Hardware Reference
In-Depth Information
(such as AC wall socket voltage), always use one hand to do the testing, not two. Either clip
one lead to one of the sources and probe with the other or hold both leads in one hand.
If you hold a lead in each hand and accidentally slip, you can very easily become a circuit,
allowing power to conduct or flow through you. When power flows from arm to arm, the path
of the current is directly across the heart. The heart muscle tends to quit working when
subjected to high voltages. It's funny that way.
I prefer the small digital meters; you can buy them for only slightly more than the analog style, and
they're extremely accurate and much safer for digital circuits. Some of these meters are not much
bigger than a cassette tape; they fit in a shirt pocket. RadioShack sells a good unit in the $25 price
range; the meter (refer to
Figure 20.6
) is a half-inch thick, weighs 3 1/2 ounces, and is digital and
autoranging, as well. This type of meter works well for most, if not all, PC troubleshooting and test
uses.
Caution
You should be aware that many older analog meters can be dangerous to digital circuits. These
meters use a 9V battery to power the meter for resistance measurements. If you use this type of
meter to measure resistance on some digital circuits, you can damage the electronics because
you essentially are injecting 9V into the circuit. The digital meters universally run on 3V-5V or
less.
Logic Probes
A logic probe can be useful for diagnosing problems in digital circuits (see
Figure 20.7
)
. In a digital
circuit, a signal is represented as either high (+5V) or low (0V). Because these signals are present for
only a short time (measured in millionths of a second) or oscillate (switch on and off) rapidly, a
simple voltmeter is useless. A logic probe is designed to display these signal conditions.
