HOW DO I MAKE A VARIABLE CAPACITOR ?

[alexzanderepair]

HOW DO I MAKE A VARIABLE CAPACITOR ?

[rockgardenlove]

Use a pot to pan between a large value and a small value.

[greenacarina]

Or salvage one out of a radio??

[alexzanderepair]

rockgardenlove, sorry if i sound retarded but how do i set up a pot like that. im confused

[JimRayden]

rockgardenlove, sorry if i sound retarded but how do i set up a pot like that. im confused

Well, you don't have to pan between caps. As two caps in parallel add up, you can use this method (I remember reading about it somewhere):

Code Select Expand


        Small cap
      -------||---------
     |                  |
-----|        ____      |----
     |       |   V      |
      ---||---\/\/\/\/---
   Large Cap   Pot


---------
Jimbo

[mcasey1]

Would this be the equivalent of a simple tone stack?

[alexzanderepair]

DOES THE VALUE OF THE POT MATTER?

[JimRayden]

Would this be the equivalent of a simple tone stack?

When connected to ground (with some additional components), it works the same as any other tone control. Note that it will not act as a real variable cap. It will not change the frequency of rolloff, it will change the slope as tone circuits usually do.

DOES THE VALUE OF THE POT MATTER?

YES IT DOES. I'LL RUN SOME SIMULATIONS WHEN I GET HOME AND DECIDE ON IT'S VALUE. IF ANYONE CAN TELL A VALUE FROM THE TOP OF THEIR HEADS, I'D APPRECIATE THAT.

UNLESS YOU HAVE A FAULTY CAPSLOCK, I'D HAVE... ahem... I'd have to ask you to stop writing in capital letters, as it is considered yelling on internet communities. No offense.

----------
Jimbo

[The Tone God]

We are missing information to give a concise answer. What value range are you looking for ? What type of signal are you attempting to control ?

Andrew

P.S. Please don't use all caps. The shift key is not that hard to use.

[alexzanderepair]

sorry for the caps thing.
the range is 0.005uf to 0.068uf
trying to control the sweep range on a wah

[Ge_Whiz]

You can't make a true variable capacitor in the nanofarad range, and the 'pan-pot' idea will not work for what you want to do here, sorry.

[zachary vex]

i seem to recall something about a beaker of fluid and some plates and wires...

[The Tone God]

i seem to recall something about a beaker of fluid and some plates and wires...

That was just foreplay.

Andrew

[rockgardenlove]

rockgardenlove, sorry if i sound retarded but how do i set up a pot like that. im confused

Well, you don't have to pan between caps. As two caps in parallel add up, you can use this method (I remember reading about it somewhere):

Code Select Expand


        Small cap
      -------||---------
     |                  |
-----|        ____      |----
     |       |   V      |
      ---||---\/\/\/\/---
   Large Cap   Pot


---------
Jimbo

That's an option too, probably better than mine.  Both will work though.

[bancika]

why don't you just use rotary switch with as many positions as you need with different cap connected to each pole. That's usual way of doing variable wah sweep

[StephenGiles]

Why can't just buy one?

[Paul Perry (Frostwave)]

Why can't just buy one?

welll...... an air cored variable cap in that value range is about 1,500 cubic inches. It's going to be a tight fit, plus not that easy to adjust
In theory, you could make something cute using a cap synthesised from OTAs, but I'm sure it wouldn't be practical. Go switched.

[R.G.]

At the risk of confusing things, this is easy, at least in theory.

Method 1: Switched Capacitors
If you put a capacitor in series with a switch and turn the switch on and off very rapidly, then the current that flows through the capacitor depends on the voltage across it when the switch is closed and the proportion of the time the switch is closed. If you run the switch at a high enough frequency, the apparent capacitance looks like the real capacitor times the duty cycle.

So I think that you can do this with a PWM setup and a CMOS switch, possibly a JFET switch.

Of course, you have to pay Mr. Nyquist his due, and make the switching frequency be more than three times the highest frequency in your signal, preferably above the audio range so you can't hear switching artifacts.

Method 2: Capacitor DAC
Make an R-2R ladder out of capacitors. Switch the ladder nodes with CMOS or JFET switches controlled by a binary encoder set up to look like a pot. The effective capcitance then varies in about 1/2% steps, appearing completely smooth to a human.

Method 3: Sythesized capacitor
Set up a synthetic capacitor circuit along the lines of a gyrator, but with capacitors.

There are others but they're much more limited in scope.

For instance, the way I sense water level in the rainwater tanks: drop a loop of insulated wire into the tank, all the way to the bottom. Hook both bare ends together at the top. Stick an electrode in the water. The capacitance between the wire sensor and the electrode is linear with the depth of the water from some non-zero minimum to some maximum when the water covers up all the wire it can. The water acts as a conformal second capacitor plate to the wire through the insulation.

[tungngruv]

[johngreene]

That is only a capacitor when the pot is shorted. Any other setting of the pot results in a RC network, not the same thing as just a 'capacitor' of a different value.

--john