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]>
List-Post: [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]
>>
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>
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