## Tuning Capacitor Rating

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• 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.

• 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.

• 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

• 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.

used slotted movable plates for adjusting the capacitance at
different places in the rotation. This was done to allow them to be
produced segments that could be bent much like fan blades so the
effect could be different at each end of that particular segment.

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.
• 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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