Hardware Reference
In-Depth Information
•
Overload protection
—If you plug the meter into a voltage or current beyond the
meter's capability to measure, the meter protects itself from damage. Cheaper meters
lack this protection and can be easily damaged by reading current or voltage values
that are too high.
•
Autoranging
—The meter automatically selects the proper voltage or resistance range
when making measurements. This is preferable to the manual range selection;
however, really good meters offer both autoranging capability and a manual range
override.
•
Detachable probe leads
—The leads can be damaged easily, and sometimes a variety
of differently shaped probes are required for different tests. Cheaper meters have the
leads permanently attached, which means you can't easily replace them. Look for a
meter with detachable leads that plug into the meter.
•
Audible continuity test
—Although you can use the ohm scale for testing continuity
(0 ohms indicates continuity), a continuity test function causes the meter to produce a
beep noise when continuity exists between the meter test leads. By using the sound,
youquicklycantestcableassembliesandotheritemsforcontinuity.Afteryouusethis
feature, you will never want to use the ohms display for this purpose again.
•
Automatic power-off
—These meters run on batteries, and the batteries can easily be
worn down if the meter is accidentally left on. Good meters have an automatic shutoff
that turns off the unit when it senses no readings for a predetermined period of time.
•
Automatic display hold
—This feature enables you to hold the last stable reading on
the display even after the reading is taken. This is especially useful if you are trying to
work in a difficult-to-reach area single-handedly.
•
Minimum andmaximumtrap
—Thisfeatureenablesthemetertotrapthelowestand
highest readings in memory and hold them for later display, which is especially useful
if you have readings that are fluctuating too quickly to see on the display.
AlthoughyoucangetabasicpocketDMMforaslittleas$20,onewithallthesefeaturesis
priced closer to $100, and some can be much higher. RadioShack carries some nice inex-
pensive units, and you can purchase the high-end models from electronics supply houses,
such as Newark or Digi-Key.
Measuring Voltage
To measure voltages on a system that is operating, you must use a technique called
back
probing
on the connectors (see
Figure 18.39
). You can't disconnect any of the connectors
while the system is running, so you must measure with everything connected. Nearly all
theconnectorsyouneedtoprobehaveopeningsinthebackwherethewiresenterthecon-
nector. The meter probes are narrow enough to fit into the connector alongside the wire
