Tracker-2800-2800S.pdf - 第71页
71 4-3. SOLID STATE SWITCHING COMPONENTS Optical Switches There are two t ypes of optical switches: phototransi stors and optocouplers. Phototransistors can be used in two modes depending on the application. It can be us…
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B-E Junction C-E Junction C-B Junction
Figure 4-27. Signature of a Darlington Transistor, TIP112 NPN Type at 20V and 20K.
Note that the B-E junction has a sloped leg bend in its signature caused by internal resistors R1 and R2.
Review
A PNP bipolar transistor consists of a layer of N-type silicon sandwiched between two layers of P-
type silicon.
A NPN bipolar transistor consists of a layer of P-type silicon sandwiched between two layers of N-
type silicon.
To test a transistor, the base-emitter (B-E), collector-base (C-B) and collector-emitter (C-E)
junctions all need to be examined.
The transistor signature resembles the diode signatures previously examined. They have polarity
and may exhibit the Zener effect.
Applying What We Have Learned
Transistors will display the same type of faulty signature as diodes, with a rounded “knee” and
non-linear or resistive current and voltage legs.
The Tracker 2800 can be used to determine the type of transistor; bipolar, Darlington, FET, etc.
The Tracker 2800 can be used to identify the polarity of a transistor (PNP or NPN).
The Tracker 2800 can be used to determine the base, collector and emitter on an unknown
transistor.
The Tracker 2800 can be used to match the gain (beta) of two transistors.
The above techniques of identification are invaluable when repairing foreign electronics and
systems without schematics.
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4-3. SOLID STATE SWITCHING COMPONENTS
Optical Switches
There are two types of optical switches: phototransistors and optocouplers. Phototransistors can be
used in two modes depending on the application. It can be used as either a light activated transistor or
as a light activated diode. In either mode, light is used to turn it on and allow current to flow.
The optocoupler consists of a light emitting diode and a phototransistor in the same package. They are
electrically isolated. When the diode is turned on by an external signal, it radiates light. This light falls
on the phototransistor base junction that results in the device turning on.
Phototransistor
Figure 4-28. Phototransistor Schematic Diagram.
Do the following to display the analog signature of a phototransistor:
1. Select the 1K and 15V.
2. Place or clip the red test lead from the Tracker 2800's Channel A jack to collector lead of the
component.
3. Place or clip the black test lead from the Tracker 2800's Common jack to emitter lead of the
component.
4. Observe the signature of the phototransistor
5. Direct a light source at the phototransistor and observe the signature change.
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MRD3056 With No Light MRD3056 With Light
Figure 4-29. Signatures of a NPN C-E Junction Phototransistor at 15V and 1K.
The phototransistor's signature is similar to a diode's signature in reverse breakdown mode when not
activated by light and as a short signature when activated by a bright external light.
SCRs and TRIACs
A SCR and TRIAC are semiconductor components that are used in switching applications. A SCR
(silicon controlled rectifier) is used for DC switching circuits. A TRIAC is used for AC switching
circuits. This section will demonstrate how to dynamically test these components.
Silicon Controlled Rectifiers (SCR’s)
The SCR is a switching semiconductor device that conducts positive current only. Its symbol and
equivalent circuit can be seen below. When the gate (G), is at the same voltage level as the cathode
(K), the SCR acts like an open. When the gate (G) is forced more positive than the cathode (K),
positive current flows between the anode (A) and the cathode (K).
Figure 4-30. Diagram of a Silicon Controlled Rectifier.
Do the following to display the analog signature of a SCR:
1. Select the 20V, 10K and 60Hz.
2. Place or clip the red test probe from the Tracker 2800's Channel A jack to gate lead (G) of the
component.
3. Observe the gate-anode signature of the SCR.
4. Move the black test probe from the SCR's anode lead to cathode lead (K) of the component.
5. Observe the gate-cathode signature of the SCR.