Capacitors

Capacitors hold an electrical charge for a predetermined amount of time. Most capacitors are made from a pair of conductive elements separated by an insulating dielectric. This dielectric can be composed of many materials including; air, paper, epoxy, plastic, and even oil. Most capacitors actually have many layers of conducting elements and dielectric.



Capacitors are rated by their capacitance, in farads, and by the breakdown voltage of their dielectric. The farad is a rather large unit of measurement, so many capacitors are rated in microfarads. A microfarads is the equivalent of one millionth of a farad (1/1000000 of a farad), an even smaller rating is the picofarad which is the equivalent of a millionth of a millionth of a farad (1/1000000 of a microfarad). The micro in the term microfarad is most often represented by the Greek mu (μ) character. The picofarad is shortened to pF. The voltage rating (absolute maximum voltage) is the highest voltage that the capacitor can withstand before the dielectric layers in the component are damaged.

Capacitors are classified by the dielectric material that they use. The most common dielectric materials are aluminum electrolytic, tantalum electrolytic, ceramic, mica, polypropylene, polyester (or mylar), paper and polystyrene. The dielectric material used in a capacitor partly determines which applications it should be used for. The larger electrolytic capacitors, which use an aluminum electrolyte, are suited for such chores as power supply filtering, where large values are needed. The values for many capacitors are printed directly on the component. This is especially true with the larger aluminum electrolytic, where the large size of the capacitor provides ample room for printing the capacitance and voltage. Smaller capacitors, such as 0.1 or 0.01 μF mica disc capacitors, use a common three-digit marking system to denote capacitance and tolerance. The numbering system is based on picofarads, not microfarads.

Example: 104 means 10 followed by four zeros, which is 100000 picofarads
Values over 1000 picofarads are most often stated in microfarads. To make the conversion, move the decimal point to the left six spaces. Values under 1000 picofarads do not use this numbering system. Instead, the actual value in picofarads, is listed.

One mark found almost exclusively on larger tantalum and aluminum electrolytic is a polarity symbol, most often a minus (−) sign. The polarity symbol indicates the positive and/or negative lead of the capacitor. If a capacitor is polarized, it is extremely important that the proper orientation is applied when the capacitor is installed. If the polarity is reversed between the positive lead (called the anode) and the negative lead (called the cathode) than the capacitor may be ruined. The results may be as minor as a small “pop” followed by a wisp of smoke, or as catastrophic as a loud bang and a small fire. Other components in the circuit may also be damaged.

Capacitor color codes:


Electrolytic capacitors usually have their maximum voltage rating and capacitance value printed right on them, with no codes to interpret. These types of capacitors are polarity sensitive and can explode if improperly installed. In order to determine the polarity look for a mark near one of the legs of the component. Large electrolytic  capacitors have a stripe down the side of the body near one leg that identifies “negative”. These type of capacitors also have a tendency to leak their stored energy.



Tantalum capacitors usually have their maximum voltage rating and capacitance value printed right on them, with no codes to interpret. Many of these types of capacitors have a stripe marked with a ''+'' sign near it that runs down near one of the leads, the lead indicated by the stripe is positive. Tantalum capacitors are polarized and an improper installation may result is a catastrophic failure.


Monolithic capacitors are not polarity sensitive. Most monolithic capacitors are labeled in a numeric fashion. Monolithic capacitors can have numbers on both sides, one side being the size code, and the other side being the date-of-manufacture code.