What is the minimum value of capacitance of a rotating capacitor ???

Tonyr1084

Joined Sep 24, 2015
4,535
A tunable (rotating) capacitor consists of plates that are spaced apart by some measure. The spacing is what derives a given capacitance. By rotating extra plates in-between the stationary plates you reduce the spacing, thus changing the capacitance. So the minimum value will be either (I'm not sure which) with the plates fully engaged or fully extracted. The varying overlap of the plates (stationary and rotary) is what changes the capacitance value.
 

OBW0549

Joined Mar 2, 2015
3,406
A tunable (rotating) capacitor consists of plates that are spaced apart by some measure. The spacing is what derives a given capacitance. By rotating extra plates in-between the stationary plates you reduce the spacing, thus changing the capacitance.
I don't think so. In all of my years, I have never encountered a variable capacitor that works that way.

In every case I've ever seen, the rotating plates of the capacitor (i.e., the rotor) acts as one terminal of the capacitor, while the stationary plates (the stator) comprise the other terminal. As the shaft is rotated, the plates on the rotor overlap either more or less with the stator plates, changing the capacitance between them.

Wikipedia has a good article that explains.
 

Tonyr1084

Joined Sep 24, 2015
4,535
I don't think so. In all of my years, I have never encountered a variable capacitor that works that way.

In every case I've ever seen, the rotating plates of the capacitor (i.e., the rotor) acts as one terminal of the capacitor, while the stationary plates (the stator) comprise the other terminal. As the shaft is rotated, the plates on the rotor overlap either more or less with the stator plates, changing the capacitance between them.

Wikipedia has a good article that explains.
Been a very long time messing with one of those caps. Yeah, I must agree, the rotary plates are one lead and the stationary plates are the other lead. Thanks for covering that.
 

MisterBill2

Joined Jan 23, 2018
6,071
What is the minimum value of capacitance of a rotating capacitor ???
The MAXIMUM value of capacitance is normally what is specified for variable capacitors. Many makers also specify a minimum value range, which is less predictable. But in all cases the minimum value is not zero. In many cases the minimum is between 5% and 10% or the maximum.
Is the question asking for a number? Or an explanation of the concept? That part is not clear.
 

KeithWalker

Joined Jul 10, 2017
895
Most tuning capacitors for analog AM and FM radio receivers have a trimming capacitors built into each tuning section, to adjust the value at minimum setting.
Keith
 

Bernard

Joined Aug 7, 2008
5,524
I have a 13 plate ,1/4" shaft, 5? to 50 pf. & 5/16" sq. 3 plate 1 to 10 pf., plastic seperators, slotted shaft.
 
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SamR

Joined Mar 19, 2019
2,126
I've seen pictures of a homebrew that was simply 2 ~4" dia. plates on wooden sticks. One fixed and handle on the other so the upper one could be swung across the fixed lower plate. Dunno if there were any calculations involved or if they were just winging it to see what they would get. Took up a lot of space but a quick way to build one in a pinch.
 

Janis59

Joined Aug 21, 2017
1,179

Danko

Joined Nov 22, 2017
928
If we can get beyond Politics
Americans can do amazing things.
LET America be Great Again.
Author: My wife.
Amazing things like that in your post?:
A tunable (rotating) capacitor consists of plates that are spaced apart by some measure. The spacing is what derives a given capacitance. By rotating extra plates in-between the stationary plates you reduce the spacing, thus changing the capacitance. So the minimum value will be either (I'm not sure which) with the plates fully engaged or fully extracted. The varying overlap of the plates (stationary and rotary) is what changes the capacitance value.
Oh, I see, Politics are guilty!
 
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MisterBill2

Joined Jan 23, 2018
6,071
To actually determine the minimum capacitance is a fairly straight-forward procedure. But you do need a means to measure the resonant frequency of a tuned circuit. This works if you know the specified maximum capacitance.
To start, you would connect a coil with an inductance suitable for the frequency of intended use, across the capacitor. You do not need to know the value of the inductance, a guess is close enough. Then, with the variable capacitance at it's maximum setting, determine the resonant frequency of the tuned circuit. Next, adjust that variable capacitor to it's minimum capacitance without changing the coil at all, and determine the resonant frequency at this position. At this point you have two equations with two unknowns, but one of those unknowns is the inductance, which is not changed. The resonant frequency in radians per second is 1/ (root) LC, that is, 2π F=1/ the square root of the product of inductance x capacitance.
So with a bit of tedious algebra you can solve for the second value of capacitance.
The most common way to measure the resonant frequency is with an instrument called a "grid dip meter", although it can also be done with a signal generator and a suitable voltage meter. One caution is that the actual value determined can only be as accurate as the measurements. So if you use a dip meter you will need to also use an accurate frequency counter.
 

Tonyr1084

Joined Sep 24, 2015
4,535
I have a 13 plate ,1/4" shaft, 5? to 50 pf. & 5/16" sq. 3 plate 1 to 10 pf., plastic seperators, slotted shaft.
Some have some sort of material such as plastic, some are "Air Core" meaning nothing between them except air. Whether they're just air or plastic - or some other material - these are just some types of capacitors. In many cases the material separating the plates has an affect on the capacitance value in both farads (micro - nano - pico or other) and voltage capability.
 

DickCappels

Joined Aug 21, 2008
6,386
A long time ago I need a very tiny capacitance to stabilize a wide band closed loop video amplifier. I told the printed circuit board designer to place two narrow traces in parallel for about 1 cm. After I received the first printed boards I trimmed the capacitor with an X-acto knife to "tune it in" and told her how long the gimmick capacitor had to be and the layout was released for production. Never had a complaint about the response of that amplifier.

@Janis59 Sometimes you need to add half a picofarad just to keep things from oscillating as was the case here.
 

Tonyr1084

Joined Sep 24, 2015
4,535
Decades ago I saw a schematic for a radar detector that was basically a capacitor whose legs were a certain length. Of course the initial capacitance was important too, along with some other circuitry - but the point is along with @DickCappels comment about tuning a capacitor. By making the legs longer (standing higher off the PCB) the frequency of the receiver would be altered, and thus tuned to a specific frequency. I think it was in Radio Electronics or Popular Electronics. Man I miss those publications.
 
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