With regards to the question at end of Macy's e-mail. Count the number of squares between probe points. Say the probes are 2" apart. If the surface containing the two points is at least 2" wide, then it only takes one square, and the resistance measured is numerically equal to the ohms/square. If the width of the surface were but 1", then there would be two squares, and the measured resistance would be twice the ohms/square of the material.
---------- >From: "Robert Macy" <[email protected]> >To: "George Tang" <[email protected]>, "Westerdahl, Eric" <[email protected]> >Cc: "'EMI-PS Group'" <[email protected]> >Subject: Re: Conductive Coating >Date: Wed, Nov 3, 1999, 10:18 PM > > > A small pedantic point here: > > Coatings are usually rated in "ohms per square" which is dimensionless. > > Looking at the resistance formula: > > R = rho * length / Area = rho * length / ( width * thickness ) > > Given fixed rho and fixed thickness (of the coating) you can see that you're > left with a dimensionless: > > R = K * length/width > > If the length and the width are the same, you have a square. The resistance > of a square of any magnitude will be the same. > > Now, to relate Ohms per square to the reading one gets when one sticks two > probes down on it? > > - Robert - > > > -----Original Message----- > From: George Tang <[email protected]> > To: Westerdahl, Eric <[email protected]> > Cc: 'EMI-PS Group' <[email protected]> > Date: Wednesday, November 03, 1999 10:17 PM > Subject: Re: Conductive Coating > > >> >>There are many different types of conductive coatings available. Silver > paint >>is >>very conductive, less than 5 ohms per square inch. But it is not as > scratch >>resistant >>as sheet metal surfaces. Electroless copper / nickel plating is very >>conductive and >>durable. You can get as low as 1 ohm per square inch. If you don't, your >>plating >>is not thick enough. This plating should pass the safety fault current > test, >>as long as >>the safety ground wire makes "surface area" contact with the plating and > not >>"point" >>contact. This plating has 60 dB shielding effectiveness for frequencies > above >>30 MHz, >>since it is much thicker than the skin depth. You need the thickness for > the >>safety fault >>current. The best feature of the plating is that it allows you to mold > your >>chassis into >>one piece of plastic with no extra metal pieces to assemble. It's kinda > nice >>that way. :-) >> >>go to www.ccoatings.com >> >>or call (972) 851-0460 >> >> >> >>George Tang >> >> >> >>"Westerdahl, Eric" wrote: >> >>> Our company has decided to use a conductive coating to mitigate some EMI >>> problems on one of our units. We have not used this method before. I > have >>> a question as to the correct resistivity of the coating. What range > should >>> I be looking at, and does the range change if the frequency of the strong >>> signal are high or low? >>> >>> The equipment is IEC 950 and EMC Directive stuff with many noisy DC motor >>> and motor controller combinations. Most of the signals we are concerned >>> about are at the lower end of the CISPR 22 region. 30 to 150 MHz. >>> >>> Eric Westerdahl >>> Regulatory Engineer >>> Roll Systems, Inc. >>> [email protected] >>> >>> --------- >>> This message is coming from the emc-pstc discussion list. >>> To cancel your subscription, send mail to [email protected] >>> with the single line: "unsubscribe emc-pstc" (without the >>> quotes). For help, send mail to [email protected], >>> [email protected], [email protected], or >>> [email protected] (the list administrators). >> >> >>--------- >>This message is coming from the emc-pstc discussion list. >>To cancel your subscription, send mail to [email protected] >>with the single line: "unsubscribe emc-pstc" (without the >>quotes). For help, send mail to [email protected], >>[email protected], [email protected], or >>[email protected] (the list administrators). >> >> > > > --------- > This message is coming from the emc-pstc discussion list. > To cancel your subscription, send mail to [email protected] > with the single line: "unsubscribe emc-pstc" (without the > quotes). For help, send mail to [email protected], > [email protected], [email protected], or > [email protected] (the list administrators). > > --------- This message is coming from the emc-pstc discussion list. To cancel your subscription, send mail to [email protected] with the single line: "unsubscribe emc-pstc" (without the quotes). For help, send mail to [email protected], [email protected], [email protected], or [email protected] (the list administrators).
