Page 398 - Robot Builder's Bonanza

TOOLS FOR ELECTRONICS YOU SHOULD HAVE 367

1. With the meter on, dial to the DCV (DC volts) setting. If your meter is not
autoranging, select a range that is one step higher than the expected voltage;
for most robot tasks this will be 20 volts or less.
2. Ensure the test leads are plugged into the proper jacks, as detailed in the previ-
ous section “Using the Meter: The Basics.”
3. Touch the black lead to the negative (–) terminal of the battery or pack; touch
the red lead to the positive (+) terminal.
4. Note the value displayed by the meter. If the battery (or pack) is good, the voltage
should be close to the expected value— for example, an AA alkaline battery should be
about 1.5 volts, give or take 0.1 or 0.2 volts.
5. For grins, switch the test leads so that black goes to the + battery terminal and red goes
to . Note the voltage again; it should now be a negative value, indicating that the
polarity of the test connection to the battery is reversed.

Using the Meter: Verifying the Value of a Resistor
Another common use of a multimeter is verifying the value of a resistor. Here’s how.

1. From your parts bin, select any four- banded resistor with the color brown as its third
band. This will ensure the resistor is between 100 and 990 ohms. (Resistors and their
values and markings are discussed in more detail in Chapter 31.)
2. Refer to the resistor color code table in Appendix D, “Electronics Reference,” to look
up the value of the resistor. For example, if the first three color bands are
orange- orange- brown, the indicated value of the resistor is 330 .
3. With the meter on, dial to the setting. If your meter is not autoranging, select
a range that is one step higher than the expected reading. For instance, if the
ranges are 2, 20, 200, 2000, and so forth, select 2000 (2k), as it is one step
higher than the expected value of 330 .
4. Ensure the test leads are plugged into the proper jacks, as detailed in the previ-
ous section “Using the Meter: The Basics.”
5. With the resistor resting on the table or workbench, apply the test leads to either side
of the resistor. Be sure not to touch the metal of the test leads, or else the natural con-
ductivity of your skin will influence the result.
6. Read the value on the meter. See Figure 30- 3 for an example:

So the resistor doesn’t read exactly what it should? There’s no cause for alarm. The fourth band
on a resistor with four color bands indicates its tolerance, or how far off the printed value it can
G be from the actual value. A gold band indicates 5 percent tolerance; a silver band, 10 percent
tolerance. If the resistor is 330 with 10 percent tolerance, its reading on the multimeter can
range from about 300 to 360 .

Meter Safeguards, Good Operating Habits
When using a meter, even on low- voltage circuits, observe these best operating habits:

• Never hold the test probes by their metal part. Not only can this give you a nasty shock
when testing AC household current, but your skin resistance can alter the test results.
• On each use of the meter, test its basic operating status with the continuity check detailed
previously.

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