Capacitor FAQ

[JBaker]

Well, I asked for it and I got it in a big way.  This list is the best!
Though I am sick as a dog, I feel it's sorta my responsibility to look
out for this FAQ.  Here's what we got so far; there are some things that
need clarification, so keep writing!
-Jeff

Q: What's a capacitor, anyway??
A: A capacitor consists of 2 metal plates seperated by an insulator.
More generically, a capacitor
consists of two or more conductors separated by a non-conductor or
'dielectric'.

Q: Plates?? Dinner plates??
A: The first caps were the infamous "plate glass and tin foil" types
used (still today) in high-
powered RF circuits, like a Tesla coil. They consisted of an alternating
stack of aluminum foil
and a piece of window glass, about 20cm square.  The odd foils were
connected to one electrode,
and the even foils to another like so:

Glass   ============
Foil 1  ******************
Glass   ============
Foil 2  ******************
Glass   ============
Foil 3  ******************
Glass   ============

The first relatively large cap was a "Bell Jar" with metal on each side
of the glass.

Q: How come mine are so small??
A: Technology has demanded ever smaller capacitors with differing
dielectrics for various
purposes.  Manufacturers figured out that if you replace the plates with
plastic film, you could
roll capacitors like a pastry to make them smaller.

Q: So why so many types: mylar, polyester, etc etc.??
A: The types describe the "quality" of the film used. That's why most
caps are grouped as:

 1) electrolytic
 2) film
 3) ceramic
 4) misc (like mica)

Capacitors are referred to by their dielelectric material.  Common types
include plastic,
ceramic, tantalum, mica, air, and electrolytes.

Q: Why do electrolytic caps have + and -?
A: Because the insulator used is a liquid (actually a gel), the the
chemisry of it requires one end
to be more positive than the other, like a battery.

Some capacitors are constructed such that they have a greater DC leakage
in one polarity. In the
proper DC polarity they offer great amounts of capacitance in a small
volume.

Q: Great, now I'm even more confused!! How do I know which one to use in
a certain appliation??
A: Well, here are some rules to go by, listed by application:

BYPASS CAPS - Use 0.1uf (microfarad = 10^-6) ceramic caps. Don't use the
"disc" types if you can
avoid it; use axial-leaded types.  The reason is they are coated with
epoxy and resist moisture.
Also, they are much smaller.

AUDIO COUPLING CAPS - Used to AC couple an audio input.  Use 0.47uf
metallized polyester or poly-
proplyene.  Values less than this will attenuate bass frequencies.

VCF CAPS - Use polypropylene film.

VCO CAPS - Use polystyrene if you can find them (the one German company
that makes the film will
stop selling it this year!!) or use metallized polypropylene.

POWER SUPPLY CAPS - For the cap between the diodes and the regulator,
use a cap rated to 105
degrees C. They are only slightly more expensive (pennies) than the 85
deg. C types, but much
higher quality.  For the output caps on 3-terminal regulators, use a 1,
2.2 or 3.3uf tantalum in
parallel with an 0.1uf ceramic.

Following are some guidelines, listed by capacitor type:

CERAMIC CAPS - Low cost capacitors in the range from about .1pf
(picofarad = 10^-12) to 1uf.  Di-
electrics can alter values with Temp (X7R, Z5U) some are Temp stable
(COG, NPO).  Relatively small
sizes but they exhibit lossy behavior and a little waveform distortion.
Good low cost, low
fidelity coupling and power supply bypassing.  Good RF caps.

TANTALUM CAPS - Moderate cost capacitors in the range from about .1uf to
470uf. They have polar-
ized dielectrics and fairly small volume wise. They have a limited
voltage capability; usually
less than 50 volts. Good capacitors for supply bypassing but have some
leakage current. Not
recommended for decoupling or signal processing.

ELECTROLYTIC CAPS - Moderate cost capacitors in the range from about
.1uf to 2.2f.  They have
polarized dielectrics and are used in a wide variety of voltages.  Some
types are optimized for
decoupling and audio signal processing.  Most have some leakage
current.  It should also be noted
that electrolytic caps SHOULD NOT be used where the DC potential across
the cap is substantially
below the rated working voltage.  Some people might assume that doing so
provides a safety margin.
But since the dielectric is "formed" by the voltage applied across the
capacitor, they will lose
capacitance when operated much below their rated voltage.

This is why voltage ranges are relatively small: 6.3, 10, 16, 25, 35,
50, etc.  Pick the voltage
rating the next step higher than the _peak_ voltage across the cap.

If you exceed the voltage rating for an electrolytic or let it get too
hot it can EXPLODE!!  Great
care should be taken to ensure that this does not happen, as capacitors
contain some pretty toxic
stuff.  Double-checking that the electrolytic is oriented the correct
way in the circuit and
measuring the voltage across it with a meter are two ways to avoid a
dangerous situation.

PLASTIC CAPS - (a.k.a. mylar, polyester, polycarbonate, polypropylene,
polystyrene, and 'metallized'
versions.)  These caps are stacked or wrapped dielectric construction
and may resonate at very high
frequencies.  Usually excellent for decoupling and bypassing of audio
signals.  Some types are
better for audio than others (e.g. polypropylene).  Values between 470pf
and 10uf usually. Fairly
moderate to large size volumes.

MICA CAPS - They are tight tolerence capacitors between 0.1pf and
.0033uf typically
used for RF coupling or bypass applications.  Fairly expensive but
useful for high frequencies.
Fairly large physical size for their values.

AIR CAPS - Used for small values from .01pf to 100pf and usually
variable capacitance types for RF
tuning. Relatively large for their values and prone to dust leakages.

Q: What about Mylar?
A: Mylar, also called polyester, is to be AVOIDED!!  Mylar is popular
for ONE REASON: PRICE. They
are not suitable for serious audio work.  Mylar was the first film cap
available (about 1953) and
most 'old-timers' associate film caps to a mylar cap.

Q: Why?
A: They have 2 problems: over time, they 'age'.  They change their
capacitance. The second problem
is mylar isn't that good of a film: it is "leaky" and the charge bleeds
off. That's why in VCO's
and S/H's a mylar cap is the WORST choice you can make.

Q: What about polycarbonate?
A: No good either. It was to be a low cost mylar cap!  Polycarbs are
used in similar applications
as polyester caps.  Some specsmanship is needed for voltage range,
tempco, tolerence, etc.

Q: Are there any strange capacitor-related effects I should be aware of?

A: All caps, particularly DC-blocking caps in the audio path, can become
microphones.  With a polar-
izing voltage across them, capacitors can be sensitive to vibration.  A
knock on your synth's case,
for example, may be heard through your amplifier.  One fix is to put a
glop of RTV adhesive over
the capacitor.  Ceramics are the worst offenders; yet another reason to
never design them into the
audio path!!

Q: Where can I get good caps?
A: In the US, try Digikey (1-800-DIGIKEY and online searchable index).
Other sources are Allied,
Newark, and Mouser.

Q: What are the best types to use in DIY synth projects?
A: In conclusion:

1) 0.1uf ceramic axials for bypassing
2) polypropylene for all audio paths
3) polystyrene for VCOs if you can find them, metallized polypropylene
if not

Hope this helps!!

Paul Schreiber, Mark Amundson, Rich Nelson, Jeff Baker