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Tuning Capacitor Rating

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  • patrick_mcstorm
    Hello When looking at a potentiometer, I would tend to believe That a 1K Ohm potentiometer would have a maximum resistance of 1000 Ohms. Are tuning capacitors
    Message 1 of 5 , Jul 11, 2014

      Hello


      When looking at a potentiometer, I would tend to believe That a 1K Ohm potentiometer would have a maximum resistance of 1000 Ohms.


      Are tuning capacitors done the same way?? Given a "365 pF" tuning capacitor, I'm assuming (I may well be wrong) that the tuning capacitor would be fully closed at maximum capacitance and be at 365 pF at that point.


      Or have I missed the boat again?


      Thanks


      Patrick


      p.s. NOTE: NO TYPO CHECK WAS DONE HERE; please forgive any that are present.




    • ahumanbeing2000
      Unlike a potentiometer, a mechanical tuning capacitor could be rotated 360 degrees (no stop limits). Its capacitance is at a maximum value when its two sets of
      Message 2 of 5 , Jul 12, 2014

        Unlike a potentiometer, a mechanical tuning capacitor could be rotated 360 degrees (no stop limits).

        Its capacitance is at a maximum value when its two sets of plates (one set is fixed while the second rotates) face fully each other. Its minimum value is reached when they become opposite.




        ---In Electronics_101@yahoogroups.com, <vw_beetle_fix_it@...> wrote :

        Hello


        When looking at a potentiometer, I would tend to believe That a 1K Ohm potentiometer would have a maximum resistance of 1000 Ohms.


        Are tuning capacitors done the same way?? Given a "365 pF" tuning capacitor, I'm assuming (I may well be wrong) that the tuning capacitor would be fully closed at maximum capacitance and be at 365 pF at that point.


        Or have I missed the boat again?


        Thanks


        Patrick


        p.s. NOTE: NO TYPO CHECK WAS DONE HERE; please forgive any that are present.




      • Andy
        Patrick, I think you are 100% correct. For all variable capacitors I ve seen that were identified with only a single capacitance value, the value refers to
        Message 3 of 5 , Jul 12, 2014
          Patrick,

          I think you are 100% correct.  For all variable capacitors I've seen that were identified with only a single capacitance value, the value refers to the maximum capacitance, when the plates are fully meshed.  (Within some tolerance, of course.)

          Whereas a rheostat or potentiometer is adjustable all the way down to zero, a variable capacitor is not.  All variable capacitors have some minimum capacitance.  There is no general rule about the difference or ratio between min. and max. capacitance.  Some variable capacitors have more than a 10:1 ratio of max. to min.  Others have a much smaller ratio.

          I suppose that a variable capacitor with a narrow range, might be specified and marked with its "average" capacitance, instead of its maximum, or both maximum and minimum.

          It is true that some variable capacitors can be rotated 360 degrees with no "stops".  But the "365 pF" ones typically found in AM receivers, can only be rotated about 180 degrees, with stops on both ends to prevent it from being rotated further.

          Most AM radio 365pF capacitors also have little trimmer capacitors in them, adjustable with a screwdriver.  This trimmer capacitor was in parallel with the main capacitor plates, and it adds a little more capacitance to the main capacitor alone.  They were adjusted when the radio was assembled, to make sure it fully covered the AM band.

          Andy


        • Paul Alciatore
          What Andy said is true. A couple more points. In addition to small trimmers, the AM radio tuning capacitors also used slotted movable plates for adjusting the
          Message 4 of 5 , Jul 13, 2014
            What Andy said is true. A couple more points.

            In addition to small trimmers, the AM radio tuning capacitors also
            used slotted movable plates for adjusting the capacitance at
            different places in the rotation. This was done to allow them to be
            adjusted to match the dial readings in a production situation. This
            produced segments that could be bent much like fan blades so the
            effect could be different at each end of that particular segment.
            Fine adjustments were possible.

            Yes, variable capacitors could be made with different rotational
            characteristics. Some go round and round with no stops. Some are
            limited, usually around 180 degrees. Others have a compression scheme
            where the plates and insulators are stacked and are compressed with a
            central screw for more capacitance or loosened for less: these could
            be multi turn.

            And on the other side, some variable resistors have been made with
            continuos rotation. This would require either a small "dead zone" or
            a small zone where the two ends are shorted. I suspect that the "dead
            zone" option was most common.
          • Paul Alciatore
            Oh, I meant to also say that to be on the safe side, you should look at the spec. sheet for the high and low end capacitance values. If you have one that was
            Message 5 of 5 , Jul 13, 2014
              Oh, I meant to also say that to be on the safe side, you should look
              at the spec. sheet for the high and low end capacitance values. If
              you have one that was intended for AM radio, then you can probably
              assume that it has a capacitance ratio that will cover that band. The
              AM band is 540 to 1700 KHz or a 3.15 to 1 ratio. Since the frequency
              is proportional to the inverse square root of the capacitance, you
              need a hi to lo end ratio of 3.15^2 or about 10:1. AM tuning
              capacitors had to have a ratio that high or more.
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