Re: Electronic Box Shielding ?

--- In CAD_CAM_EDM_DRO@egroups.com, Hugh Currin <currinh@O...> wrote:
> Mariss:
>
> Thanks for responding.
>
> >3) Use an RFI line filter on the 115VAC where it enters the box.
> OK, but what's an RFI line filter? Is this like an RC filter
> set at somewhere above 60Hz or some more sophisticated
> circuit? Are these purchasable and can you give a source
> or two? :-)

An RFI filter is an LC low pass filter. It passes 60Hz while stopping
high-frequency signals from passing thru. Try the Corcom series B or
series R filters; Digikey page 304 of their Apr-Jun 2000 catalog.

> >4) Use a 5-conductor shielded cable / motor. The 5th conductor goes
> >from motor case to ground inside the box.
> This would be a fifth wire in addition to shielding? Wouldn't this
> form a ground loop, ground/motor case/fifth wire/ground?

The windings of a step motor forms one plate of a capacitor. The
nylon bobbin forms the insulator and the motor case is the other
capacitor plate. It measures in the 100pF to 1,000pF range. This is a
dead short at the frequencies of concern. The fifth wire returns this
high-freq current back to its source.

> >6) Establish a "holy point" ground inside the box. All internal
> >ground wires go to this single point. Everything that needs a
ground
> >wire gets a seperate, individual wire from this point.
> OK. Someone previously suggested the (-) terminal of the
> power supply capacitor. Is this good? Should this point in turn
> be taken to an earth ground?

The "-" terminal is handy because innumerable ground wire lugs fit
under the 10-32 screw terminal. Connect the earth ground to the box
itself.

> >8) Shield input cables the same way as (5).
> I was planning to use twisted shielded pairs for wiring.
> Then use the twisted pair for signal and return. Is this
> appropriate and/or important?

That will work if there is at least 1 twist per inch.

> >Mariss
> >Geckodrive Inc.
>
> Thank you greatly, it is already a big help. And also thanks
> for your Geckodrive site, it has answered a few other questions
> for me. :-)
>
> Hugh Currin
> Klamath Falls, OR