I have a bunch of variable air capacitors. I want to find out how I can figure out their range. I think if I measure the plates, spacing, and amount of plates, I can figure out the rating. Is there an easier way if it has no writing?
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You could build an oscillator using the variable capacitor, the frequency range of the oscillator is equal to the square root of (C max/ C min)
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The capacitance will be about 0.22*A*(N-1) / T, in picofarads. A is the area of a plate, N is the number of plates, and T is the thickness of the dielectric (air between two plates). All dimensions are in inches. The last number will be difficult to measure, though. Most air capacitors are a few hundred picofarads at the most, so a capacitance meter may not give an accurate reading. Roger's idea may be the best way to get the range and at least a relative value.
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Hook it into a capacitor tester. Turn capacitor plates complete inside other opposite plates, take a reading. This is the maximum capacitance. Now completely turn the plates out from other opposite plates,take a reading. This is the minimum capacitance.
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If you could rig up a circuit so that it is powered with AC in series with a resistor and you could measure both current and voltage drop the calculations would be simple.