PlacementPrepAn SCR (thyristor) turns on when:
Solution
Once on, it stays on until the current drops below the holding current.
Answer: (B) It is forward biased and a gate pulse is applied
Technical · Electrical and Electronics (EEE)
Power Electronics MCQs for placement tests
20 technical questions of moderate difficulty, as asked in campus placement and company tests. Each has a short explanation.
An SCR turns off when its anode current falls below the:
Solution
Holding current is the minimum current to stay on.
Answer: (D) Holding current
Which device is commonly used in modern inverters and motor drives for medium power?
Solution
IGBTs combine easy gate drive with high current capacity.
Answer: (C) IGBT
A rectifier converts:
Solution
Rectifiers convert alternating current to direct current.
Answer: (A) AC to DC
An inverter converts:
Solution
Inverters produce AC from a DC source.
Answer: (A) DC to AC
A chopper converts:
Solution
Choppers are DC-DC converters.
Answer: (A) Fixed DC to variable DC
A cycloconverter converts:
Solution
It changes frequency directly without a DC link.
Answer: (B) AC at one frequency to AC at another frequency
A buck converter with input 24 V and duty cycle 0.5 gives an output of:
Solution
Vo = D × Vin = 0.5 × 24 = 12 V.
Answer: (C) 12 V
A boost converter with input 12 V and duty cycle 0.5 gives an ideal output of:
Solution
Vo = Vin/(1 − D) = 12/0.5 = 24 V.
Answer: (B) 24 V
The average output of a single-phase half-wave controlled rectifier with resistive load at firing angle α is:
Solution
Integrating the conducting part of the half cycle gives this result.
Answer: (D) (Vm/2π)(1 + cos α)
Freewheeling diodes are used in rectifiers with inductive loads to:
Solution
They prevent negative voltage across the load.
Answer: (A) Provide a path for load current and improve the output
PWM (pulse width modulation) in inverters is used to:
Solution
Varying pulse widths shapes the output.
Answer: (C) Control output voltage and reduce harmonics
A TRIAC can conduct:
Solution
A TRIAC is like two SCRs in anti-parallel.
Answer: (C) In both directions
A DIAC is commonly used to:
Solution
It gives a sharp trigger pulse in light dimmers and fan regulators.
Answer: (C) Trigger TRIACs
The power MOSFET is preferred at:
Solution
It switches fast with low gate power.
Answer: (A) High switching frequencies
Snubber circuits protect power devices against:
Solution
An RC snubber limits the rate of voltage rise.
Answer: (C) High dv/dt
The latching current of an SCR is:
Solution
Latching current is needed to switch on; holding current to stay on.
Answer: (B) Greater than its holding current
A UPS mainly uses:
Solution
The battery is charged by the rectifier and supplies the inverter.
Answer: (A) A rectifier, battery and inverter
Harmonics in power electronic converters mainly cause:
Solution
Harmonic currents increase losses and distort the supply.
Answer: (B) Extra heating and interference
A three-phase full-wave bridge rectifier uses how many diodes?
Solution
Two diodes per phase.
Answer: (A) 6
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