4 Replies Latest reply on Dec 9, 2010 8:46 AM by Les

    Resistive plating

    Les

      I have a design that requires resistive plating.  The board will basically function as a potentiometer. I assume the the material is graphite but I cannot find any information on it.  Has anyone designed anything like this.  Any input would be appreciated.

       

      Thanks,

       

      Les...

        • 1. Re: Resistive plating
          jim.granville

          I have seen carbon used for keypads, for the contacts (Elastomer keypad + PCB fingers + Carbon)

          - but if you want to have reliable long term operation, that likely to be a challenge for a sliding contact

          .

          These days, capacitive rotary encoders would be more usual approach, or if you want a precise linear action,

          you could work on the copper resistance, and in the Millivolt region ?

          • 2. Re: Resistive plating
            Les

            Thanks Jim.  The sample I have is definitely a hard substance over copper.  The spec they have is 10K ohms over a length of 38mm.  In this application, it may be used many times and for many years.

             

            Thanks again,

             

            Les...

            • 3. Re: Resistive plating
              sauer

              Les:

               

              When desigining resistors or potentiometers from the sheet resistance of some metal plating, remember that it is the ratio of trace length to trace width that matters. Making the trace longer is like putting a bunch of little resistors in series so the overall resistance goes up. But making the trace wider is like putting strings of resistors in parallel so the resistance goes down. This is why many semi-resistive coatings are characterized in units of "ohms per square". The first question anyone has is... "per square what?" inches... millimeters? The answer is that it does not matter. If you build a trace resistor that is 7 times longer than it is wide, it will have the same resistance if it is 7 millimeters long by 1 mm wide, or 7 inches long by 1 inch wide... the length units cancel. So when you see 10K ohms per 38mm, please remember that the shape (aspect ratio) is critically important!

               

              Good Luck.

              • 4. Re: Resistive plating
                Les

                Understood. The wiping contacts are defined so the width is known, as is the length.  The only variable I have is the z dimension to dial in the resistance.  All I need to know now is what kind of magic mouse milk they put on top of the copper.