Various capacitors

Capacitance is measured in units called farads. By definition, if you force one coulomb of charge (approximately 6,241,509,480,000,000,000 electrons) into a capacitor, and after that, the capacitor has 1 volt of potential across its electrodes, that capacitor has a capacitance of 1 farad. Until fairly recently, capacitors with high values tended to be physically large and expensive. A traditional one-farad capacitor may be as large as a liter bottle. Therefore, typical capacitors tend to have low values. Capacitors labeled in the microfarad (μF) or even the picofarad (pF) range. A capacitor with a value in the millifarad range will usually be labeled in thousands of microfarads. A capacitor in the nanofarad range will usually be labeled in thousands of picofarads. It is rare to see capacitors labeled in the millifarad or nanofarad range. (see Units of Measure above).

Today, new technologies, such as the electric double-layer effect, can be used to make high value capacitors in a very small size. One example is a 1 farad, 2.5V aerogel capacitor that is 7/16 inch long and 5/16 inch in diameter and sells for about $5.00.

Sometimes, especially in material published before the mid 1960s, capacitors in the picofarad range are specified in micro-microfarads (μμF). A micro-microfarad is the same as a picofarad.

Large capacitors are usually clearly marked with their capacitance in microfarads. Usually, the Greek letter mu (μ) is used to represent the micro in microfarads. However, sometimes the modern letter U is used. Therefore, a 10,000 microfarad capacitor may be marked 10,000UF.

Confusion Alert!

Many capacitors may be labeled with “mF” (lower case “m”). In this case "mF" means microfarads, not millifarads. This is supposedly because the labeling equipment owned by the manufacturers is not capable of printing the Greek letter mu (μ). Capacitors are also commonly labeled with “MF” (upper case “M”). These are not megafarads. They are still microfarads.

1000MF   1000mF   1000μF

All these labels represent 1,000 microfarads

With smaller capacitors, particularly ceramic capacitors the capacitance is labeled as nothing more than a number printed on the capacitor. If this number is a whole number, the value is in picofarads. If the value is a decimal fraction, the value is in microfarads. Therefore, a ceramic capacitor labeled ".47" has a value of 0.47 microfarads. A ceramic capacitor labeled "47" has a value of 47 picofarads.

If the number on a ceramic capacitor has three digits, the capacitance is decoded much like the resistor code with the capacitance in picofarads. The first two numbers are the significant digits and the third number is the number of zeros to put after them. Therefore, a capacitor labeled 103 would be have a value of 10,000 picofarads, which is the same as .01 microfarads. A ceramic capacitor labeled 103 and one labeled .01 have the same capacitance.

A letter after the number on a ceramic capacitor represents the tolerance, the percentage that the actual value may differ from the printed value. The tolerance values are: J=5%, K=10% and M=20%.

Capacitor voltage ratings

As stated above, if you put too much voltage across a capacitor, the insulation will break down, destroying the capacitor. Certain types of capacitors will explode if too much voltage is applied to them. The voltage printed on a capacitor is its voltage rating, which should not be exceeded.

Factors affecting capacitance

Any time you have two conductors separated by an insulator you have a capacitor. Pick up any two pieces of metal and hold them up, making sure they are not touching. This is two conductors (metal) separated by an insulator (air). Is this a capacitor? You betcha.

A capacitance demonstrator consisting of two parallel plates of metal separated by air.

Capacitance is primarily determined by the area of the conducting surfaces and their distance from each other. If you increase the size (area) of the conductors you will have more capacitance. If you increase the distance between the conductors you will have less capacitance. Therefore, if you want a lot of capacitance you need conductors with a lot of surface area very close to each other.

If enough voltage is placed across a capacitor the insulating material will break down. This typically causes permanent damage in the form of a short circuit (usually accompanied by an explosion). Generally, the more distance between the conductors, the more voltage the capacitor can handle. Higher voltage capacitors tend to be larger than lower voltage capacitors.