Hi All
When you want to build a circuit and be able to play with the Resistance of a Resistor, you buy an Adjustable Resistor, or in 1 word - Potentiometer.
There are many kinds, in many sizes, and in many stores.
Adjustable Capacitors on the other hand, I can't seem to find.
I would like to buy some, in order to be able to play with its value when it's connected to a circuit, and then see the effect on the result.
Anyone knows where can I buy such?
Thank you very much
When you want to build a circuit and be able to play with the Resistance of a Resistor, you buy an Adjustable Resistor, or in 1 word - Potentiometer.
There are many kinds, in many sizes, and in many stores.
Adjustable Capacitors on the other hand, I can't seem to find.
I would like to buy some, in order to be able to play with its value when it's connected to a circuit, and then see the effect on the result.
Anyone knows where can I buy such?
Thank you very much
You mean a Variable Capacitor. They exist, but the available values are way too small be be of any use in the audio electronics realm. There are, or at least used to be capacitor substitution switch boxes, which had a rotary switch and a bunch of different value caps, in a box with alligator leads. That's something you could build yourself...you know, DIY! 😉
Mike
Mike
Hi Mike
Thank you very much for your reply.
I wonder why bigger ones are not manufactured..
I remember when I was a child, I used to take apart small portable radios,
an they had inside a variable capacitor..
Made of many layers of metal, intertwined with many layers of a transparent material that reminds nylon.. and a knob, that gradually takes the metals out of the nylons..
Quite an easy build, and customizable to any needed values,
yet with only 1 disadvantage:
The transition between each Capacitance will not be gradual..
In fact, it will even include a momentary disconnection..
This means, that unlike a real variable capacitor,
here, changing the capacitance cannot always be performed while the whole circuit is running,
and instead, the circuit should be powered off before changing the capacitance, and then turned on again..
BTW,
when thinking of it,
If one wants to be really creative, he can DIY build a capacitor like in the first part of this post..
Just a big one..
I do prefer buying something ready, If anyone might happen to have a source for such..
Thank you very much for your reply.
Oh, sad to hear,
I wonder why bigger ones are not manufactured..
I remember when I was a child, I used to take apart small portable radios,
an they had inside a variable capacitor..
Made of many layers of metal, intertwined with many layers of a transparent material that reminds nylon.. and a knob, that gradually takes the metals out of the nylons..
Right..
Quite an easy build, and customizable to any needed values,
yet with only 1 disadvantage:
The transition between each Capacitance will not be gradual..
In fact, it will even include a momentary disconnection..
This means, that unlike a real variable capacitor,
here, changing the capacitance cannot always be performed while the whole circuit is running,
and instead, the circuit should be powered off before changing the capacitance, and then turned on again..
BTW,
when thinking of it,
If one wants to be really creative, he can DIY build a capacitor like in the first part of this post..
Just a big one..
I do prefer buying something ready, If anyone might happen to have a source for such..
Trimmer capacitors are still available, but usually have values of a few dozen picofarads or less, for example:
https://nl.farnell.com/w/search?capacitance-max=50pf&st=variable capacitor
Mouser has some trimmer types with an unusually large capacitance:
Find Electronic Components | Mouser Europe
Variable capacitors used to be available up to a few hundreds of picofarads. This shop in the Hague is specialized in antique electronics, maybe they still have them:
Radio Twenthe – on- en offline electronica winkel, verzending na vooruitbetaling
If you need more capacitance, you could combine a switchable capacitor bank with a variable or trimmer capacitor for fine tuning.
https://nl.farnell.com/w/search?capacitance-max=50pf&st=variable capacitor
Mouser has some trimmer types with an unusually large capacitance:
Find Electronic Components | Mouser Europe
Variable capacitors used to be available up to a few hundreds of picofarads. This shop in the Hague is specialized in antique electronics, maybe they still have them:
Radio Twenthe – on- en offline electronica winkel, verzending na vooruitbetaling
If you need more capacitance, you could combine a switchable capacitor bank with a variable or trimmer capacitor for fine tuning.
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Thank you very much..
I will try to email the store on the second link..
(altho it seems that as you 2 both showed, there's mostlikely not going to be any bigger capacitance)
OK, I guess for audio this is not really possible.
I might build a small switch box with several capacitors and a code switch, and use it carefully (turning off the circuit before changing)..
It's a nice idea..
I will try to email the store on the second link..
(altho it seems that as you 2 both showed, there's mostlikely not going to be any bigger capacitance)
OK, I guess for audio this is not really possible.
I might build a small switch box with several capacitors and a code switch, and use it carefully (turning off the circuit before changing)..
It's a nice idea..
https://www.surplussales.com/index/Variable-Capacitors.html
https://www.surplussales.com/Variables/AirVariables/AirVar1.html
45-1000 pF (5-1/2" long body) $107
10-100pF and 10-130pF $14.50
21 - 820 pF 4"L -- Government cost: $1,700 -- $149 each
Air is "thinner" than copper, iron, or carbon. A "variable" capacitor can't be wound super-tight and also be variable.
See also "Decade boxes":
Test Equipment: Decades & Bridges
While that shop mostly does high-class gear, you can build a fine cap-decade out of two one-dollar 12-way switches and a capacitor assortment.
3nF is about the largest you get off the shelf it seems, the size of a shoebox and rated for kilovolt or so.
If you want more capacitance you've have to get it made, and it would be huge and probably quite microphonic if you go for small plate separations. The hum pick up could be quite a serious issue too with all that surface area
Variable capacitors are made for RF and RF power applications. Often they use vacuum variable caps for high power since air is all sparky and dust collecting, and you don't want your transmitter taken out by a vaporizing insect.
Switched capacitance is the way to go with audio, definitely!
I understand.
My original thought was to have a nice set of variable capacitors, like I have in potentiometers..
I will create something myself then.
BTW,
In capacitance, connecting capacitors in parallel, adds their capacitance together.
So Sachin's idea
The nice thing about a DIP Switch is the fact that it's a set of several switches, in 1 component.
But 1 disadvantage of it, is that the switches are small.
Is there maybe something similar (1 component, with many switches) but that is bigger?
So you can more easily play with it
My original thought was to have a nice set of variable capacitors, like I have in potentiometers..
I will create something myself then.
BTW,
In capacitance, connecting capacitors in parallel, adds their capacitance together.
So Sachin's idea
is a really good option here, which enables getting ore values, than If I use a Code Switch (which will only select one Capacitance, and not allow creating mixes).
The nice thing about a DIP Switch is the fact that it's a set of several switches, in 1 component.
But 1 disadvantage of it, is that the switches are small.
Is there maybe something similar (1 component, with many switches) but that is bigger?
So you can more easily play with it
In the 1920s-1940s, nearly every radio maker had the tooling to make variable caps; and a couple factories offered a wide range of stock and custom-order caps.
I bet ALL that tooling got thrown-out in the 1970s. Oh, maybe one maker for the limited radio-transmitter market. Looks like Sprague.
Find Electronic Components | Mouser
Also Comet-PCT:
Comet PCT - Variable Capacitors
Comet PCT - Uni-Select
The once-common crystal AM radio cap is now $20 (and does not have the 2nd gang for superhet radio); I'm actually shocked because these were un-findable a few years ago.
Capacitor - Variable, Single, 365 pF, CW Rotation: Amazon.com: Industrial & Scientific
*Small* trim caps are still a thing, dozens pFd and Volts. Quite useless for most audio. (Compensation in hi-Z NFB amps, but we don't want these to be remotely close to squeal, and we use standard values not trims.)
Back in the 1920s we made everything by hand. (Meaning more variable coils than caps.) You can still DIY a cap:
Air Variable Capacitor From Scrap Aluminum Sheets: 18 Steps (with Pictures)
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It all depends on what you want to do with it. The last two times I used a variable capacitor for audio, I used it to trim a phase correction network for a subtractive test. That would have been a nightmare with a switchable capacitor bank, but a switchable capacitor bank plus a continuous control would have been OK.
Attachments
Amazing how what I thought was a simple request, turns out to be such a rare and almost non-existing thing..
At least for the Audio field..
At least for the Audio field..
Look at it from a practical side, any "audio range" variable capacitor will be a monster.
Think **room size** or larger. No kidding.
Think **room size** or larger. No kidding.
There are tricks with op-amps, like making an inductance from a capacitor.
Active circuitry can do magic.
Why not make a large capacitor from a small adjustable cap. I do not have a schematic to propose, I bet it is doable and may be simple.
As an example a large liaison cap can be replaced by a servo that has a quite small cap.
Back to variable caps. Old radio receiver had those beautifull rotary caps. Gone to the drain when PLL became popular.
I remember a 50pF adjustable that was two cylinders mesched together with a screw inside, rotating was doing this adjustment. A popular cap of the 60s. I used one in a 27Mhz tube power stage, before I replaced that cap by a 27.12000 Xtal.
Active circuitry can do magic.
Why not make a large capacitor from a small adjustable cap. I do not have a schematic to propose, I bet it is doable and may be simple.
As an example a large liaison cap can be replaced by a servo that has a quite small cap.
Back to variable caps. Old radio receiver had those beautifull rotary caps. Gone to the drain when PLL became popular.
I remember a 50pF adjustable that was two cylinders mesched together with a screw inside, rotating was doing this adjustment. A popular cap of the 60s. I used one in a 27Mhz tube power stage, before I replaced that cap by a 27.12000 Xtal.
You can use the Miller effect: make an inverting amplifier with a gain of -A and connect the capacitor with value C between input and output, and you will see a capacitance of (1 + A)C at the input. The inverting amplifier could be made with two op-amps, one that acts as a buffer and one connected as the usual op-amp inverting amplifier. The trick is to keep it stable with two op-amps in the loop and to keep them from clipping.
In principle, you could use a fixed capacitor and tune the gain -A with a variable resistor.
As an alternative, you could make a non-inverting amplifier with a gain adjustable between 0 and 1 and bootstrap out a part of the capacitance. Basically the same trick but with the opposite sign of the gain, leading to a reduction rather than an increase of the capacitance.
In principle, you could use a fixed capacitor and tune the gain -A with a variable resistor.
As an alternative, you could make a non-inverting amplifier with a gain adjustable between 0 and 1 and bootstrap out a part of the capacitance. Basically the same trick but with the opposite sign of the gain, leading to a reduction rather than an increase of the capacitance.
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you can use small breadbroad and start with lowest value of the range of capacitance you need and keep on adding in parallel till you sort out the proper value. you can fine tune later after narrowing it down. breadboard will not require any soldering I guess.
Why does the "variable" part increase the size?
With capacitors that have fxed capacity, we have audio-range capacitors in small component size..
mchambin:
Yes, those are the capacitors I mentioned in the beginning.
I was not aware, back then, that their capacitance is small.
thimios and Hiten:
Thank you
For each layer of a capacitor, you have the parallel plate capacitance equation:
C = epsilon0 epsilonr A/d, with A the area of and d the distance between the plates.
Fixed capacitors often have extremely thin insulation layers, so very small d. As long as the voltage is not too high, the electrodes stay in place and the insulation can't get dirty, that works fine. In a variable capacitor, the distance between the plates can vary more and there is more chance of dirt getting in, so d has to be made much bigger.
C = epsilon0 epsilonr A/d, with A the area of and d the distance between the plates.
Fixed capacitors often have extremely thin insulation layers, so very small d. As long as the voltage is not too high, the electrodes stay in place and the insulation can't get dirty, that works fine. In a variable capacitor, the distance between the plates can vary more and there is more chance of dirt getting in, so d has to be made much bigger.
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