All Free Electrical Engineering MCQs with Answers

• A. Is inexpensive
• B. Is mechanically strong
• C. Has small leakage current
• D. Is tetravalent

Explanation: Silicon is preferred for SCRs because its reverse leakage current is much smaller than that of germanium, improving blocking performance.

• A. Equal to
• B. Less than
• C. Greater than
• D. None of the above

Explanation: The supply is normally kept below the SCR's breakover voltage so it does not turn on unintentionally.

• A. Breakover voltage
• B. Appropriate anode current
• C. Appropriate gate current
• D. None of the above

Explanation: An SCR is normally triggered by applying an appropriate positive gate current with respect to the cathode.

• A. A rectifier and resistance
• B. A rectifier and transistor
• C. A rectifier and capacitor
• D. None of the above

Explanation: An SCR combines the one-way current control of a rectifier with the switching and control action associated with a transistor.

• A. Reducing anode voltage to zero
• B. Reducing gate voltage to zero
• C. Reverse biasing the gate
• D. None of the above

Explanation: Reducing the anode voltage to zero removes forward bias and turns the SCR off.

• A. Cathode
• B. Anode
• C. Anode supply
• D. Gate

Explanation: The gate is the control terminal of an SCR and receives the triggering pulse.

• A. At zero potential
• B. Negative
• C. Positive
• D. None of the above

Explanation: For forward operation, the anode must be positive with respect to the cathode, establishing forward bias across the SCR.

• A. Unidirectional
• B. Bidirectional
• C. Mechanical
• D. None of the above

Explanation: An SCR permits controlled current flow mainly in one direction, from anode to cathode, so it behaves as a unidirectional switch.

• A. Positive
• B. Negative
• C. At zero potential
• D. None of the above

Explanation: To trigger a conventional SCR, the gate is made positive with respect to the cathode by applying gate current.

• A. Triac
• B. Diac
• C. Unijunction transistor
• D. Thyristor

Explanation: SCR stands for silicon-controlled rectifier and is a member of the thyristor family.

• A. Triode
• B. Pentode
• C. Gas-filled triode
• D. Tetrode

Explanation: An SCR is the solid-state counterpart of a gas-filled triode, commonly called a thyratron.

• A. Cathode, anode, gate
• B. Anode, cathode, grid
• C. Anode, cathode, drain
• D. None of the above

Explanation: An SCR is a three-terminal thyristor with anode and cathode for the main current path, plus a gate for triggering.

• A. Germanium
• B. Silicon
• C. Carbon
• D. None of the above

Explanation: SCRs are fabricated from silicon because it withstands high blocking voltage, temperature, and power levels.

• A. Two
• B. Three
• C. Four
• D. None of the above

Explanation: An SCR has four alternate semiconductor layers: P-N-P-N. These layers form the three internal pn junctions of the device.

• A. Two
• B. Three
• C. Four
• D. None of the above

Explanation: The four-layer P-N-P-N structure contains three interfaces between adjacent layers, so an SCR has three pn junctions.

• A. Anode current is reduced to zero
• B. Gate voltage is reduced to zero
• C. Gate is reverse biased
• D. None of the above

Explanation: An SCR turns off when its anode current falls below the holding current, which is commonly approximated as reducing the current to zero.

• A. Is decreased
• B. Is increased
• C. Remains the same
• D. None of the above

Explanation: Increasing gate current assists regenerative turn-on, so the required anode-cathode breakover voltage decreases.

• A. Changing anode voltage
• B. Changing gate voltage
• C. Reverse biasing the gate
• D. None of the above

Explanation: The firing instant, and therefore the conduction angle, is controlled by the gate trigger signal, commonly adjusted through gate voltage…

• A. Exactly zero
• B. Small leakage current
• C. Large leakage current
• D. None of the above

Explanation: An OFF SCR blocks the applied voltage but permits a small leakage current through its reverse- or forward-blocking junctions.

• A. Two SCRs in series
• B. Two SCRs in parallel
• C. Two SCRs in parallel opposition
• D. None of the above

Explanation: Two SCRs connected in anti-parallel, or parallel opposition, conduct on alternate half-cycles and allow bidirectional AC power control.