Tracker-2800-2800S.pdf - 第56页
56 SECTION 4 T ESTING D ISCRETE SEMICO NDUCT OR S 4-1. DIODES The most basic type of soli d state semiconductor component is the diode . Diodes are formed by creating a junction between p-t ype and n-type semi conductor …
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Figure 3-25. Using the Tracker 2800 DC Voltage Source for Relay Testing
Review
The Tracker 2800 can test switches in real time. This makes an excellent test for microswitches,
power switches, control switches, pressure and heat sensor switches.
As the mechanical switch closes, watch for erratic or discontinuous signature. Switch bounce will
display as multiple closure signatures. Resistive contacts will display a resistive signature at 50.
Use the arrow buttons to increase/decrease
voltage level applied to relay
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SECTION 4 TESTING DISCRETE SEMICONDUCTORS
4-1. DIODES
The most basic type of solid state semiconductor component is the diode. Diodes are formed by
creating a junction between p-type and n-type semiconductor material. The pn junction gives diodes
and semiconductor components polarity characteristics that allow them to conduct current when an
external voltage is applied. They conduct current in one direction, but not in the other. Current flows in
a diode when the positive terminal (anode) is made more positive than the negative terminal (cathode).
Figure 4-1 shows how the diode symbol indicates the polarity of the diode.
Figure 4-1. Diode and Schematic Symbol.
Diode Signatures
Diode signatures demonstrate the fundamental operation of a semiconductor junction. There is a
threshold or forward voltage V
F
(about 0.6V for a silicon diode) at which the diode begins to conduct
current. The diode acts as an open circuit and no current flows as long as the voltage differential
between the anode and cathode is below that threshold. As the anode to cathode voltage becomes more
positive, the diode will begin to conduct current. Once current begins to flow in the diode, very small
increases in anode voltage will cause very large increases in current. In analog signature analysis, this
is called the “knee” effect in which is characteristic of a good semiconductor junction.
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Figure 4-2. Tracker 2800 Core Circuit Block Diagram with a Diode.
You can see this "knee" signature on some diodes in the next section.
Do the following to display the analog signature of a diode:
1. Select 50, 10V and 60Hz.
2. Place or clip the red test lead from the Tracker 2800's Channel A jack to anode lead of the diode.
3. Place or clip the black test lead from the Tracker 2800's Common jack to anode lead of the diode.
V
S
= 10 Volts V
S
= 3 Volts
Figure 4-3. Signatures of a 1N914 type Silicon Diode at 50 and 60Hz.
The diode signatures are similar to each other. In the 50 Ohm range, the test signal voltage is 10 V
P
.
Each horizontal division on the display equals approximately 2.5 V. In this range the diode's signature
shows that its threshold or forward voltage is approximately 0.6 Volts. By lowering the test voltage to
3 V with the encoder, the 0.6 volt threshold is clearly visible for easier analysis.