CAPACITOR

                            Capacitors come in a bewildering variety of different types.  The specific type may be critical in some applications, where in others, you can use anything you please.  Capacitors are the second most common passive component, and there are few circuits that do not use at least one capacitor.

A capacitor is essentially two conductive plates, separated by an insulator (the dielectric).  To conserve space, the assembly is commonly rolled up, or consists of many small plates in parallel for each terminal, each separated from the other by a thin plastic film.  See below for more detailed information on the different constructional methods.  A capacitor also exists whenever there is more than zero components in a circuit - any two pieces of wire will have some degree of capacitance between them, as will tracks on a PCB, and adjacent components.  Capacitance also exists in semiconductors (diodes, transistors), and is an inescapable part of electronics.

There are two main symbols for capacitors, and one other that is common in the US, but rarely seen elsewhere.  Caps (as they are commonly called) come in two primary versions - polarised and non-polarised.  Polarised capacitors must have DC present at all times, of the correct polarity and exceeding any AC that may be present on the DC polarising voltage.  Reverse connection will result in the device failing, often in a spectacular fashion, and sometimes with the added excitement of flames, or high speed pieces of casing and electrolyte (an internal fluid in many polarised caps).  This is not a good thing. 

 
 Capacitor Symbols

Capacitors are rated in Farads, and the standard symbol is "C" or "F", depending upon the context.  A Farad is so big that capacitors are most commonly rated in micro-Farads (uF).  The Greek letter (lower case) Mu is the proper symbol, but "u" is available on keyboards, and is far more common.  Because of the nature of capacitors, they are also rated in very much smaller units than the micro-Farad - the units used are ...

• mF: Milli-Farad, 1x10^-3 Farad (1,000th of a Farad) - uncommon
• uF: Micro-Farad, 1x10^-6 Farad (1,000,000th of a Farad)
• mF: Micro-Farad, a very, very old term, still sometimes used in the US (True!) - Causes much confusion.
• ufd: Micro-Farad, another very old term, still used in the US
• mfd (or MFD): Yet another antiquated term - US again!
• nF: Nano-Farad, 1x10^-9 Farad (1,000,000,000th of a Farad) - Common everywhere except the US
• pF: Pico-Farad, 1x10^-12 Farad (1,000,000,000,000th of a Farad)
• mmF: Micro-Micro-Farad, another extremely old term, also still used sometimes in the US

The items in bold are the ones I use in all articles and projects, and the others should be considered obsolete and not used - at all, by anyone !

Milli-Farads (mF) should be used for large values, but are generally avoided because of the US's continued use of the ancient terminology.  When I say ancient, I mean it - these terms date back to the late 1920s or so.  Any time you see the term "mF", it almost certainly means uF - especially if the source is the US.  You may need to determine the correct value from its usage in the circuit.

A capacitor with a value of 100nF may also be written as 0.1uF (especially in the US).  A capacitor marked on a schematic as 2n2 has a value of 2.2nF, or 0.0022uF (mF ??).  It may also be written (or marked) as 2,200pF.  These are all equivalent, and although this may appear confusing (it is), it is important to understand the different terms that are applied.

A capacitor has an infinite (theoretically!) resistance at DC, and with AC, it has an impedance.  Impedance is defined as a non-resistive (or only partially resistive) load, and is frequency dependent.  This is a very useful characteristic, and is used to advantage in many circuits.

In the case of a capacitor, the impedance is called Capacitive Reactance generally shown as Xc.  The formula for calculating Xc is shown below ...

6.1.1   Xc = 1 / 2 π F C  where π is 3.14159..., F is frequency in Hertz, and C is capacitance in Farads

The Transposition Triangle can be used here as well, and simplifies the extraction of the wanted value considerably.

 
Capacitance Triangle

As an example, what is the formula for finding the frequency where a 10uF capacitor has a reactance of 8 Ohms?  Simply cover the term "F" (frequency), and the formula is

6.1.2     F = 1 / 2 π C Xc

In case you were wondering, the frequency is 1.989kHz (2kHz near enough).  At this frequency, if the capacitor were feeding an 8 ohm loudspeaker, the frequency response will be 3dB down at 2kHz, and the signal going to the speaker will increase with increasing frequency.  Since the values are the same (8 ohm speaker and 8 ohms reactance) you would expect that the signal should be 6dB down, but because of phase shift (more on this later), it is actually 3dB.

With capacitors, there is no power rating.  A capacitor in theory dissipates no power, regardless of the voltage across it or the current through it.  In reality, this is not quite true, but for all practical purposes it does apply.

All capacitors have a voltage rating, and this must not be exceeded.  If a higher than rated voltage is applied, the insulation between the "plates" of the capacitor breaks down, and an arc will often weld the plates together, short circuiting the component.  The "working voltage" is DC unless otherwise specified, and application of an equivalent AC signal will probably destroy the capacitor.