Thyristors UJT and Optoelectronic Devices: Two Marks Important Questions and Answers

Electron Devices

Chapter 6: Thyristors UJT and Optoelectronic Devices

 

Two Marks Questions with Answers

 

1. What is LED? Which material is used for LED ?

 LED is a light emitting diode. When forward biased this diode emits light of different colours depending on the material used.

 

2. List any four applications of light emitting diode.

1. All kinds of displays used in watches and calculators.

2. In the optical devices such as opto‒couplers.

3. As on‒off indicator in various electronic circuits.

4. In remote controls and burgler alarms.

 

3. What are the advantages of LCD ?

1. Less power consumption.

2. Low cost.

3. Uniform brightness with good contrast.

4. Low operating voltage and current.

 

4. What are the disadvantages of LCD.

1. Poor reliability.

2. Limited temperature range.

3. Poor visibility in low ambient temperature.

4. Slow speed.

5. Requires an a.c. drive.

 

5. State the three main categories of LCD.

1. Smectic 2. Nematic 3. Cholesteric.

 

6. What is SCR? Explain its construction in brief.

SCR is a silicon controlled rectifier.

The SCR is a four layer p‒n‒p‒n device where P and n layers are alternately arranged. The outer layers are heavily doped. There are three p‒n junctions called J₁, J₂ and J₃. The outer p layer is called anode while outer n layer is called cathode. Middle p layer is called gate. The three terminals are taken out respectively from these three layers.

 

7. Draw the characteristics of SCR.

 

8. State any four specifications of SCR.

1. Forward Breakover Voltage (V_BO): It is the voltage above which the SCR enters the conduction region ('ON' state). The forward breakdown voltage is dependent on the gate bias.

2. Holding Current (I_H): It is that value of current below which the SCR switches from the conduction state (ON state) to the forward blocking state.

3. Latching Current (I_L): This is the minimum current flowing from anode to cathode when SCR goes from OFF to ON state and remains in ON state even after gate bias is removed. It is greater than, but very close to holding current.

4. Reverse Breakdown Voltage (V_BR): It is the voltage (Anode‒negative cathode‒positive) above which the reverse breakdown occurs, breaking J₁ and J₃ junctions. When the SCR is reversed biased, the thickness of the J₂ depletion layer during the forward bias condition is greater than the total thickness of the two depletion layers at J₁ and J₃. Therefore, the forward breakover voltage V_BO is greater than the reverse breakover voltage V_BR.

 

9. State the applications of SCR.

1. Controlled rectifiers.

2. A.C. voltage stabilizers.

3. D.C. to D.C. converters called choppers.

4. D.C. to A.C. converters called inverters.

5. Dimmerstats to control light intensity.

6. For speed control schemes of d.c. and a.c. motors called drives.

 

10. Draw the equivalent circult of UJT.

 

11. What Is Interbase resistance of UJT ?

When the emitter diode is not conducting then the resistance between the two bases B₁ and B₂ is called interbase resistance denoted as R_BB.

 R_BB = R_B1+R_B2

Its value ranges between 4 kΩ and 12 kΩ.

 

12. Define intrinsic stand off ratio for UJT.

For UJT intrinsic stand off ratio is defined as

 

13. Draw the characteristics of UJT.

 

14. State the applications of UJT.

1. Triggering of other devices like SCR.

2. In a sawtooth waveform generator.

3. In a relaxation oscillator.

4. In timing circuits.

5. In automobile ignition circuits.

 

15. Draw the diac characteristics.

 

16. State the applications of diac.

1. Triggering of triac

2. Motor speed control

3. Temperature control

4. Light dimming circuit

5. In air conditioning and other domestic applications.

 

17. Draw the characteristics of triac.

 

18. State the applications of triac.

1. Static d.c. switching of applications.

2. In a three position static switch.

3. A.C. power flashers.

4. Heater or temperature controllers.

5. As a triggering device for SCRS.

6. Light dimmer circuits.

 

19. Compare SCR and triac.

SCR

1. The SCR is unidirectional device.

2. The terminals are called anode‒cathode due to unilateral property.

3. The gate current can be only in direction to turn on SCR

4. The characteristics are lying in first quadrant only, hence are nonsymmetrical.

Triac

1. The triac conducts in both the directions hence bilateral device.

2. The terminals are called MT₁ and MT₂ with gate.

3. For both the directions of gate current, triac conducts.

4. The characteristics are lying in first and third quadrant and are symmetrical.

 

20. State the applications of phototransistor.

1. Punch card readers

2. Computer logic circuitry

3. Lighting control

4. Level indicators

5. Relays.

 

21. What is photoconductive cell?

The photoconductive cell is semiconductor device whose resistance varies inversely with the intensity of light that falls upon its photosensitive material. These devices are also called photoresistive cells or photoresistors, since the change in conductivity appears as a change in resistance.

 

22. State the applications of photoconductive cell.

1. Smoke detecters

2. Burglar alarm

3. Outdoor lighting control.

4. Photographic instruments

5. Punch card readers.

 

23. What is photovoltaic cell ?

A solid state device which converts light energy into an electrical energy based on photovoltaic effect is called photovoltaic cell or solar cell. It generates voltage proportional to the light incident on it.

 

24. State the applications of photovoltaic cell.

1. Satellites and space vehicles.

2. Automated street lights.

3. Power supply to calculators.

4. Emergency lights.

5. For charging the batteries.

6. Solar panels for domestic purposes, water pumps etc.

 

25. Compare LED and LCD.

 

26. Draw the basic structure of TRIAC and its symbol.

 

27. Write down the significance of opto coupler.

A package which is a combination of light source like LED and a light detector as photodiode or phototransistor is called optocoupler. In optocoupler if input voltage changes then light emitted by light source changes. This changes the characteristics of a light detector hence output voltage changes. Thus output voltage is optically coupled with the input voltage, though electrically isolated. Thus the device couples input to output circuit optically, providing electrical isolation.

 

28. Draw the two transistor equivalent circuit of an SCR.

 

29. A solar cell is a pn junction device with no voltage directly applied across the junction". If it is so, how does a solar cell deliver power to a load ?

A solar cell is a photovoltaic cell which converts the light energy incident on it into an electrical energy based on photovoltaic effect. This electrical energy is in the form of internally generated voltage across the junction due to the movement of minority charge carriers across the junction due to light energy. This voltage delivers power to the load hence no external voltage is required to be applied across the junction.

 

30. Draw the circuit diagram of opto coupler.

 

31. What is meant by regenerative action of SCR?

• Consider that the voltage is applied between gate and cathode when the SCR is in forward blocking state.

• The gate is made positive with respect to the cathode.

• The electrons from n‒type cathode which are majority in number, cross the junction J₃ to reach to positive of battery.

• While holes from p type move towards the negative of battery, this constitutes the gate current.

• This current increases the anode current as some of the electrons cross junction J₂. As anode current increases, more electrons cross the junction J₂ and the anode current further increases.

• This action is called regenerative action in SCR. Due to this within short time junction J₂ breaks and SCR conducts heavily,

 

32. Glue the symbol, structure and equivalent clrcult of DIAC ?

 

33. What is the effect of temperature on solar cell?

The efficiency of solar cell decreases as the temperature increases. The effect of temperature on solar cell is known as temperature coefficient. For silicon solar cells, the efficiency decreases by 0.45 % for every degree rise in temperature.

 

34. Draw the symbol and equivalent circuit of TRIAC ?

The equivalent circuit is shown in the Fig. 6.13.2.

 

35. Define quantum efficiency in an LED.

The fraction of the electrons that are injected into the depletion (active) region which results in photons getting produced is known as internal quantum efficiency in LED. It is defined by the number of photons generated from a given active region divided by the number of injected electrons.