat ;^)

But of course.  I'd argue no need for the
optocoupler...
simple resistive termination should be Ok... depending
on the actual cable length. Anything more than a
couple
of feet and the cable should be driven and terminated
in a reasonable manner  (perhaps even an active MOS
driver circuit).

A series resistor to the gate must be there to prevent
oscillation, some form of gate protection (aka back to
back zeners etc) and some load resistance to assure
that
the mosfets will STAY off.

What we usually do... is bias the Mosfet gates
negative
when they are in the off state.... then drive them
positive (and FAST) to turn them on.

I usually don't do pure MOSFET... I use IGBTs in the
maybe 1000A range... so I am quite aware of what could
happen if one turns on just a little BIT at the wrong
time :^P  (can you say WMD ?)

H^) harry