📡 Electronics and Communication Engineering (ECE) Solved MCQs with Answers

Practice 406 approved 📡 Electronics and Communication Engineering (ECE) questions with verified answers from the 🛠️ Engineering Essentials subject for Pakistan exams.

406 solved MCQs

Assertion (A): Inverters and choppers use fast switching thyristors Reason (R): Fast switching SCR has low turn off time.Answer: Both A and R are correct and R is correct explanation of A The value of capacitor C for dynamic equalising circuit of series connected thyristors is determined byAnswer: reverse recovery characteristics of thyristors A chopperAnswer: converts constant voltage dc into variable voltage dc directly Assertion (A): If the duty cycle of chopper is low the current would not become discontinuous Reason (R): If the time constant L/R of the load in chopper is low the current may become discontinuous.Answer: A is correct but R is wrong In a single phase full wave regulator, the firing angles in the positive and negative half cycles are generallyAnswer: equal The terminals of a power BJT are calledAnswer: emitter, base, collector In single phase half wave regulator, the average current over one full cycleAnswer: is always negative The dynamic equalising circuit consists of a series combination of capacitor C and resistor RC across each thyristor. This resistance RC (along with parallel connected diode)Answer: limits discharging current A single phase semiconverter is feeding a highly inductive load and has freewheeling diode across the load. The waveshapes of output voltage and output currentAnswer: are not similar Current derating may be necessary when thyristors are operating inAnswer: parallel Assertion (A): Power transistor has lower switching time than SCR Reason (R): Power transistor has greater switching time than SCRAnswer: A is wrong but R is correct A 3 kV circuit uses SCR of 800 V rating. If derating is 25%, the number of SCRs in series isAnswer: 5 In a circuit using a full wave converter (M - 2 connection) the PIV of each thyristor is 400 V. For the same output voltage and fully controlled bridge converter, PIV will beAnswer: 200 V A dc separately excited motor has constant field current. The armature is fed from a single phase supply through a full converter. When a = 0, speed is 500 rpm. If a = 45°, the speed is likely to beAnswer: about 125 rpm In a multiphase chopper, all choppers operate together.Answer: False The terminals of a power MOSFET are calledAnswer: source, gate, drain Assertion (A): The surge current which an SCR can withstand is much higher than rms on state current. Reason (R): The duration of surge current is very small.Answer: Both A and R are correct and R is correct explanation of A Assertion (A): A transistor requires a continuous base signal for conduction but a thyristor requires a gate pulse. Reason (R): Transistor find widespread application in power electronic circuits.Answer: A is correct but R is wrong In a thyristor the gate current is increased, thenAnswer: anode current will remain constant Thyristors are not suitable for logic circuits.Answer: False The number of leads in an SCR are :Answer: 3 A thyristor has a turn on time of 6 μs. If the anode circuit is inductive, the turn on time will beAnswer: more than 6 μs The total number of leads in reverse conducting thyristor isAnswer: 3 Assertion (A): A TRIAC is a bidirectional SCR. Reason (R): A TRIAC is a four layers 3 terminal device and can conduct in both directions.Answer: Both A and R are correct and R is correct explanation of A A semi converter operates in two quadrants.Answer: False Assertion (A): An ideal rectifier should have unity harmonic factor Reason (R): An ideal rectifier should have unity displacement factor.Answer: A is wrong but R is correct To prevent a DC return between source and load, it is necessary to useAnswer: capacitor between source and load For a base current of 10 μA, what is the value of collector current in common emitter if βdc = 100Answer: 1 mA Which of the following oscillators is suitable for frequencies in the range of mega hertz?Answer: Hartley An RC coupled amplifier has an open loop gain of 200 and a lower cutoff frequency of 50 Hz. If negative feedback with β = 0.1 is used, the lower cut off frequency will beAnswer: about 2.38 Hz