Re: Re: Shielding

> >Seeing as how I spent several years employed as an EMI/RFI engineer I
> >cant help but wonder a few things...
> >
> >I have seen several posts saying the shield should be tied to the
> >chassis of the driver and left unconnected at the motor... I cant
> >help but think this would make a marvelous antenna.

It was common practice in the professional audio industry to connect the shield
at the "preamp" end only, especially when talking about the equipment for a
live performance. ( I say was because it's been a few years since I was
that business )

The issue here is that when the two pieces of equipment are separated wide
enough, there is a good chance that the "grounds" at both ends are not at
the same potential. A current then flows along the ground/shield wire which
could be induced into the signal wires.

As an aside, a number of years ago, our band did a Saturday morning live
show at the end of the cul-de-sac of a new subdivision.

I arrived late to connect everything up ( leaving the stage setup and
all other heavy lifting to those dumb enough to show up early ).

Extension cords to power all this had been run from houses on either side
of the street. Without even thinking about it I plugged the PA gear into
some of the extensions and the instrument amps into others.

Upon completing setup, I dutifully tested everything, one at a time, and
all was well.

Time to start. The lead singer, grabbing his guitar strings so as to keep
them quiet, then "kissed" his mic in preparation for getting things going,
and promptly got a shock bad enough to send a STREAM of AMPLIFIED profanity
across the neighbourhood, mothers rushing to cover the ears of their
innocent children.

The problem turned out to be a difference in ground potential ( of 20 or 30
volts ? .. my memory is dim ) on opposite sides of the street. Unplugging
the extensions and plugging them back into just one house solved the problem.


Another aside. When in the professional audio industry, it was also common
practice among (good) technicians to wire their shops with a separate
ground wire from each 110v duplex outlet back to the ground rod, eschewing
the normal practice of just daisychaining things.

If you didn't so wire things, it could easily be shown that with high powered
amps running full out, distortion measurements changed, depending upon which
outlets the test equipment and amp were plugged into.

Again, the problem was differences in ground potential.

My point is that grounding the shield at the motor end could make things
better or could make things worse, and needs to be determined experimentally.

A third possibility is to connect the ground/shield lead to the motor frame
through a small ( .01uf to .1 uf ) capacitor.

Again, experimentation is in order.

Alan

--

Alan Rothenbush | The Spartans do not ask the number of the
Academic Computing Services | enemy, only where they are.
Simon Fraser University |
Burnaby, B.C., Canada | Agix of Sparta