Here is one way to make a rough measurement: Take a coated surface. Take a knife and carve a 3" x 1" plated area on the surface. Use a DMM to verify that the isolated rectangular surface is electrically separate from the rest of the plating. Cut 2 pieces of copper foil into 1" x 1" square. Solder the square copper foils to the 2 lead wires on the DMM. Now press the 2 square foils onto each end of the 3" x 1" plated area firmly with the palm of your hand (with 3.14 lb of force exactly :-) ) so the current flows through the 1" x 1" plated surface in the middle of the 3" x 1" area. Take a reading on the DMM. It would probably say 1.3 ohms. Press the 2 copper foils onto each other firmly with the same force and take a reading. It should say about 0.4 ohms. Assuming your hand is a poor conductor and has minimal effects on the readings, the resistance of the plating is about 1 ohm per "square inch." You really should use a micro-ohm meter to measure this to get accurate readings. But accurate readings are not terribly important to me as long as they are less than 1 ohm.
George Robert Macy wrote: > 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).
