uses for variable capacitor?

[calpolyengineer]

I was taking apart a bunch of electronic gizmos that were lying around the house and came across a variable capacitor and I was wondering what it could be used for.

[Sir H C]

most are so low in value (100pF max) that you can not use them for much unless it is in a circuit that amplifies their value or is extremely high impedence.

[nelson]

you *could* use it to change the clocking speed of  a BBD in a chorus/flanger, if its big enough in value...if not you could always add some caps in paralell.

About the only use I can think of in my limited knowledge.

[Harry]

Why can't these be made in larger values and in potentiometer form?  :arrow:

[niftydog]

there are much bigger capacitors, some in what you might call "the form of a potentiometer" but they are expensive, large, mechanical, fragile beasts and the capacitance is still comparitively low because of the air dielectric. Certainly wouldn't fit one in a hammond 1590.

[Penguin]

actually you could make your own variable capacitor quite easily althouhg you have to understand there are some limitations.

using an ordinary pot [look up rg's the secret life of pots] you can actually monkey with the resistave strip and modify the pot to be a switching pot if you put say 9 taps on it and apply 9 different capacitors you essentialy have a 9 way capacitance switch without the clunkyness of a switch there for making it a variable capacitor



Ed

[calpolyengineer]

They obviously don't have to use an air dielectric though. It seems like they could easily be made as small as a pot and have "useable" capacitance by using a different dielectric. That could potentially add a whole new dynamic to guitar effects.

[calpolyengineer]

That's interesting Penguin. Then you could pick precise values for capacitors to make it linear, log, reverse log, etc.

[Penguin]

That's interesting Penguin. Then you could pick precise values for capacitors to make it linear, log, reverse log, etc.

thanks yep i did that in my tremolo pedal i made only it was for the electrolytics like a fine control for the oscilator

[niftydog]

They obviously don't have to use an air dielectric though. It seems like they could easily be made as small as a pot and have "useable" capacitance by using a different dielectric. That could potentially add a whole new dynamic to guitar effects.

If that could have been done, it would have been done already.

One of the largest influences on a capacitors capacitance is the distance between the plates. Then, the area of the plates is also highly important. The dielectric does have a significant effect, but not so much that it could outweigh the effects of those factors above.

To put it in perspective you could carefully dismantle a 100µF electrolytic just to see the size of the plates and the thinness of the dielectric, but I wouldn't recommend you do that!

Instead, run a few examples through these equations;

The capacitance of a parallel plate capacitor with a dielectric material filling the region between its plates is given by

C = (k x e0 x A)/d

where k is the dielectric constant for the material, e0 is the permittivity of free space, A equals plate area and d equals the distance between the plates.

So, say you want 10µF and you can accurately position the plates 0.1mm apart with Mica sheets in between. You would require a plate size of;

- insert hold music here -

9 square meters... give or take.

[SonicVI]

This has nothing to do with anything, but at first sight I thought somebody's name was 'calopygianeer' (which would be clever and funny though spelled incorrectly    

[R.G.]

Paperweight
Archeological electronic relic
Business card file (big, open ones)
soft fruit macerator (with some modification)

and always, if nothing else, anything may be used as a bad example

[SonicVI]

......

[Vsat]

For use in active filters for audio a variable cap with a max capacitance in the range of 500 pF to 0.005 uF would be quite useful. Immerse it in dielectric oil to increase the capacitance....

My old Philips sine gen used a dual 500 pF variable cap to vary the frequency within each decade range.

Get a 4-gang cap and build a pedal-controlled phase shifter... should be scratch free, might have microphonic problems, but would be able to withstand huge signal levels, and avoids the small amount of distortion that LDRs have. Make sure the capacitance is variable over a wide enough range though.... the Philips had a 10:1 range which would provide just over 3 octaves of sweep.
Cheers, Mike

[calpolyengineer]

If that could have been done, it would have been done already.

Well, IF there is no common use for variable caps with values above a few hundred pF, then there is no need to make them. Plus, just because it can be done doesn't mean it can be done cheaply, it may not be finacially reasonable for any company to make them commercially available on a wide scale.

Just because it can be done does not mean it has to.

Also, the dielectric has a huge influence on the capacitor. For instance, the dielectric constant of mica is about 7; the dielectric of water on the other hand is 80! If your .1mm gap capacitor used water between the plates, the area of the plates could be reduced from 16.2 square meters to 1.4 sq meters. And if the gap was reduced to .05mm, the area goes down to .7 sq meters. Still large, but much better than 16.

[PharaohAmps]

I remember Eric Barbour had a design for a tube-powered wah pedal that used variable caps to sweep the filter frequency.  It used an EF86 pentode tube and required a lot of fabrication to make it all go, but it was a very cool idea.

I can't find my copy of the project, but some Googling would probably turn it up.

Matt Farrow

[aaronkessman]

the Rat uses an externally compensated opamp, and calls for a standard 18pF cap there. you could use a variable cap for THAT. wondering what it'd be like.

[Penguin]

If that could have been done, it would have been done already.

Well, IF there is no common use for variable caps with values above a few hundred pF, then there is no need to make them. Plus, just because it can be done doesn't mean it can be done cheaply, it may not be finacially reasonable for any company to make them commercially available on a wide scale.

Just because it can be done does not mean it has to.

Also, the dielectric has a huge influence on the capacitor. For instance, the dielectric constant of mica is about 7; the dielectric of water on the other hand is 80! If your .1mm gap capacitor used water between the plates, the area of the plates could be reduced from 16.2 square meters to 1.4 sq meters. And if the gap was reduced to .05mm, the area goes down to .7 sq meters. Still large, but much better than 16.

i like what you are saying there man i had an idea once on a water based capacitor it was actually a basic grade school project [salt water capacitor refill the water as needed to change the capacitance and make this audio generator freak out] but i had thought about that as a kind of hit or miss Osc. thought it would be quite interesting

also i agree what you were talking about not being done but being possible. along the same vein did you ever watch the theremin story on dvd.  quite interesting he had rigged up a vacum tube color monitoring system before they had Black and White TV.   makes you wonder why we had B&w first then doesn't it.   A friend of mine has a theory on it that people woudlnt' accept it cause it was too radical so they had to break us into it slowly.  i am still out to lunch on that one.