_________________________________________________________________ Semiconductor Diodes
measurements. However, as a rule of thumb, a small signal diode will have a ratio of
several hundred to one, while a power rectifier can operate satisfactorily with a ratio of 10
to 1.
1-101. Remember that the ohmmeter check is NOT conclusive. It is still possible for a
diode to check well under this test. However, it can still breakdown when replaced in the
circuit. The problem is that the meter used to check the diode uses a lower voltage than the
diode usually operates at in the circuit.
1-102. Another thing to remember is that a diode should not be condemned because two
ohmmeters give different readings. This occurs due to the different internal resistance of
the ohmmeters and the different states of charge on the ohmmeter batteries. Since each
ohmmeter sends a different current through the diode, the two resistance values read on the
meters will not be the same.
Figure 1-28. Checking a Diode With an Ohmmeter
1-103. Another way of checking a diode is with the substitution method. In this method, a
good diode is substituted for a questionable diode. Use this technique only after you have
made voltage and resistance measurements to make sure that there is no circuit defect that
might damage the substitution diode. If more than one defective diode is present in the
equipment section, where trouble has been localized, this method becomes cumbersome,
23 June 2005
TC 9-62
1-33
Previous Page
