Tracker-2800-2800S.pdf - 第59页
59 Composite Diode Signatures A composite analog signature is a combination of several compone nts connected together in an electronic circuit. Up to this point, we ha ve been showing you what the basic component signatu…
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Effects of Changing Frequency on Diode Signatures
With the 3V, 50 selected and the test signal frequency of 60 Hz, the signature of the diode is shown
on the left figure below. Changing only the test signal frequency to 2kHz displays the signature on the
right. At F
S
= 2kHz, the diode's signature has slight circular loop added to it. This loop in the signature
is due to a physical characteristic of diodes called junction capacitance.
60Hz 2KHz
Figure 4-4. Signature of a 1N914 Diode at Different Frequencies at 3V and 50.
Effects of Changing Resistance on Diode Signatures
Changing Tracker 2800's internal resistance R
S
moves the vertical knee portion of the diode's analog
signature. As R
S
increases, the knee of the signature moves inward toward the origin. R
S
controls the
current that's flowing through the diode so the forward diode voltage changes in response to the current
change.
R
S
= 50 R
S
= 100K
Figure 4-5. Signature of a 1N914 Diode at Different Resistances at 3V and 60Hz
59
Composite Diode Signatures
A composite analog signature is a combination of several components connected together in an
electronic circuit. Up to this point, we have been showing you what the basic component signatures
look like out of circuit. In the real world of electronics troubleshooting, components are connected
together in a circuit and when testing with ASA, the signatures are a composite that may appear quite
complex. However, with knowledge of ASA fundamentals and experience you will find that even the
most complex looking signatures can be analyzed quickly and efficiently.
This section will introduce you to some examples of composite diode signatures.
Figure 4-7. Composite Model of a Diode and Capacitor in Parallel.
V
S
= 10 V, F
S
= 20 Hz V
S
= 10 V, F
S
= 1KHz V
S
=200 mV, F
S
= 1KHz
Figure 4-8. Composite Signature - 1N914 Diode and 1F Capacitor in Parallel.
The signature on the left shows only the diode signature because the test signal frequency is set below
any visible contribution due the capacitive reactance. The composite signature in the center consists of
the distinctive loop of the capacitor and the "knee" pattern of the diode. The signature on the right
shows only the capacitor signature because the test signal voltage is below the diode's turn on level.
When multiple components are connected together, it's important to realize that the Tracker 2800 has
the ability to selectively display the signature of a single component.
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Figure 4-9. Composite Model of a Diode and Resistor in Parallel.
V
S
= 10 V, R
S
= 50 V
S
= 10 V, R
S
= 1K V
S
= 200 mV, R
S
= 1K
Figure 4-10. Composite Signature - 1N914 Diode and 1KResistor in Parallel.
The signature on the left shows only the diode signature because the test signal resistance is set below
any visible contribution due the 1k resistor. The composite signature in the center consists of the
distinctive slope of the resistor and the "knee" pattern of the diode. The signature on the right shows
only the resistor signature because the test signal voltage is below the diode's turn on level. Again,
when multiple components are connected together, it's important to realize that the Tracker 2800 has
the ability to selectively display the signature of a single component.
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.