Question RE variable caps

You should check a book to see how to connect capacitors so they add up.

The reality is if you put them in series, the total goes down. If you put them in parallel, the capacitance is the sum of all the capacitors in parallel.

You'd want to check up on resistors, since they don't work the same way, and clearly that must be what you are thinking of here.

So this will increase the maximum capacitance of the variable. But it will also increase the minimum capacitance. So if the capacitors are

5 to 100pF each, the range will be 10 to 200pF.

Parallel would be both capacitors in parallel, ie take a lead from one capacitor, and connect it to a lead on the other capacitor. Then take the remaining two leads, and connect them. Given that these are variables, it makes sense to put the rotors together and then the stators together.

Now, if you really wanted serial (so the sum is less than the values of each capacitor) then you'd connect a stator to a rotor and then the rotor of the first would be a lead and the stator of the second would be a lead.

Michael

"Dave"

** To add values, the caps must be in parallel.

Caps in series never divide values but follow the formula:

1 / Ct = 1 / C1 + 1 / C2

....... Phil

Which is correct and what you want.

Wow. Thanks to all who replied. Sounds like I have it the way it needs to be, and I just thought it was backwards. Thanks. Now if I can just figure out what it doesn't seem to tune...

Dave

You want then in parallel for the values to add.

Graham

1 f = ----------------- 2pi (sqrt (LC))

where:

f = frequency in hertz L = inductance in henrys C = capacitance in farads

--
JF

Oh yeah, I got that. But for some reason it doesn't seem to actually select the frequency in question. Suspect it is something in the amplification circuit the tank circuit is feeding, but I can't figure out what. And it

*has* worked, wonderfully, just not consistantly. Think I need to back away from it for a while, and let myself get a new perspective.

Thanks,

Dave

Have you considered stray capacitance ?

Graham

...

I'd connect the rotors together, and let the stators be the terminals of the series pair.

But I think Dave still needs to read up on series and parallel capacitors.

Cheers! Rich

I don't really understand this question. If you're wondering why putting capacitors in parallel adds the capacitance, you need to look at the capacitor formula: "The Simple Capacitor C=(8.8542 x 10-12 K A)/D

If a person were to make a homemade parallel-plate capacitor, he or she can use this equation. C is the capacitance in farads. K is the dielectric constant for the material (including air if nothing else is used) between the plates. A is the area of each plate (in square meters). D is the distance between the plates in meters. Air has a dielectric constant of 1. At least most other substances have significantly higher dielectric constants, most of which range from around 3-6." ---

You see that capacitance is directly proportional to the area of the plates. When capacitors are put in parallel, it effectively just adds the areas of the plates, as if it were one capacitor (all other things being equal, of course.)

You can map the formulas for series and parallel resistors one to one on the formulas for parallel and series capacitance (i.e., you just swap them.)

Hope This Helps! Rich

The word ". . . seem to actually SELECT the frequency in question?"

Are you trying to calculate something like a tank circuit center freq based on LC? That works but you have to consider all the "stray" capacitances - proximity to a ground - leads draped near the tuning cap - gate capacitance in a mosfet, fet, etc.. - and your inductor? In the old days that meant how close the coil was to the shielding - today it mean how close the inductor/resonator is to the ground plane or traces on the board.

What frequency are you working at????

And if it did work and won't - and it was "bread boarded" with a push in plastic circuit gizmo - those things are unreliable at radio frequencies - even the ones with ground planes built in.

This could be a layout problem - not a calculation problem.

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Huh. No, I haven't. And am not even sure where to start looking for such. There is a .11 microfarad cap coupling the input to the tank circuit, and I'm not sure what else. Where would one look for stray capacitance?

Thanks,

Dave

Yes, I see this now. Think I have been working too closely on this for too long to be able to back away from it and see it as it really is. Tunnel vision, I guess. Working on that...

Dave

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