times 1 position. Ensure that the meter has been properly zeroed on this scale. If the meter reads down-
scale, reverse the polarity switch. If the meter reads less than midscale, the diode under test is either open
or shorted. The related circuit impedance of this test is less than 25 ohms.
RESISTANCE-CAPACITANCE-INDUCTANCE (RCL) BRIDGES
Resistance, capacitance, and inductance can be measured with precise accuracy by alternating-
current bridges. These bridges are composed of capacitors, inductors, and resistors in a wide variety of
combinations. These bridges operate on the principle of the Wheatstone bridge; that is, an unknown
resistance is balanced against known resistances and, after the bridge has been balanced, the unknown
resistance is calculated in terms of the known resistance.
The universal Impedance Bridge, Model 250DE (shown in figure 4-18) is used to measure
resistance, capacitance, and inductance (RCL) values. It is also used to make other special tests, such as
determining the turns ratio of transformers and capacitor quality tests. This instrument is self-contained,
except for a source of line power, and has an approximate 500-hour battery life expectancy. It has its own
source of 1,000-hertz bridge current with a sensitive bridge balance indicator and an adjustable source of
direct current for electrolytic capacitor and resistance testing. The bridge also contains a meter with
suitable ranges to test for current leakage on electrolytic capacitors.
Figure 4-18.Resistance-capacitance-inductance bridge.