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

[Colin Hinz]

On Wed, 4 Feb 2004, Harry Bissell Jr wrote:

> If you want low noise, the mosfets should be
> loacted as physically close to the solenoids
> as possible... minimizing the 'loop' area for the
> radiated EMI
>
> Don't forget to have some form of snubbing for the
> solenoid coils when you turn them off... (diode,
> g-mov etc) of there will be a very high voltage
> generated which will make the noise MUCH worse.
>
> Best if the power for the solenoids comes from the
> solenoid box... and the ground return on the 36 pin
> cable has nothing more than the reference for the mos
> switches...

[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.

- Colin Hinz
  Toronto, Canada