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Electronics Measurements: Capacitance

By Doug Lowe

Capacitors are among the most useful of all electronic components. And capacitance is the term that refers to the ability of a capacitor to store charge. It’s also the measurement used to indicate how much energy a particular capacitor can store. The more capacitance a capacitor has, the more charge it can store.

Capacitance is measured in units called farads (abbreviated F). The definition of one farad is deceptively simple. A one-farad capacitor holds a voltage across the plates of exactly one volt when it’s charged with exactly one ampere per second of current.

Note that in this definition, the “one ampere per second of current” part is really referring to the amount of charge present in the capacitor. There’s no rule that says the current has to flow for a full second. It could be one ampere for one second, or two amperes for half a second, or half an ampere for two seconds. Or it could be 100 mA for 10 seconds or 10 mA for 100 seconds.

One ampere per second corresponds to the standard unit for measuring electric charge, called the coulomb. So another way of stating the value of one farad is to say that it’s the amount of capacitance that can store one coulomb with a voltage of one volt across the plates.

It turns out that one farad is a huge amount of capacitance, simply because one coulomb is a very large amount of charge. To put it into perspective, the total charge contained in an average lightning bolt is about five coulombs, and you need only five, one-farad capacitors to store the charge contained in a lightning strike. (Some lightning strikes are much more powerful, as much as 350 coulombs.)

It’s a given that Doc Brown’s flux capacitor was in the farad range because Doc charged it with a lightning strike. But the capacitors used in electronics are charged from much more modest sources. Much more modest.

In fact, the largest capacitors you’re likely to use have capacitance that is measured in millionths of a farad, called microfarads and abbreviated μF. And the smaller ones are measured in millionths of a microfarad, also called a picofarad and abbreviated pF.

Here are a few other things you should know about capacitor measurements:

  • Like resistors, capacitors aren’t manufactured to perfection. Instead, most capacitors have a margin of error, also called tolerance. In some cases, the margin of error may be as much as 80%. Fortunately, that degree of impression rarely has a noticeable effect on most circuits.

  • The μ in μF isn’t an italic letter u; it’s the Greek letter mu, which is a common abbreviation for micro.

  • It’s common to represent values of 1,000 pF or more in μF rather than pF. For example, 1,000 pF is written as 0.001 μF, and 22,000 pF is written as 0.022 μF.