"Basic Electronics"

Basic Electronics

Objective Questions from Measurements Instrumentation  [1] A CRO can display (a) AC signals (b) DC signals (c) Both AC and DC signals (d) Time invariant signals Ans: C Answer[2] The internal resistance of an ammeter should be (a) Very small (b) medium (c) High (d) Infinity Ans: A Answer[3] A galvanometer has (a) air friction damping (b) fluid friction damping (c) Spring coil damping (d) Eddy current damping Ans:D Answer[4] Electrostatic instruments are normally used for (a) Low current measurements (b) High current measurements (c) Low voltage measurements (d) High voltage measurements Ans:D Answer[5] In a moving coil instrument, the deflecting torque is proportional to (a) Current (b) Square of the current (c) Squareroot of the current (d)  sine of the measurand Ans:A Answer[6] The absolute measurement of resistance is done by (a) Ohm's law method (b) Bridge Method (c) Rayleigh method (d) Lorenz method Ans: D Answer[7] To measure the flux, devices used are based on (a) Voltaic effect (b) Piezo-electric effect (c) Hall effect (d) Photo-voltaic effect Ans:C Answer[8] The form factor in AC is the ratio of (a) peak value to average value (b) peak value to rms value (c) rms value to average value (d) rms value to peak value Ans:C Answer[9] PMMC type instruments normally use (a) Air friction damping (b) Fluid friction damping (c) Eddy current damping (d) None of the above Ans: C Answer[10] In a moving iron meter, the deflecting torque is proportional to (a) Current through the coil (b) Square of the current through the coil (c) Sine of the measurand (d) Squareroot of the measurand Ans: B Answer[11] In a gravity controlled instrument, the deflection angle is proportional (a) Measurand (b) Square of the measurand (c) sine inverse of measurand (d) sine of the measurand Ans: C Answer[12] Ampere is one of the (a) Supplementary units (b) Derived units (c) Base units (d) Units used to measure charge Ans: C Answer[13] Direct method is used to measure (a) Length (b) Temperature (c) Pressure (d) Voltage Ans: A Answer Please leave your comments and questions below...

Basic Electrical objective type questions: Part-13 Posted: 23 Jul 2014 10:19 AM PDT Electrical Objective Questions with Answers: [1] In a uniform electric field, field lines and equipotentials [Gate 1997 ] a) Are parallel to one another b) Intersect at 45 deg c) Intersect at 30 deg d) Are orthogonal Ans:D Answer[2] For a pulse transformer, the material used for its core and the possible turn ratio from primay to secondary are respectively (a) ferrite; 20:1 (b) laminated iron; 1:1 (c) ferrite; 1:1 (d) powdered iron; 1:1 Ans: C Answer [3] The capacitance of a reverse biased junction of a thyristor is 20 picofarad. The charging current of this thyristor is 4mA. The limiting value of dv/dt is .....V/uS Ans:200 Answer [4] When the induction motor runs at synchronous speed the voltage induced in the rotor of it is (a) Very near input voltage to stator (b) Slip time the input voltage (c) Zero (d) None of the above Ans:C Answer [5] Chopper control for the DC motor provides variation in (a) Input voltage (b) Frequency (c) None of the above Ans: A Answer [6] While comparing SCR with BJT, which of the following is true (a) SCR requires turn off circuit while BJT does not (b) The voltage drop of SCR is less than BJT (c) SCR requires a continuous gate current (d) BJT draws continuously base current Which of the above statements are true? (1) a only (2) d only (3) a, d (4) b, d Common Data Questions: 7 & 8  In the 3-phase inverter circuit shown, the load is balanced and the gating scheme is 180°-conduction mode.All the switching devices are ideal Ans: 3 Answer [7] The rms value of load phase voltage is [GATE2012] a. 106.1V b. 141.4V c. 212.2V d. 282.8V Ans: B Answer [8] If the dc bus voltage Vd=300V, the power consumed by 3-phase load is  [GATE2012] a. 1.5kW b. 2.0kW c. 2.5kW d. 3.0kW Ans: D Answer [9] The typical ratio of latching current to holding current in a 20A thyristor is [GATE 2012] a. 5.0 b. 2.0 c. 1.0 d. 0.5 Ans: B Answer [10] A half-controlled single-phase bridge rectifier is supplying an R-L load. It is operated at a firing angle  α and the load current is continuous. The fraction of cycle that the freewheeling diode conducts is [GATE 2012] a. 1/2 b. (1-α/π) c. α/2π d. α/π Ans: D Answer [11]  For power factor correction of power systems which one is used: (a) capacitor (b) inductor (c) resistors Ans: A Answer [12] Which type of motor used in home ceiling fan: (a) capacitor motor  (b) shaded pole motor  (c) universal motor (SPLIT PHASE CAPACITOR MOTOR) Ans: A Answer [13] Op-amp (operational amplifier) input resistance: (a) Zero (b) Infinity (c) Relative resistance (d) Same output resistance Ans:B Answer [14] For an inverter op-amp, what will be the equivalent output resistance? (a) Zero (b) Infinity (c) Relative forward resistance (d) Same as the input resistance Ans: A Answer

Power Electronics Objective Type Questions: Part-13 Posted: 11 Feb 2014 03:23 AM PST Power Electronics Multiple Choice Questions [1] During forward blocking state, the SCR has (a) Low current, medium voltage (b) Low current, large voltage (c) Medium current, large voltage (d) Large current, low voltage Answer: B [2] Once SCR starts conducting a forward current, its gate loses control over (a) Anode circuit current only (b) Anode circuit voltage only (c) Anode circuit voltage and current (d) Anode circuit voltage, current and time Answer: C [3] In a SCRs (a) Both Latching current and holding current are associated with turn-off process (b) Latching current is associated with turn-off process and holding current with turn on process (c) Holding current is associated with turn-off process and Latching current with turn-on process (d) Both Latching current and holding current are associated with turn on process Answer: C [4] The SCR can be termed as (a) DC switch (b) AC switch (c) Square-wave switch (d) Either A or B Answer: A [5] In a thyristor, the magnitude of anode current will (a) Increase if gate current is increased (b) Increase if gate current is decreased (c) Decrease if gate current is decreased (d) Not change with any variation in gate current Answer: D [6] Turn on time of an SCR can be reduced by using a (a) Rectangular pulse of high amplitude and narrow width (b) Rectangular pulse of low amplitude and wide width (c) Triangular pulse (d) Trapezoidal pulse Answer: A [7] Turn off time of an SCR in series with RL circuit can be reduced by (a) Increasing circuit resistance R (b) Decreasing circuit resistance R (c) Increasing circuit inductance L (d) Decreasing circuit inductance L Options: 1) B, C 2) A, D 3) B, D 4) D only Answer: 4 [8] A forward voltage can be applied to an SCR after its (a) Anode current reduces to zero (b) Gate recovery time (c) Reverse recovery time (d) Anode voltage reduces to zero Answer: B [9] Turn-off time of an SCR is measured from the instant (a) Anode current becomes zero (b) Anode voltage becomes zero (c) Anode voltage and anode current become zero at the same time (d) Gate current becomes zero Answer: A [10] In an SCR, anode current flows over a narrow region near the gate during (a) Delay time td (b) Rise time tr and spread time tp (c) td and tp (d) td and tr Answer: D [11] Gate characteristic of a thyristor (a) Is straight line passing through origin (b) Is of the type Vg = a + bIg (c) Is a curve between Vg and Ig (d) Has a spread between two curves of Vg-Ig Answer: D [12] Surge current rating of an SCR specified the maximum (a) Repetitive current with sine wave (b) Non-repetitive current with rectangular wave (c) Non-repetitive current with sine wave (d) Repetitive current with rectangular wave Answer: C [13] The di/dt rating of an SCR is specified for its (a) Decaying anode current (b) Decaying gate current (c) Rising gate current (d) Rising anode current Answer: D [14] For an SCR, dv/dt protection is achieved through the use of (a) RL in series with SCR (b) RC across (c) L in series with (d) RC in series with Answer: B [15] For an SCR, di/dt protection is achieved through the use of (a) R in series with SCR (b) RL in series with SCR (c) L in series with SCR (d) L across SCR Answer: C Solution Hint: [4] SCR will allow current flow in one direction only. So the ideal SCR is similar to DC switch [7] By the decreasing the Inductor L value, the di/dt is increasing. ie, fast turn off process [14] Snubber circuit ( RC circuit across device) is used for dv/dt protection [15] Inductor will oppose sudden change in current (di/dt). For inductor, series resistance is not required. But in snubber circuit, R is mandatory for dissipating the stored energy in the capacitor.

Power Electronics Multiple Choice Questions: Part-12 Posted: 11 Feb 2014 03:40 AM PST Multiple Choice Questions Based on Power Electronics and Drives [1] When a line commutated converter operates in the inverter mode (a) It draws both real and reactive power from the AC supply (b) It delivers both real and reactive power to the AC supply (c) It delivers real power to the AC supply (d) It draws reactive power from the AC supply Answer: C [2] A chopper operating at a fixed frequency is feeding an RL load. As the duty ratio of the chopper is increased from 25% to 75%, the ripple in the load current is (a) Remains constant (b) Decreases, reaches a minimum at 50% duty ratio and then increases (c) Increases, reaches a maximum at 50% duty ratio and then decreases (d) Keeps on increasing as the duty ratio is increased Answer: A [3] To turn off a SCR, the reverse bias should be applied for a period ....... the turn-off time of the SCR (a) Equal to  (b) Longer than (c) Less than (d) Irrespective of Answer: B [4] In class A and class B commutation the resonating circuit has to be (a) Over damped (b) Critically damped (c) Under damped (d) Negatively damped Answer: C [5] In phase controlled rectification power factor (PF) (a) Remains unaffected (b) Improves with increase of firing angle α (c) Deteriorates with increase of α (d) Is unrelated to α Answer: C [6] Comparing with the full wave rectifier using two diodes, the four diode bridge rectifier has the dominant advantage of (a) Higher current carrying  (b) Lower peak inverse voltage requirement (c) Lower ripple factor (d) Higher efficiency Answer: B [7] A SCR is rated at 75A peak, 20A average. The greatest possible delay in the trigger angle if the dc is at rated value is (a) 47.5° (b) 30° to 45° (c) 74.5° (d) 137° Answer: B [8] The applied sine voltage to a SCR is VM=200V and R=10Ohm. If the gate trigger lags the ac supply by 120°, the average load current is (a) 15/π A (b) 5/π A (c) -5/π A (d) -15/π A Answer: A [9] A sine voltage of 200Vrms, 50Hz is applied to an SCR through 100ohm resistor. The firing angle is 60°. Consider no voltage drop. The output voltage in rms is (a) 89.7 V (b) 126.7 V (c) 166.7 V (d) 200√2 V Answer: B [10] A 100VDC is applied to the inductive load through a SCR. The SCR's specified latching current is 100mA. The minimum required width of gating pulse to turn on the SCR is (a) 100μS (b) 100S (c) 1mS (d) 50μS Answer: A [11] A cycloconverter is a (a) Frequency changer from higher to lower frequency with one-state conversion (b) Frequency changer from higher to lower frequency with two-stage conversion (c) Frequency changer from lower to higher frequency with one-stage conversion (d) Either a or c Answer: D [12] The cycloconverter require natural or forced commutation as under (a) Natural commutationin bothstep-up and step down cycloconverter (b) Forced commutation in both step-up and step-down cycloconverter (c) Forced commutation in step-up cycloconverter (d) Forced commutation in step-down cycloconverter Answer: C [13] In synchronized UJT triggering of an SCR, voltage VC across capacitor reaches UJT threshold thrice in each half cycle so that there are three firing pulses during each half cycle. The firing angle of the SCR can be controlled (a) Once in each half cycle (b) Thrice in each half cycle (c) Twice in each half cycle (d) Four times in each half cycle (e) None of the above  Answer: A [14] In a GTO, anode current begins to fall when gate current (a) Is negative peak at time t=0 (b) Is negative peak at t = storage period tS (c) Just begins to become negative at t = 0 (d) Is negative peak at t = (tS + fall time) Answer: B [15] The SCR can be turned on by (a) Applying anode voltage at a sufficiently fast rate (b) Applying sufficiently large anode voltage (c) Increasing the temperature of SCR to a sufficiently large value (d) Applying sufficiently large gate current OPTIONS: 1) A, B 2) C, D 3) B, C 4) A, B, C, D 5) None of the above options Answer: 4 Solution Hint: [2] VL= V[(Ton/(Ton+Toff)]          =V. f .Ton   There are two ways to vary VL. Either by varying f or Ton. Here f is fixed and Ton is     varied. As the frequency is kept constant ripple remains constant. [6] The PIV of diodes used in the full wave rectifier using two diodes is 2 times that of  the four diode bridge rectifier [7] RMS Current/ Average Current = Form Factor [8] Idc = [Vm/2πR](1+cosα)         α = delay angle [15] a= dv/dt triggering, b = forward voltage triggering, c= temperature triggering, d = gate triggering. So all are correct

Objective Type Questions on Power Electronics: Part-11 Posted: 11 Feb 2014 03:23 AM PST Power Electronics Objective Type Questions with Answers: Part-11 [1] In a SCR, (a) Gate current is directly proportional to forward breakover voltage (b) As gate current is raised, forward breakover voltage reduces (c) Gate current has to be kept on continuously for conduction (d) Forward breakover voltage is low in the forward blocking state. Answer: B [2] In a SCR, at switch on, there is a sudden increase of current, then (a) The tolerance limit of di/dt varies from 3 to 30A/us (b) di/dt upper limit is 3A/us (c) di/dt rating can be increased by having an inductive load (d) di/dt rating can be increased by a resistive load Which of the above statements are true? (1) a, c (2) c, d (3) a, b, c, d (4) a, b Answer: 1 [3] The following PNPN device has a terminal for synchronising purpose (a) SUS (b) DIAC (c) TRIAC (d) Schmitt trigger Answer: A [4] In single phase full wave controlled rectifier, maximum output voltage is obtained at conduction angle....and minimum at conduction angle... (a) 0°, 180° (b) 180°, 0° (c) 0°, 0° (d) 180°, 180° Answer: A [5] A thyristor is triggered by a pulse train of 5kHz. The duty ratio is 0.4. If the allowable average power is 100W the maximum allowable gate drive power is (a) 100√2 W (b) 50 W (c) 150 W (d) 250 W Answer: D [6] The consequence of introducing inductance in the load circuit of SCR is (a) Anode current through SCR rises slowly (b) Anode current reached is more than without it (c) SCR has to be derated for anode current (d) SCR's current rating can be raised Which of the above statements are true? (1) a, c (2) a, b, c, d (3) b, d (4) a, d Answer: 4 [7] When a SCR is in the forward blocking state, (a) All the 3 junctions are reverse biased (b) The anode and cathode junctions are forward biased but the gate junction is reverse biased (c) The anode junction is forward biased but the cathode and gate junctions are reverse biased (d) The anode and gate junctions are forward biased but the cathode junction is reverse biased Answer: B [8] When we need to drive a dc shunt motor at different speeds in both directions ( forward and reverse) and also to brake it in both the directions, which one of the following would you use? (a) A half-controlled SCR bridge (b) A full controlled thyristor bridge (c) A dual converter (d) A diode bridge Answer: C [9] The frequency of the ripple present in the output voltage of the 3 phase half controlled bridge rectifier depends on the (a) Firing angle (b) Load inductance (c) Load resistance (d) Supple frequency Answer: D [10] A fully controlled thyristor bridge drives a DC motor. The system is capable of (a) Motoring and braking in both directions (b) Only motoring in both directions, no braking (c) Motoring in forward direction and braking in reverse direction (d) Only motoring in forward direction, no braking Answer: D [11] Three-phase to three-phase cycloconverter employing 18 SCRs and 36 SCRs have the same voltage and current ratings for their component thyristors. The ratio of VA rating of 36 SCR device to that of 18 SCR device is (a) 1/2 (b) 1 (c) 2 (d) 4 Answer: C [12] Three phase to three phase cycloconverter employing 18 SCRs are 36 SCRs have the same voltage and current ratings for their component thyristors. The ratio of power handled by 36 SCR device to that handled by 18 SCR device is (a) 4 (b) 2 (c) 1 (d) 1/2 Answer: A [13] The number of SCRs required for single phase to single phase cycloconverter of the mid point type and for three phase to three phase type cycloconverter are respectively (a) 4, 6 (b) 8, 18 (c) 4, 18 (d) 4, 36 Answer: C [14] For dynamic equalizing circuit used for series connected SCRs, the choice of C is based on (a) Reverse recovery characteristics (b) Turn-on characteristics (c) Turn-off characteristics (d) Rise-time characteristics OPTIONS: 1) A 2) B, C 3) D 4) A, B, C, D 5) None of the above options Answer: 1 [15] The function of connecting zener diode in an UJT circuit, used for the triggering of SCRs, is to (a) Expedite the generation of triggering pulses (b) Delay the generation of triggering pulses (c) Provide a constant voltage to UJT to prevent erratic firing (d) Provide a variable voltage to UJT as the source voltage changes. OPTIONS: 1) B, C 2) C 3) A 4) D 5) None of the above options Answer: 2 Solution Hint: [4] When the conduction angle is 0, it will not suppress any part of the input, whereas during 180 degree conduction angle, it will suppress entire input waveform. [5] PMAX >= Pavg/(f.TON) Duty cycle D =TON/T = f.TON PMAX >= Pavg/D =100/0.4 =250W [7] When a SCR is in the forward blocking state, outer junctions (j1 & j3) are forward biased and middle junction(j2) is reverse biased [15] Remember that the use of zener diode in a circuit is to provide voltage regulation ( constant voltage)

Power Electronics Part- 10 Objective Questions with Answers Posted: 11 Feb 2014 03:22 AM PST Power Electronics Objective Questions with Answers: Part-10 1. SCR is the solid state equivalent of (a) Transistor (b) Thyratron (c) Vacuum diode (d) Crystal diode Answer: B 2. With gate open, if the supply voltage exceeds the breakover voltage of SCR, then SCR will conduct (a) False (b) True (c) For DC (d) For AC Answer: B 3. The SCR is turned off when the anode current falls below (a) Forward current rating (b) Break over voltage (c) Holding current (d) Latching current Answer: C 4. In a SCR circuit, the angle of conduction can be changed by changing (a) Anode voltage (b) Anode current (c) Forward current rating (d) Gate current Answer: D 5. The normal way to close a SCR is by appropriate (a) Gate current (b) Cathode current (c) Anode current (d) Forward current Answer: A 6. An SCR has ......PN junctions (a) Two (b) Four (c) Three (d) One Answer: C 7. If gate current is increased, the anode-cathode voltage at which SCR closes is..... (a) Increased (b) Decreased (c) Maximum (d) Least Answer: B 8. A conducting SCR can be opened by reducing ..... to zero (a) Supply voltage (b) Grid voltage (c) Grid current (d) Anode current Answer: A 9. With gate open, a SCR can be turned-on by making supply voltage.... (a) Minimum (b).Reverse (c) Equal to cathode voltage (d) Equal to breakover voltage Answer: D 10. If firing angle in a SCR rectifier is increased, output is.... (a) Increased (b) Maximum (c) Decreased (d) Unaffected Answer: C 11. A SCR is a ...switch (a) Two directional (b) Unidirectional (c) Three directional (d) Four directional Answer: B 12. The anode of SCR is always maintained at........potential with respect to cathode (a) Positive (b) Zero (c) negative (d) Equipotential Answer: A 13. IF the chopper switching frequency is 200Hz and Ton time is 2ms, the duty cycle is (a) 0.4 (b) 0.8 (c) 0.6 (d) None of the above Answer: A 14. Speed control of induction motor can be effected by varying (a) Flux (b) Voltage input to stator (c) Keeping rotor coil open (d) None of the above Answer: B 15. In a Dc motor if the field coil gets opened, the speed of the motor will (a) Decrease (b) Come to stop (c) Increase (d) None of the above Answer: C Solution Hint: [11] SCR will conduct in one direction only. So it act as a unidirectional switch. [13] T=1/f, D = TON/T