2-20
damage the substitute semiconductor device. If more than one defective semiconductor is present in the
equipment section where trouble has been localized, the semiconductor replacement method becomes
cumbersome. Several semiconductors may have to be replaced before the trouble is corrected. To
determine which stage(s) failed and which semiconductors are not defective, you must test all the
removed semiconductors. You can do this by observing whether the equipment operates correctly as you
reinsert each of the removed semiconductor devices into the equipment.
TESTING DIODES
Semiconductor diodes, such as general-purpose germanium and silicon diodes, power silicon diodes,
and microwave silicon diodes, can be tested effectively under actual operating conditions. However,
crystal-rectifier testers are available to determine dc characteristics that provide an indication of crystal-
diode quality.
A common type of crystal-diode test set is a combination ohmmeter-ammeter. Measurements of
forward resistance, back resistance, and reverse current can be made with this equipment. Using the
results of these measurements, you can determine the relative condition of these components by
comparing their measured values with typical values obtained from test information furnished with the
test set or from the manufacturer’s data sheets. A check that provides a rough indication of the rectifying
property of a diode is the comparison of the back-and-forward resistance of the diode at a specified
voltage. A typical back-to-forward-resistance ratio is on the order of 10 to 1, and a forward-resistance
value of 50 to 80 ohms is common.
Q-11. What is the typical back-to-forward resistance ratio of a good-quality diode?
Testing Diodes with an Ohmmeter
A convenient test for a semiconductor diode requires only an ohmmeter. The back-and-forward
resistance can be measured at a voltage determined by the battery potential of the ohmmeter and the
resistance range at which the meter is set. When the test leads of the ohmmeter are connected to the diode,
a resistance will be measured that is different from the resistance indicated if the leads are reversed. The
smaller value is called the FORWARD RESISTANCE, and the larger value is called the BACK
RESISTANCE. If the ratio of back-to-forward resistance is greater than 10 to 1, the diode should be
capable of functioning as a rectifier. However, keep in mind that this is a very limited test that does not
take into account the action of the diode at voltages of different magnitudes and frequencies. (NOTE:
This test should never be used to test crystal mixer diodes in radars. It will destroy their sensitivity.)
Testing Diodes with Oscilloscopes
An oscilloscope can be used to graphically display the back-and-forward resistance characteristics of
a crystal diode. A circuit used in conjunction with an oscilloscope to make this test is shown in figure 2-
15. This circuit uses the oscilloscope line-test voltage as the test signal. A series circuit (composed of
resistor R1 and the internal resistance in the line-test circuit) decreases a 3-volt, open-circuit test voltage
to a value of approximately 2 volts peak to peak.
