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72 MRD3056 With No Light MRD3056 With Light Figure 4 -29 . Signatures of a NPN C-E J unction Phototransistor at 15V and 1K  . The phototransistor's signature is simila r to a diode's signature in reverse bre a…

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.

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.

6. Place the red test probe to the SCR's anode lead and the black test probe to the SCR's cathode lead.

7. Observe the SCR's anode-gate signature.

Gate-Anode Gate-Cathode Anode-Cathode

Figure 4-31. Signatures of a SCR - C106B Type at 20V and 10K.

8. Connect the Tracker 2800 DC Voltage Source to the Gate. Increase the DC Voltage Source level

while observing the anode-cathode signature.

Figure 4-32. Testing a SCR using the DC Voltage Source

Review

Solid state photosensitive switch components are turned on by light.

SCRs and TRIACs are solid state switches that are turned on by a control input pin called a gate. SCRs

conduct current in one direction while TRIACs conduct current in both directions.

Applications

The SCR and TRIAC can also be a problem to troubleshoot. They may be used to switch large

currents. Quite often these components are susceptible to degradation and eventual failure. The Tracker

can easily show these failure.

Using the arrows buttons to

increase/decrease the DC voltage level