Power Supplies - Upgrading and Repairing PCs - page 925

connected to the motherboard and peripheral devices.
Selecting a Meter
You need a simple digital multimeter (DMM) or digital volt-ohm meter (DVOM) to perform voltage
and resistance checks on electronic circuits (see Figure 18.39 ). Only use a DMM instead of the older
needle-type multimeters because the older meters work by injecting 9V into the circuit when
measuring resistance, which damages most computer circuits.
Figure 18.39. A typical DMM.
A DMM uses a much lower voltage (usually 1.5V) when making resistance measurements, which is
safe for electronic equipment. You can get a good DMM with many features from several sources. I
prefer the small, pocket-size meters for computer work because they are easy to carry around.
Some features to look for in a good DMM are as follows:
• Pocket size —This is self-explanatory, but small meters that have many, if not all, of the
features of larger ones are available. The elaborate features found on some of the larger meters
are not really necessary for computer work.
• 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