1-12
shunt resistor and measure the calibration signal voltage developed across it with an accurate voltmeter.
The calibration signal current can then be calculated by means of Ohm’s law. Since the oscilloscope
merely indicates the voltage developed across the shunt resistor, the measurements for alternating or
direct current will be similar to voltage measurements using an oscilloscope.
RESISTANCE MEASUREMENTS
A high percentage of technical manuals contain point-to-point resistance charts that list correct
resistance readings for major test points. These resistance charts are extremely useful when you
troubleshoot faulty equipment. Without them, equipment resistance measurements within a complicated
circuit would not mean much. Many circuits contain other circuit elements, such as capacitors, coils, or
other resistors in parallel with the resistances being measured. This, of course, is a possible source of
measurement error that you eliminate when you disconnect or unsolder one side of the resistor or a group
of resistors under test.
You should be thoroughly familiar with the calibration of your ohmmeter. Analog meters are
typically more accurate and easier to read at midscale. With the exception of bridge circuits, a meter may
provide only approximate resistance readings. However, these readings may be adequate when you also
consider the wide tolerances of resistors themselves. An ohmmeter that you use in field testing should be
portable, convenient, and simple to operate - factors that usually are more important than extreme
accuracy.
When an ohmmeter is used, completely de-energize the circuit under test and remove any current-
sensitive elements before the resistance measurement is performed. Low-resistance measurements that
require precision readings should be taken with a bridge type of instrument.
An ohmmeter consists of a galvanometer, batteries, and resistors of known value that are connected
in such a way that unknown resistors to be measured are compared with standard values. Figure 1-10
illustrates three basic ohmmeter circuits: (A) single range type, (B) series multirange type, and (C) shunt
type.
Figure 1-10A.—Basic ohmmeter circuits.
