Audio Engineering

Components 125

numbers to represent capacitance. It’s much easier to understand the system by way of an
example. Let’s look at the code 104. This is a capacitance value expressed in picofarads.
The fi rst and second numbers relate to the actual fi rst two digits of capacitance. The fi nal
number indicates the number of zeros following. So 104 is 100,000 picofarads. Because this
number is a bit unwieldy, multiply it by 10 ^6 to convert to μ F, which works out to 0.1 μ F, a
much more convenient number to work with. This is a very common capacitor value.

Variable capacitors do exist, but they are used less frequently than variable resistors. But
variable capacitors are still two-terminal devices. Why? Variable capacitors operate on
the principle of varying the overlap between two metal plates, separated by either air or
an insulator—the greater the overlap, the greater the capacitance. So you see, just two
terminals are needed. There are no variable capacitors used in the projects in this book.

Radial lead capacitors have leads emerging from one side of the body, and if you don’t
have any height restrictions in your project case, this is the type I recommend you use.
Axial lead capacitors, however, have leads emerging one from each end of the body of
the component. They take up an awful lot of board space and are used only when the
assembly board profi le has to be as low as possible, but this is hardly a requirement for
simple single-IC hobby projects. (An example of a requirement where you would need a
very low profi le would be for a pager. Pagers are thin as we know and therefore need an
assembly board with a low profi le.)

Like resistors, capacitors can also be connected in series and in parallel to form different
values. However, the rules are different from those for resistors. To increase a capacitor
value, we connect two together in parallel. So two 0.1- μ F capacitors connected in parallel
give us 0.2 μ F. Three capacitors of 0.1 μ F value each connected in parallel give us 0.3 μ F,
and so on. If the capacitors were to be connected in series, then

1112total capacitance. capacitance 1 capacitance , and so on

For example, two 0.1- μ F capacitors connected in series result in a 0.05- μ F capacitor, since

1101101101020total capacitance..μμ. F F

Hence the capacitance is 1/20  0.05 μ F. Sometimes for timing applications in an
oscillator circuit, you might want to change the output frequency a little, and this is one
way of obtaining a 0.05- or 0.2- μ F capacitor if you don’t have one handy (and it’s too late
to run out to your local component store).