6-
8
Diode Failures
Diodes can fail in a number of ways, and each type of failure will cause the signature to change.
The defective diodes often appear as open and short signatures. Two other types of flaws are
internal resistance and leakage.
Internal Resistance Flaw in a Diode
Figure 6-12. Defective Diode Model with a Small Series Resistor
V
s
=10V, R
s
=100Ω
ΩΩ
Ω V
s
=15V, R
s
=1kΩ
ΩΩ
Ω V
s
=15V, R
s
=100kΩ
ΩΩ
Ω
Figure 6-13. Defective Diode Signature With A 50Ω series Resistor
The 10V,100Ω
ΩΩ
Ω range shows that there is a resistive component to the signature when the diode is
conducting. This is the result of a defect in the diode's internal PN junction. The resistance is
visible only in this range because the voltage drop across it is small. In the other two ranges, the
diode’s series resistance is masked due to the internal resistances being too large to show such a
small voltage drop.
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