Paul' capacitor FAQ

[Mark Amundson]

Paul Schreiber wrote:
> 
> OK, here is the first stab at a capacitor FAQ. JH, MD and others please join in!!
> 
> Q: What's a capacitor, anyway??
> A: A capacitor consists of 2 metal plates seperated by an insulator.

More generically, two or more conductors seperated 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 were a stack of alternation alum. foil and a piece of window glass, aout 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: Because people figured out if you replace the plates with plastic film, you could roll them up like a pastry.

Technology has demanded ever smaller capacitors with differing
dielectrics for various
purposes.

> 
> Q: So why so many types: Mylar, polyester, etc etc.
> A: Because they 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)

Too many dielectric types to accurately list. Plastic types, Ceramic
types, 
Tantalum types, Electrolytic types, Mica types, and Air types.

> 
> 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 so rules to go by.

Experiance and training. My Groupings:

Ceramic Caps - Low cost capacitors in the range from about .1pf to 1uf.
Dielectrics
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 polarized 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.

Plastic Caps - (a.k.a. mylar, polyester, polycarbonate, polypropylene,
polystyrene,
and metalized 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 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.

> 
> BYPASS CAPS
> 
> Use 0.1mfd ceramic caps. Don't use the "disc" types if you can: 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 0.47mfd metallized polyester or polyproplyene. 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 deg. 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 3.3mfd tantalum in parallel with a 0.1mfd ceramic.
> 
> 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: is is "leaky" and the charge bleeds off. THat's why in VCOs and S/H a Mylar cap is the WORSE
> choice you can make.
> 
> Q: What about polycarbonate?
> A: No good either. It was to be a low cost Mylar cap!
> 
> Q: Where can I get good caps?
> A: In the US, try Digikey (1-800-DIGIKEY). Other sources are Allied, Newark, and Mouser.
> 
> In conclusion:
> 
> 1) 0.1mfd 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
> Synthesis Technology
> 
> ----------
> From:   scopey at hooked.net[SMTP:scopey at hooked.net]
> Sent:   Saturday, October 25, 1997 12:03 PM
> To:     synth DIY
> Subject:        cap varieties
> 
> I've been wondering this for a while, and no one seems to know:  Is
> there any difference between polyester and polycarbonate caps?  When it
> says 'poly', what am I to assume?  While we're on the subject, what's
> the advantage of poly-whatever anyway?  Temperature stability?  Improved
> voltage specs?  Also, is it generally OK to substitute a mylar cap for a
> ceramic one?  Again, what is the advantage of ceramic?
> 
> -Jeff

Polycarbs are used in similar applications as polyester caps. Some
specsmanship
is needed for voltage range, tempco, tolerence, etc.

Mark Amundson,