Re: RFI isolation

Mere 0.01 uF caps on the leads next to the chip probably won't do much for
RFI.  You need to use a good feed through filter for ALL wires. Opto
isolating eliminates common mode problems but unless it is an optical
fiber, the wires going to the opto can still carry RFI through the wall of
the box to the opto, where it capacitively couples across the opto body.

For low power leads, I'd go with some feed through filters like those from
Spectrum Control. For high power leads (like going to the motor and ac
line), I'd use a good quality EMI line filter, making sure that the case of
the filter is connected to the box all the way around, with NO holes or
gaps.  3M Copper foil tape (available in a variety of widths, 1" is most
common) makes this fairly easy. (That is, you have a rectangular hole in
the box which the filter sticks through. The gap between the box and filter
is entirely filled with a conductor).

For line filters, the modular ones from a PC power supply might work well,
since they have both a male and female connector, and are shielded.  In
fact, building your motor controller into a good quality PC power supply
housing might not be a bad idea.

Don't neglect such interference paths as things like metal potentiometer
shafts which pass through an insulating bushing, and the metal handles on

> From: Tesla List <tesla-at-pupman-dot-com>
> To: tesla-at-pupman-dot-com
> Subject: RFI isolation
> Date: Sunday, September 05, 1999 7:16 PM
> Original Poster: "Matt Behrend" <electronxlc-at-earthlink-dot-net> 
> Hi all,
> I have built a digital motor controller for my sync RSG.  It is 
> malfunctioning due to picking up RFI on each firing of the gap.  The 
> circuit is enclosed in an aluminum box.  There several wires that go 
> outside the box, but they are optoisolated and I have ruled out the 
> possibility of RFI getting in on these.  The RFI is definitely passing 
> through the box and hitting the logic circuitry.  I have 0.01 uF caps 
> next to each chip and the power supply is well-filtered.
> I have tried grounding the box and leaving it unconnected to 
> anything.  Could you please give me some suggestions for 
> blocking this RFI.
> Matt Behrend