Hardware Reference
In-Depth Information
Caution
Whenever you are using a multimeter to test any voltage that could potentially be 50V or
above(suchasACwallsocketvoltage),alwaysuseonehandtodothetesting,nottwo.Eith-
ercliponeleadtooneofthesourcesandprobewiththeotherorholdbothleadsinonehand.
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
metersarenotmuchbiggerthanacassettetape;theyfitinashirtpocket.RadioShacksells
a good unit in the $25 price range; the meter (refer to
Figure 20.6
) is a half-inch thick,
weighs31/2ounces,andisdigitalandautoranging,aswell.Thistypeofmeterworkswell
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.
Thesemetersusea9Vbatterytopowerthemeterforresistancemeasurements.Ifyouusethis
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
(switchonandoff)rapidly,asimplevoltmeterisuseless.Alogicprobeisdesignedtodis-
play these signal conditions.
Figure 20.7
A typical logic probe.
