120 Chapter 4
Often people will make up their own jingle to remember the color codes—you know,
something that has meaning for you, such asB ye B ye R eba O ff Y ou G o B e V aliant G o
W ell. You get the idea.
The next most common format for resistors, and one that you’ll come across very
often in the circuit projects, is the variable resistor or, as it is more usually called, the
potentiometer. Relatively speaking, the potentiometer is a much larger device than the
resistor; it is more mechanical as opposed to electrical, and it is a three-terminal device.
A rotating shaft coupled internally to a movable wiper track follows an arc-shaped path
over a track of resistive material. The movable wiper terminal is brought out to a fi xed
electrical connection point. Further, two fi xed terminals are connected electrically to the
other two ends of the resistive track. As you can probably tell, the resistance measured
across the wiper terminal and either of the other ends will vary continuously as the
shaft is rotated. The maximum resistance value will be the value marked on the device;
typically, values of 1, 10, and 100 kohms are used.
Resistor values will typically run from 1 ohm to 1 Mohm. I fi nd that with most circuit
applications you can get away with using just a few “ good ” resistor values. My own
personal preference is 10 ohms, 100 ohms, 470 ohms, 1 kohm, 2.7 kohms, 4.7 kohms,
10 kohms, 27 kohms, 47 kohms, 100 kohms, 470 kohms, and 1 Mohm. If I had to choose
the four most useful values, these values can be further distilled down to 100 ohms, 1
kohm, 10 kohms, and 100 kohms. Look at the circuits later in the book and see how often
these values turn up. Intermediate values can be built up by juggling a handful of basic
values and learning a bit of “ resistor math. ” Two resistors of equal value connected in
parallel produce half the resistor value. So two 1-kohm resistors produce 500 ohms, and
two 10-kohm resistors give you 5 kohms. So if a circuit called for a 5.5-kohm resistor and
it’s late at night and you desperately need that last component to fi nish, join two
1-kohm resistors connected in parallel to two 10-kohm resistors connected in parallel, and
you’ve got what you need. A useful trick indeed.
The more general rule to follow when the resistors are not equal in value is that for two
resistors of unequal value connected in parallel, the total value is the product divided by
the sum of the two values. For example, a 1- and a 10-kohm resistor connected in parallel
will yield the product 10 1 1, divided by the sum of the resistor values, 10 1 1 1 ,
yields 10 1 1 0.9 kohm. Another useful trick to remember when connecting two
resistors in parallel is that the total is always less than the smaller of the two values. In the