There are many kinds of capacitors and several popular ones are pictured here. Generally, capacitors have two leads. Some are axial leaded, like resistors, and others are radial leaded, with both leads at one end. Stompbox layouts seem to use radial leaded capacitors most often, but axial leaded are just as good. Unlike resistors, some capacitors are polarized, with positive and negative leads: the voltage across such capacitors must agree with the polarity of the leads. Take care to orient polarized capacitors correctly in a circuit.
Generally, polarized capacitors have lead markings on the casing, like a colored band of minus (negative) signs. The radial electrolytic capacitor pictured above has a black casing with a gray band and you can just see one minus sign. Note also that the negative lead is shorter than the positive lead. On axial electrolytic capacitors, arrows often point toward the negative lead. The blue capacitor shown here is an example. When the arrows are not present, note that the aluminum can (housing) shows on the negative end while the positive end has a (black) seal insulated from the housing. Often, the indentation or groove around one end of the casing is on the positive end. You can also see some of these features on the blue capacitor below.
The other capacitors pictured are mylar film (the brown and green ones above), box film (yellow), and ceramic disc (light brown).
There are many types of capacitors because there are many ways to make them and each has its advantages. There are accompanying disadvantages of course. For stompboxes, important considerations are size and effects on an audio signal. There is a lot of discussion about the latter, with many different views. I offer my interpretation of good advice for beginners below under Values.
There are also a variety of schematic symbols for capacitors. Two popular symbols are shown here, one for non-polarized and one for polarized capacitors. Often, the non-polarized symbol is made into a polarized symbol by adding a plus sign to show polarity. The polarized symbol shown here does not always have its polarity marked, but the curved side is always the negative lead. There is a nice table of other capacitor schematic symbols on the Capacitor page of Wikipedia.
Capacitors are labelled with the capital letter C and a number, like C2. In the Fuzz Face schematic described in Eagle 1: Description, C1 is a polarized capacitor with its positive lead on the input. C2 is another polarized capacitor, positive lead going to ground. C3 is non-polarized. This circuit has a positive ground (note that the voltage supply has a negative symbol) and this explains the orientation of the polarized capacitors. In a negative ground circuit, they would be reversed.
The layout symbols for capacitors are ovals, boxes, circles, and the schematic symbol.
Capacitor values are called capacitance, which is measured in farad units that are denoted by the capital letter F. In stompbox circuits, the largest capacitances are on the order of 10-6 farads and their units are microfarads, denoted by μF. For typing convenience,
μF and uF and mF
are equivalent, where u has a similar appearance to μ and m is the Latin character that corresponds to μ. (The mF notation is awkward because m often denotes “milli” or “one-thousandth” as in mA for milliamps or mm for millimeter. Nevertheless you will see it occasionally, as in the Small Bear catalog.) The smallest capacitances one sees are picofarads or 10-12 farads, denoted pF. In between, there are nanofarads (10-9 farads denoted nF). Many schematics avoid nF, writing .01μF instead of 10nF. If there are no units for the capacitors on a stompbox schematic, one generally assumes that the units are μF.
Although the notation is usually reserved for resistors, one occasionally sees the decimal point in a capacitor value replaced with the capacitor’s magnitude. For example, 2.2nF is sometimes noted as 2n2. Also the F (for farad) is also frequently dropped even when there is a decimal point: 2.2n instead of 2.2nF.
Tolerances are generally ±20%, much less accurate than common resistors. As a result, capacitors generally come in fewer values than resistors, but the values are organized in the same way. Capacitance values proportional to 10, 15, 22, 33, 47, and 68 are quite common. See the Values section of Resistors 1: Description for additional information or this Wikipedia entry.
- ceramic: often used for small capacitances in the 1pF to 1000pF range.
- polarized electrolytic: typically appear in power supply filters with values 10μF and higher.
- film: come in various kinds and their values cover a large range, say 1,000pF to several μF.
- mica: used for small capacitances like ceramic capacitors.
- tantalum: polarized and used in the signal path for their character. For examples, see the schematic of the tubescreamer clone or the schematic of the Red Llama clone.
Capacitors are rated for the voltage potential across their leads. 16V are typically used for 9V stompbox circuits. Move up to 25V for an 18V power supply. Many builders just buy 25V capacitors because they can be used in either case. Some capacitor values are not available without moving up to higher voltage ratings like 50V. There is no apparent problem with using such higher voltage ratings in stompbox circuits.
To learn more about types of capacitors and their audio properties, read “the Cap FAQ on muzique.com”. There is much discussion about the special character of certain capacitors and Aron’s stompbox forum is a good place to get a sampling.