Tracker-2800-2800S.pdf - 第62页
62 50 10V 1K 15V 50K , 20V Figure 4-14 . Signature of A 1N914Diode W ith Internal Leakage (10K In Parallel). Notice that in the 50 range, there does not seem to be a problem. In the 1K and 10K range…
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Internal Resistance Fault in a Diode
Figure 4-11. Defective Diode Model with a Small Series Resistor.
50 10V 1K 15V 20K, 20V
Figure 4-12. Defective Diode Signature with a 50 Series Resistor.
The 50 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 50
range because the voltage drop across it is small. In the other two ranges, the resistance is masked due
to the internal resistances being too large to show such a small voltage drop.
Internal Leakage Fault in a Diode
Figure 4-13. Defective Diode Model with an Internal Leakage Resistance.
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50 10V 1K 15V 50K, 20V
Figure 4-14. Signature of A 1N914Diode With Internal Leakage (10K In Parallel).
Notice that in the 50range, there does not seem to be a problem. In the 1Kand10K ranges, you
can see the diode conducting when it should be acting like an open. This is called leakage. The diode
acts like a diode when it is forward biased. When reverse biased, the diode acts like a resistor when it
should be acting as an open.
Zener Diodes
Normal switching and signal diodes conduct when forward biased only. When reverse biased, they act
as opens unless they are operated outside design limits. If this condition occurs then so much voltage is
applied that they break down and can no longer prevent current flow.
A zener diode is a different type of diode and is designed for operation when reverse biased (diode's
cathode connected to positive and anode to negative), but under carefully controlled conditions. When
the zener diode is forward biased (diode's cathode connected to negative and anode connected to
positive), they act as regular diodes and begin to conduct at approximately 0.6V.
When reverse biased, they act as an open until the applied voltage reaches their specified zener
voltage, at which time the zener diode begins to conduct current. Even if the reverse voltage is
increased, the voltage across the zener remains constant. It is this feature of zener diodes that allows
them to be used as voltage regulators and references. Because they conduct in both directions, the
zener diode's analog signature has two knees, one at 0.6V and the other at the zener voltage of the
diode. In ASA terminology, this two knee signature is known as the classic "chair" pattern that is
common in many solid state semiconductor components.
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Figure 4-15. Tracker 2800 Core Circuit Block Diagram with a Zener Diode.
Figure 4-16. Single Zener Diode and two Zener Diodes in Series.
Single 1N5239B Zener Diode 2 Zener Diodes in Series, 1N5239B
Figure 4-17. Signatures of A Zener Diodes at 20V, 10K
Since each horizontal division on the Signature graticule (in 20V range) is approximately 5 Volts, from
the signature on the left you can estimate that this is about a 9 volt zener diode. The signature at the
right is the signature of two zener diodes connected in series. The Zener voltage (V
Z
) of this circuit is
the sum V
Z
of each of the separate diodes. The signature shows this voltage to be approximately 18
Volts.
Review
Diodes conduct current in one direction (forward biased) and not the other. The diode's analog
signature displays this characteristic as the “knee” effect or pattern.
Diodes have polarity, an anode and a cathode.
Diode defects, other than opens and shorts, are usually resistive.
A diode in series or parallel with a resistor or capacitor will create a composite signature displaying
both characteristics. The Tracker 2800 makes it easier to separate these characteristics from
composite signatures using the three parameters F
S
, R
S
and V
S
.
Zener diodes are special diodes that conduct when reverse biased at a specific voltage.