Have a Jameco catalog at your side, or use on-line at www.jameco.com
Capacitors come in polarized and non polarized versions. No cap is ideal. Real caps have parasitic resistance and inductance. For most applications, this doesn't matter. For active filters and high end audio it does.
Cap specs include:
Value: In micro-farads or u-F if no units are listed. 100 uF is a big capacitance, 10 pF is a tiny capacitance. 0.1-100 uF are used for power supply filtering, smaller values used for active filters and the like.
Voltage: The max voltage that can be placed across the capacitor. The higher the voltage rating the larger the capacitor in physical size. Always pick the minimum voltage that is greater than the power supply in your circuit. Common voltage ratings are 25 V and 10 V.
Accuracy: How close the cap is to its nominal value. Typically spec'd as +/-%. When you see +-20% on a data sheet it means the actual value can vary by plus or minus 20 per cent around the nominal value. High accuracy costs you. For all applications except active or passive filters, +-20% is fine. For filters, use +-5% or less.
Temp stability: How much the capacitance varies with temperature.
When listing a cap on a bill of materials, always list the type, the value and the voltage.
Polaraized caps must be oriented properly in the circuit and their plus side must never be at a lower voltage than their minus side. Polarized caps have the advantage of lots of capacitance in a small package, but tend to be leaky and lossy.
Polarized caps come in these common flavors:
Electrolytic: General purpose, cheap, can get large. Use to filter power lines. Radial or axial lead. For example, Jameco 29962CF is 100 uF, 50V.
Tanalum: General purpose, nice and small, less cheap. Use to filter power lines when space counts. E.G. Jameco 94078CF is 10 uF, 25V.
Non-polarized caps can be used in any orientation. They are expensive and large for big cap values and/or big voltage values.
Non-polarized caps come in these common flavors:
Monolithic cereamic: The cap of choice for 0.1 uF power lead filtering at chip leads. Small and cheap. E.G. Jameco 25523CF is 0.1 uF, 50V.
Ceramic disc: Larger than the monolithics, wider range of values. The type that's pictured when textbooks have a picture of a capacitor. Good when you need caps with values in pFs. E.G. Jameco 15510CF is 47 pF, 50V.
Active filters and high-end audio circuits demand fancier (and more expensive) non-polarized capacitors. In filter applications, look for caps with tight tolerance on the nominal value (e.g. +/- 5% or +/- 10%) and with a low temperature coefficient. Type NPO (same as COG) ceramic capacitors are popular. COG-type caps have the tightest temperature tolerance followed by type X7R. Types Z5U and Y5V should be avoided in filter circuits. Some flavors of filter caps are:
Polypropylene: And excellent cap for accurate active filters. Digi-Key has a good selection of high accuracy polypropylene caps. For example, P3222-ND is a .0022 uFD cap. The larger values get physcially large.
Ceramic: Type COG for lowest values, type X7R for higher. Smaller and cheaper than polypropylene and will do for most, non-demanding applications. For example, Digi-Key P4863-ND is a 1500 pF type COG and P4910-ND is a .10 uF type X7R.
Mylar: Metalized polyester film. E.G. Jameco 26930CF is .047, 100V. Very large values are used in the cross-over network for speakers.
Silver Mica: Also OK for filters. Small values only. E.G. Jameco 16002CF is 100 pF, 500V.
Polystyrene: Highly accurate. Expensive. Good for very precise active filters.
