Important Short Questions and Answers: Power Devices and Display Devices

POWER DEVICES AND DISPLAY DEVICES

1.  Define Photovoltaic effect.

When Light is incident on a photodiode, an internal voltage is generated, it causes the current floe through internal circuit even though no external source is applied. This generated EMF is proportional to the frequency and intensity of the incident light.

2.     Mention the applications of DIAC, SCR, LED and photoconductive cell.

DIAC:

1.     It is used as a trigger device in TRAIC power control system.

2.     It is used in Lamp dimmer circuit.

3.     It is used in Heater control circuits

4.     It is used for Speed control of universal motor.

SCR:

1. It can be used as a speed controller in DC and AC motors.

2. It can be used as an inverter.

3. It can be used as a converter

4. It is used in battery chargers.

5. It is used for phase control and heater control.

6. It is used in light dimming control circuits.

LED:

1.     It is used in optical switching application.

2.     It is used in seven segment and dot matrix displays.

3.     It is used in the field of optical communication.

4.     It is used in Digital watches and calculators.

Photoconductive Cell:

1.     To measure the intensity of illumination.

2.     As a voltage regulator.

3.     As a volume controls that is itself controlled by light control.

4.     As On-OFF switch.

3.     What is Photodiode?

The photo diode is a diode in which the current sensitivity to radiation can be made much larger by the use of the reverse biased PN junction. Thus this diode conducts heavily in the reverse bias when there is some radiation allowed to fall on the PN junction.

4.       Draw the symbol of UJT, Photodiode, TRIAC, DIAC, LED and SCR.

5.       Define intrinsic standoff ratio of UJT.

It is defined as the ratio of the voltage drop across RB1 (VA) to the battery voltage (VBB)

Its value is generally lying in the range of 0.5 to .08.

6.  Define holding current of SCR and draw the two transistor model of SCR.

Holding current is defined as the minimum value of anode current to keep the SCR ON.

7.     Define dark current in photodiode.

When there is no light the reveres e bias photodiode carries a current which is very small and is

called dark current.

8.       Differentiate photodiode and phototransistor.

Photodiode :It is a light sensitive device used to convert light signal into electrical signal. It is also

called photo detector

Phototransistor:The photo transistor is a light detector, it combines a photodiode and phototransistor. The phototransistor cannot be directly used in control applications

9.       Draw the VI characteristics of  TRIAC.

10.                        What are the regions in the VI characteristics of UJT?

1. Cut-off region

2. Negative resistance region.

3. Saturation region

11. What is meant by negative resistance region of UJT?

In a UJT when the emitter voltage reaches the peak point voltage, emitter current starts flowing. After the peak point any effort to increase in emitter voltage further leads to sudden increase in the emitter current with corresponding decrease in emitter voltage, exhibiting negative resistance. This takes place until the valley point is reached. This region between the peak point and valley point is called negative resistance region.

12. Mention the applications of UJT.

1. It is used in timing circuits

2. It is used in switching circuits

3. It is used in phase control circuits

4. It can be used as trigger device for SCR and triac.

5. It is used in saw tooth generator.

6. It is used for pulse generation.

13.                        What is a Ohmic contact?

An ohmic contact is a low resistance junction providing conduction in both direction

between metal and the semiconductor. Ohmic contact acts as contact between any semiconductor device and outside world.

14. What does UJT stands for? Justify the name UJT.

UJT stands for uni junction transistor. The UJT is a three terminal semiconductor device having two doped regions. It has one emitter terminal (E) and two base terminals (B1and B2 ). It has only one junction, moreover from the out look, it resembles to a transistor hence the name unijunction transistor.

15. What is inter base resistance of UJT?

The resistance between the two bases (B1and B2) of UJT is called as inter base resistance. Inter base resistance = RB1 + RB2

RB1- resistance of silicon bar between B1 and emitter junction.

RB2 - resistance of silicon bar between B2 and emitter junction

16.                        Give the expression for peak point voltage for UJT?

VP = ή VBB + VD Where

VP - peak point voltage

ή - intrinsic stand -off ratio

VBB - voltage applied between the bases VD – barrier potential of UJT

17. What is backward diode?

The backward diode is a diode in which the doping level is moderate. The forward current in this case is very small, very much similar to that of the reverse current in the conventional diode.

18. What is a LED?

A PN junction diode which emits light when forward biased is known as Light emitting diode (LED).

19. What is a TRIAC?

TRIAC is a three terminal bidirectional semiconductor switching device. It can conduct in both the directions for any desired period. In operation it is equivalent to two SCR‟s connected in antiparallel.

20. Give the application of TRIAC.

·        Heater control

·        Motor speed control

·        Phase control

·        Static switches

21.                        What are the different operating modes of TRIAC?

·        Keeping MT2 and G positive

·        Keeping MT2 and G negative.

·        Keeping MT2 positive and G negative.

·        Keeping MT2 negative and G positive.

22.                        What is a DIAC?

DIAC is a two terminal bidirectional semiconductor switching device. . It can conduct in

either direction depending upon the polarity of the voltage applied across its main terminals. In operation DIAC is equivalent to two 4 layer diodes connected in anti parallel.

23. Give some applications of DIAC.

·        To trigger TRIAC

·        Motor speed control

·        Heat control

·        Light dimmer circuits

24.                        What is a SCR?

A silicon controller rectifier (SCR) is a three terminal, three junction semiconductor

device that acts as a true electronic switch. It is a unidirectional device. It converts alternating current into direct current and controls the amount of power fed to the load.

25. Define break over voltage of SCR.

Break over voltage is defined as the minimum forward voltage with gate open at which the SCR starts conducting heavily.

26. Why SCR cannot be used as a bidirectional switch.

SCR can do conduction only when anode is positive with respect to cathode with proper gate current. Therefore, SCR operates only in one direction and cannot be used as bidirectional switch.

27.                        How turning on of SCR is done?

·        By increasing the voltage across SCR above forward break over voltage.

·        By applying a small positive voltage at gate.

·        By rapidly increasing the anode to cathode voltage.

·        By irradiating SCR with light.

28.                        How turning off of SCR is done?

·        By reversing the polarity of anode to cathode voltage.

·        By reducing the current through the SCR below holding current.

·        By interrupting anode current by means of momentarily series or parallel switching

29.                        Define holding current in a SCR.

Holding current is defined as the minimum value of anode current to keep the SCR ON.

30.                        List the advantages of SCR.

·        SCR can handle and control large currents.

·        Its switching speed is very high

·        It has no moving parts, therefore it gives noiseless operation.

·        Its operating efficiency is high.

31.                        List the application of SCR.

·        It can be used as a speed controller in DC and AC motors.

·        It can be used as an inverter.

·        It can be used as a converter

·        It is used in battery chargers.

·        It is used for phase control and heater control.

·        It is used in light dimming control circuits.

32.                        What is meant by latching.

The ability of SCR to remain conducting even when the gate signal is removed is called as latching.

33. Define forward current rating of a SCR.

Forward current rating of a SCR is the maximum anode current that it can handle without destruction.

34. Compare SCR with TRIAC.

SCR

unidirectional current

triggered by positive pulse at gate

fast turn off time

large current ratings

TRIAC

bidirectional current

triggered by pulse of positive or negative at gate

Longer turn off time

lower current ratings

35. Differentiate BJT and UJT.

BJT  

It has two PN junctions

Three terminals present are emitter, Base, collector

basically a amplifying device

UJT

It has only one PN junctions

Three terminals present are emitter, base1,base2

basically a switching device

36.            Give the various triggering devices for thyristors.

·        SCR

·        UJT

·        DIAC

·        TRIAC

37.            Draw the opto-coupler circuit?