small variable capacitors (measured in uF)? measuring capacitance?

[Mad Scientist Jr]

I would like to hook up a variable capacitor to some audio circuits I
built for learning about synths and amps
(schematics are here:
http://www.geocities.com/usenet_daughter/tone_generators.htm )
http://www.runoffgroove.com/littlegem.html
http://www.runoffgroove.com/ruby.html
http://makezine.com/09/crackerboxamp/
)

because in some of these the capacitors change the pitch, tone or
other qualities.

These circuits run off a 9V battery and use capacitors with ratings
such as
220 uF
100uF,
0.1 uF,
0.01 uF
0.047 uF
so I would want variable caps in this range.
Do these exist? Online all I am seeing is ones rated in pF.

Also, assuming I find these, and get them working in the circuit, and
find a desired setting for a capacitor, how do you measure the
capacitance? I have a multimeter but have really only used it to
measure ohms.

Thanks...

 

[Lostgallifreyan]

These circuits run off a 9V battery and use capacitors with ratings
such as
220 uF
100uF,
0.1 uF,
0.01 uF
0.047 uF
so I would want variable caps in this range.
Do these exist? Online all I am seeing is ones rated in pF.

They'd be HUGE. Easily as big as a fridge or a car, even a truck in the
case of 220 µF.

What matters is the timing, as it's timing that matters in amps and
synthesizers, except for power supplies or sample and hold circuits where
capacity is more important.

To get long timing with small capacitors, use large value resistors, and
make sure you have low leakage capacitors.

 

[Lostgallifreyan]

Also, use a fixed capacitance and vary the resistance. That's what is
usually done, and is why variable capacitors are rare.

 

[Mad Scientist Jr]

They'd be HUGE. Easily as big as a fridge or a car, even a truck in the

case of 220 µF.

Are you serious? That's crazy! The fixed capacitors themselves are so
tiny!

What matters is the timing, as it's timing that matters in amps and
synthesizers, except for power supplies or sample and hold circuits where
capacity is more important.
To get long timing with small capacitors, use large value resistors, and
make sure you have low leakage capacitors.

Aha! Thanks for the advice.

 

[Lostgallifreyan]

Are you serious? That's crazy! The fixed capacitors themselves are so
tiny!

Well, I was slightly overdoing it, on the assumption that you'd be looking
at something rigid enough to support the weight, and given that it would
need air or solid dielectric separation.

 

[David]

I would like to hook up a variable capacitor to some audio circuits I
built for learning about synths and amps
(schematics are here:
http://www.geocities.com/usenet_daughter/tone_generators.htm )
http://www.runoffgroove.com/littlegem.html
http://www.runoffgroove.com/ruby.html
http://makezine.com/09/crackerboxamp/
)

because in some of these the capacitors change the pitch, tone or
other qualities.

These circuits run off a 9V battery and use capacitors with ratings
such as
220 uF
100uF,
0.1 uF,
0.01 uF
0.047 uF
so I would want variable caps in this range.
Do these exist? Online all I am seeing is ones rated in pF.

Also, assuming I find these, and get them working in the circuit, and
find a desired setting for a capacitor, how do you measure the
capacitance? I have a multimeter but have really only used it to
measure ohms.

Thanks...

\
http://www.stormwise.com/page3.htm

 

[John A]

220 uF
100uF,
0.1 uF,
0.01 uF
0.047 uF
so I would want variable caps in this range.
Do these exist? Online all I am seeing is ones rated in pF.

Also, assuming I find these, and get them working in the circuit, and
find a desired setting for a capacitor, how do you measure the
capacitance? I have a multimeter but have really only used it to
measure ohms.

You cannot buy variable capacitors with these values, but you could make up
or purchase an arrangement of switches and fixed capacitors quite cheaply
which a) would be the next best thing and b) would directly address your
"how do you measure the capacitance" question. Commercially they are called
Capacitance Substitution Boxes or Capacitance DecadeBoxes . They're not
infinitely variable, of course, but are finitely practical!

John A

 

[Lostgallifreyan]

You cannot buy variable capacitors with these values, but you could
make up or purchase an arrangement of switches and fixed capacitors
quite cheaply which a) would be the next best thing and b) would
directly address your "how do you measure the capacitance" question.
Commercially they are called Capacitance Substitution Boxes or
Capacitance DecadeBoxes . They're not infinitely variable, of course,
but are finitely practical!

Nice. I'm not sure that's what the OP wanted, (more likely a single
continuous control of something), but if this switched-cap box were built
to 1 nF resolution, for $40 extra or so, you can add a variable capacitor
shown on the page David linked to: http://www.stormwise.com/page3.htm

That way you can have any infinitely variable value, just not in one sweep.

 

[John A]

Nice. I'm not sure that's what the OP wanted, (more likely a single
continuous control of something), but if this switched-cap box were built
to 1 nF resolution, for $40 extra or so, you can add a variable capacitor
shown on the page David linked to: http://www.stormwise.com/page3.htm

That way you can have any infinitely variable value, just not in one

sweep.



Well, I intentionally didn't suggest adding a variable as a) it would
re-open the Pandora's Box of how to calibrate the thing, and, b) the
combined tolerances of the fixed components would make a nonsense of such
calibration anyway.

As the OP sounds as though he may want several capacitors to be
simultaneously variable his best homebrew move may be to build up
a number of simple, two or three decade BCD-style, successive approximation
systems. Each will need 4 switches and capacitor combos (15 caps of one
value) per decade - a nice little homebrew project.

John A via rec.radio.amateur.homebrew

 

[John A]

Nice. I'm not sure that's what the OP wanted, (more likely a single
continuous control of something), but if this switched-cap box were built
to 1 nF resolution, for $40 extra or so, you can add a variable capacitor
shown on the page David linked to: http://www.stormwise.com/page3.htm

That way you can have any infinitely variable value, just not in one

sweep.



Well, I intentionally didn't suggest adding a variable as a) it would
re-open the Pandora's Box of how to calibrate the thing, and, b) the
combined tolerances of the fixed components would make a nonsense of such
calibration anyway.

As the OP sounds as though he may want several capacitors to be
simultaneously variable his best homebrew move may be to build up
a number of simple, two or three decade BCD-style, successive approximation
systems. Each will need 4 switches and capacitor combos (15 caps of one
value) per decade - a nice little homebrew project.

John A via rec.radio.amateur.homebrew

 

[Lostgallifreyan]

Well, I intentionally didn't suggest adding a variable as a) it would
re-open the Pandora's Box of how to calibrate the thing, and, b) the
combined tolerances of the fixed components would make a nonsense of such
calibration anyway.

Initially, I didn't assume the OP wanted a large variable capacitor at all,
but was mistakenly trying to match fixed sizes with variable sizes without
considering why such things are not common practise. Calibration aside, the
variable I suggested adding IS a viable idea, assuming your switched-cap
box is valid. After all, if you CAN get any value, then you set it as you
need. If it's tuning frequency, you measure the frequency.

But as I said WAy early in the thread, first reply, if you really want a
sweep of possible timing values, just use a fixed cap and a variable pot,
as standard. The OP was talking about synthesizers, after all, he mentioned
them explicitly.

 

[Lostgallifreyan]

Well, I intentionally didn't suggest adding a variable as a) it would
re-open the Pandora's Box of how to calibrate the thing, and, b) the
combined tolerances of the fixed components would make a nonsense of such
calibration anyway.

Another thing: That variable cap on the page David (and I) linked to, is
more than 1 nF. So long as the fixed caps had a tolerance tight enough, it
should cover (1.398 nF allows for greater than 20% tolerance on nominal 1
nF increments). Not that any of this matters, in practise.