122 Chapter 4
of the nominally marked value (the three color bands) that the resistor is allowed to
read and still remain within specifi cation. This tolerance designation gives the resistor
manufacturer greater latitude in offering resistors with a nominal value than would be
otherwise possible. From the user’s point of view (you and me), this means a 100-kohm
resistor might not exactly read that value when measured, but it is perfectly acceptable
from the manufacturer’s point of view. For example, if you have a 5% 100-kohm resistor
and you measure the actual resistance, it could lie anywhere between
100 5 100 5 105
100 5kohms percent kohms kohms kohms, or
kohms per ccent 100 kohms 5 kohms 95 kohms.If this were a 20% 100-kohm resistor, then the limits would run from 120 to 80 kohms,
which is an extraordinarily wide variation. All the projects described later in the book use
5% tolerance resistors.
The third parameter associated with resistors is their power rating. The value typically
used is 1/4 watt, which is also the wattage specifi ed for the project circuits in this book.
The power rating of a resistor refers to its ability to dissipate power, which in turn
translates to its ability to dissipate heat. The more current you pass through a resistor, the
hotter it gets, and the resistor power rating must be suffi cient to stand up to the dissipated
power. Larger resistors go up to1/2 W and more. It’s a waste to use these for the projects
in this book because these resistors take up more space, cost more, and are unnecessary.
However, for the sake of demonstrating the calculations involved, I’ll describe what
happens to the power rating when we join resistors in series or parallel. In the simple case
of two 100-ohm 1/4 watt resistors joined in series, the total resistance is 200 ohms, and
the power rating is still 1/4 watt. However, when these resistors are joined in parallel, the
resistance drops to 50 ohms, and the power rating increases to1/2 watt—a nice technique
to remember if you want to increase your power rating. Let’s say you wanted a 10-ohm
1-watt resistor and the shops are closed. This is quite a large beast. You’ve got a bunch
of common 100-ohm 1/4-watt resistors. Take 10 of these 100-ohm resistors and connect
them in parallel. The total resistance is now 10 ohms (one-tenth of the individual values),
and the power is increased to 10 1/4 1.25 watt. This is another good trick to remember.
4.1.2 Capacitors
Capacitors, like resistors, are two-terminal devices and are distinctive in terms of their
ability to block DC signals and pass AC signals. For example, a DC signal, that is,