Hardware Reference
In-Depth Information
continuity exists between the meter test leads. By using the sound, you quickly can test cable
assemblies and other items for continuity. After you use this 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 and maximum trap
—This feature enables the meter to trap the lowest and 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.
Although you can get a basic pocket DMM for as little as $20, one with all these features is priced
closer to $100, and some can be much higher. RadioShack carries some nice inexpensive 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 18.40
)
. You can't disconnect any of the connectors while the system is
running, so you must measure with everything connected. Nearly all the connectors you need to probe
have openings in the back where the wires enter the connector. The meter probes are narrow enough
to fit into the connector alongside the wire and make contact with the metal terminal inside. The
technique is called
back probing
because you are probing the connector from the back. You must use
this back-probing technique to perform virtually all the following measurements.
Figure 18.40. Back probing the power supply connectors.
To test a power supply for proper output, check the voltage at the Power_Good pin (P8-1 on AT,
Baby-AT, and LPX supplies; pin 8 on the ATX-type connector) for +3V to +6V of power. If the
