E-Music DIY Reference Archive

The Capacitor Reference Guide

Q: What's a capacitor, anyway??

A: A capacitor consists of 2 metal plates separated 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 jar with a metal coating on the outside and inside, the official name for which is a 'Leyden Jar'.

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. Ceramic
2. Electrolytic
3. Film
4. Miscellaneous (like mica)

Capacitors are referred to by their dielectric 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 chemistry 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 application??

A: Well, here are some rules to go by, listed by application (note: all values given are just typical suggestions):

BYPASS CAPS - Bypass cap recommendations may vary from designer to designer. The famous 0.1uf (microfarad = 10^-6) ceramic is a good value for audio supply bypassing; 0.01uf may be a little better if you mix higher speed circuits like logic on the same supplies. Values also depend on load and input impedance. 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. Do not forget to retain a 10uf or greater aluminum or tantalum electrolytic on circuits as well. They act as local energy storage reserviors when the DC supply is more than a few inches away.

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

VCF CAPS - Use polypropylene film.

VCO CAPS - Use polystyrene if you can find them (try Mouser); the one German company that makes the film will stop selling it this year!! Metallized polypropylene, and polycarbonate are also fine. See "What about polycarbonate caps?" below.

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 1.0, 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 0.1pf (picofarad = 10^-12) to 1uf. Dielectrics can alter values with temperature (X7R, Z5U types), some are temperature stable (COG, NPO types), but some vary extremely with temperature. They exhibit lossy behavior and a little waveform distortion. Good for power supply bypassing and RF applications. Low self inductance makes them good for use as bypass in digital circuits. Avoid ceramics in the audio path.

TANTALUM CAPS - Moderate cost capacitors in the range from about 0.1uf 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 0.1uf 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. Avoid a dangerous situation by double-checking the orientation of and the voltage across your electrolytics. Also, don't place electrolytics near heat sinks.

PLASTIC CAPS - (a.k.a. Mylar, polyester, polycarbonate, polypropylene, polystyrene, and 'metallized' versions.) These caps are stacked or wrapped (stacked - parallel plates, wrapped - cylindrical shaped plates) 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. Available in fairly moderate to large size volumes.

MICA CAPS - They are tight tolerance capacitors between 0.1pf and 0.0033uf 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 0.01pf 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: This type is known for good performance (i.e. stable) over a wide temperature range. Polycarbs are used in similar applications as polyester caps. The Illinois Capacitor company makes a line of polycarbonate caps with temperature coefficient (tempco) of 50 ppm, similar to good metal film resistors. The primary concern for VCO timing caps is tempco. Polycarbonates are specified for Electronote VCO's. They are also specified directly on the schematics for Buchla VCO's.

Q: Are paper caps OK to use?

A: Paper capacitors predate Mylar by about 60 years. They are prone to leaking and other bad age-related effects, and are therefore not recommended. In case you want to experiment with "bad" capacitors, there are new paper-dielectric capacitors being made for the more extreme audiophiles. Audio Note is the major brand. Also, Sprague/Vishay still manufactures the old phenolic-impregnated-paper 'Vitamin Q' types. All very expensive, and very leaky. Definitely not recommended for time constants in VCOs or for temperature-sensitive applications.

Q: What about oil-filled caps??

A: Oil-filled capacitors still used in high-voltage AC applications. Some audiophiles build DIY tube amplifiers using oil caps to filter the high-voltage plate supply; users report that oil-filled caps have some major advantages over electrolytics, sound quality being one of them.

Q: Do teflon caps really exist?

A: Yes. Made by 2 companies at present. Used in very specialized military and industrial applications, and in overpriced audiophile tube preamps.

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 transducers, like microphones. A vibration may induce voltages due to internal "piezo-electric" effects. 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 and some plastic caps are the worst offenders; yet another reason to never design them into the audio path!! Some ceramic dielectrics are worse than others regarding this effect.

Q: Any other pitfalls to avoid?

A: Yep. A high-valued cap (electrolytic, tantalum) is often used to bypass relatively low frequencies. Such a capacitor, however, may not bypass expected high frequencies (based on the circuit's RC time constant for an ideal cap) because the equivalent series resistance (ESR) of these caps becomes excessive at high frequencies. The remedy is to parallel the capacitor with a small (0.1uf) high frequency cap.

Q: Where can I get more information?

A: If you care about this stuff you need the analog designers bible, "the Art of Electronics" by Horowitz and Hill, Cambridge University Press, readily available everywhere and pirated into most Asian and East European languages.

Q: I wish I had a quick reference I could tack on the wall next to my workspace!

A: Umm, yeah, funny you should ask...This data comes from Analog Devices' "Analoq Dialog" 30-2 from 1996. The data is tabular and concentrates on dielectric absorption (DA) criteria.

INSERT GIANT TABLE HERE

Q: Where can I get good caps?

A: In the US, try Digikey (http://www.digikey.com has an online searchable index). Other sources include Allied, Newark, and Mouser (http://www.mouser.com). Both Digi-Key and Mouser'll send ya a free catalog, requestable at their website. Delivery is quite prompt.

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

A: In conclusion:

1. ceramic axials for bypassing
2. metallized polyester or polypropylene for audio coupling
3. polypropylene for all audio paths (VCF's, VCA's, etc.)
4. polystyrene, polycarbonate, or metallized polypropylene for VCO's
5. tantalum and ceramic for voltage regulators

Thanks go out to the following people for compiling this information:

Paul Schreiber, Mark Amundson, Rich Nelson, Jeff Baker, Paul Perry, Eric Barbour, Grant Richter, Tony Clark