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Reading Capacitance Values

The basic unit of capacitance is the farad, named after British physicist and chemist Michael Faraday (1791 - 1867). For you physics types, the basic equation for capacitance is:

C = q/V ,

Verbally, a capacitance of one farad will exhibit a voltage difference of one volt when an electrical charge of one coulomb is moved from one plate to the other through the capacitance.

To help put this in perspective, one ampere of current represents one coulomb of charge passing a given point in an electrical circuit in one second.

In practical terms, the farad (f) represents a extremely large amount of capacitance. Real-world circuits require capacitance values very much smaller. Therefore, we use microfarads (µf) and picofarads (pf) to represent practical capacitance values. The use of the micro- and pico- prefixes is standard. 1 µf = 1 × 10-6 f and 1 pf = 1 × 10-6 µf. Sometimes you will see the designation µµf in place of pf; they have the same meaning.

Like resistors, capacitors are generally manufactured with values to two significant digits. Also, small capacitors for general purposes have practical values greater than 1 pf and less than 1 µf. As a result, a useful convention has developed in reading capacitance values. If a capacitor is marked "47," its value is 47 pf. If it is marked .047, its value is .047 µf. Thus, whole numbers express capacitance values in picofarads while decimal fractions express values in microfarads. Any capacitor manufactured with a value of 1 µf or greater is physically large enough to be clearly marked with its actual value.

A newer nomenclature has developed, where three numbers are printed on the body of the capacitor. The third digit in this case works like the multiplier band on a resistor; it tells the number of zeros to tack onto the end of the two significant digits. Thus, if you see a capacitor marked "151," it is not a precision component. Rather, it is an ordinary capacitor with a capacitance of 150 pf. In this nomenclature, all values are given in picofarads. Therefore you might well see a capacitor marked 684, which would mean 680000 pf, or 0.68 µf.

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