Paul' capacitor FAQ

[Paul Schreiber]

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.

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   ============

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.

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)

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.

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.

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