oose the letter that best answers each question: 1. Refer to Fig. 14-5. What is

Question

oose the letter that best answers each question: 1. Refer to Fig. 14-5. What is the purpose of the gate switch? 6. Refer to Fig. 14-5. Assume that the source voltage has just been a. To turn the transistor switch on and off b. To commutate the NPN transistor c. To provide an emergency shutdown feature (safety) d. To turn on the transistor switch Refer to Fig. 14-5. The transistors are on. How can they be shut7. applied and the gate switch has not been closed. What can you conclude about the load current? a. It will equal c. It will be mainly determined by V and the load resistance d. It will flow until the gate switch is closed zero b. It will gradually increase 2. Refer to Fig. 14-5. Assume the gate switch has been closed and a. Open the gate switch b Close the gate switch c. Open the load circuit d. Increase the source voltage then opened again. What can you conclude about the load current? a. It will go off and then on. b. It will go on and then off 3. How is a silicon-controlled rectifier similar to a diode rectifir? c. It will come on and stay on d. None of the above a. Both can be classed as thyristors b. Both support only one direction of current flow e. Both are used to change alternating current to pulsatingb current (rectify) d. Both have one PN junction 8. How are SCRs normally turned on? a. By applying a reverse breakover voltage b.By applying a forward breakover voltage c. By a separate commutation circuit d. By applying gate current 4. Why is thyristor control more efficient than resistance l9. The three triac connections are a. Thyristors are less expensive b. Thyristors are easier to mount on a heat sink .Thyristors vary their resistance automatically d. Thyristors are solid-state switches What is the chief advantage of a triac as compared with a silicon-controlled rectifier? a. It costs less to buyb.It runs much cooler e. The triac is bidirectional d. All of the above a. anode, cathode, gate b. anode, main terminal I,gate c. main terminal , main terminal 2, gate d. cathode, main terminal 1, main terminal 2 10. Which of the following events will turm on a triac? 5. a. A rapid increase in voltage across the main terminals b. A positive gate pulse (with respect to terminal 1) c. A negative gate pulse (with respect to terminal)

Explanation / Answer

1) d The purpose of gate switch is to turn on the transiter switch

Explanation: When sourse is connected across anode and cathode terminal of scr it will not come into conduction state because two transisters will not have base current until we close gate switch. After closing gate switch two transiters come into conduction and devise remains in on state even though we open gate switch because two tranisters will maintain base drive for each other.

2) C:we can turn off the device by opening load circuit

Explanation: Gate switch is only intended for turning on of the devise and which has no effect on turn off process. Also by increasing sourse voltage we cannot bring device into off state. The reason behind this inorder turnoff scr load current should bring beloow holding current which is possible by opening load circuit.

3)B: Both devices supports only one direction of current flow

Explanation:As both devices supports one direction of current flow they will conduct either in positive half cycle of supply or in negatie half cycle of supply based on baising. From this we can conclude both devices acts as rectifiers.

.4)C.thyristors vary resistance autoatically

Explanation: By changing the firing angle of thyristor we can control ON state of the device so that we can hae a smooth control over available supply voltage which is similar to resistance control. But in resistance contol smooth variation is not possible  

5) C. TRIAC is bidirectional

Explanation : As TRIAC is bidirectional. A single TRIAC is enough for AC voltage regulators where as two unidirecational devices are required for the same purpose. A sigle gate circuit is enough for controling TRIAC in both directions.

6) C mainly depends on Vsourceand load resistace

Explanation if V source is greater than forward breakover voltage and load resistanc within the limits then devive will gets turn on this method of triggering is called forwad voltage triggering .

7)C it will come and stay on

Explanation : same as Explanation for question 1

8)D by applying gate current

Explanation : when the device in forward blocking state ,gate current acts as base drive for NPN transistor . NPN transistor will turn on and collector current will inturn acts as base drive for PNP trasisitor so that a path will form fron Anode to Cathode of SCR.We can use gate triggering only when the device in forward blocking state only..

9)C Main terminal 1 main termnal 2 and gate

Explanation : unlike SCR anode cathode gate terminals TRIAC will have Main terminal 1 main termnal 2 and gate because TRIAC is a bidirectional device we cannot assingn anode and cathode terminals rather than main terminals

10 ) D any of the above

Explanation: it will turn on with positive gate pulse or negative pulse with respect to MT1 & MT2 .and also TRIAC will come into ON state even by rapid increaseof voltage across MT1 & MT2 but which is nt desirable because due to rapid rasing of voltage Avalanche breakdown occur across the junctons which causes the device to damage

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