0.1 Discrete components  (Page 2/6)

Resistors are typically the easiest discrete components to understand and use. They can be found in all sorts of sizes and configurations. Rather than have an exhaustive discussion on resistors, let’s just do a quick overview.

The film resistor is probably the easiest-to-recognize form of a resistor. Besides having to know the color code for determining the value of the resistor, it is also important to know the tolerance and wattage.

Color code

There are several ways to remember the color code. The mnemonic I used to learn it many years ago is not politically correct, so I will not use it here as an example. But you can find several on the Web or from a professor that can help you learn the color code.

So the first three colors for a 1K-ohm resistor would be brown, black and red. Figure 2 is a 1K ohm, ¼ watt, 5 percent resistor.

Notice that the fourth band in the picture is gold, which means it has a 5 percent tolerance.

Tolerance

Most resistors found in a lab have a 5 percent tolerance. This means that a 1K-ohm resistor could vary anywhere from 950 to 1,050 ohms. An interesting observation is that the 5 percent resistor series is set up such that two adjacent resistor values are within 10 percent of each other. That means that a resistor, once manufactured, will always fit into one of the tolerances.

In practical terms, the two resistor values adjacent to the 1-K resistor are the 910 ohm and 1.1-K ohm. Expressed as in Table 1, you can see how this works.

The 910-ohm resistor could be as much as 955 ohms on the high side of its allowable tolerance and the 1K-ohm resistor could be as little as 950 ohms on the low side of its allowable tolerance, giving a 5-ohm overlap.

Knowing this piece of information can help when picking out the next-size-larger resistor. But in case you don’t want to do this exercise, below is a chart with the values for a 5 percent tolerance resistor. Notice that it also includes a chart for 10 percent tolerance capacitors.

A tighter tolerance resistor might be wire-wound in order to guarantee its tolerance. But due to the nature of its manufacture, it may tend to have an inductive component at higher frequencies. So when picking a resistor for the circuit, keep this in mind. I’ll discuss this issue further in the Capacitors section.

Wattage

One mistake you should try to avoid in circuit design is using a resistor that is not capable of handling the power dissipation it must handle. It is easy to assume that a ¼ watt resistor can handle ¼ of a watt. But this is incorrect on two counts: failing to determine the power dissipation of the resistor in the active circuit, and assuming that a ¼ watt resistor will handle a ¼ watt load.

You’d realize the first error relatively fast, once you’d turned on the power. The resistor in question would become warm to the touch, then get really hot to the touch, begin to smoke or blow up. All of these conditions indicate that you need a larger-wattage resistor. To determine the power through a resistor, it is best to use Ohm’s law (Equation 1) to determine the power: