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Power Supply Design & Repair Flashcards

7 cards from real CET practice questions. Tap to flip, then mark Knew It or Still Learning — missed cards come back until you master them.

Read the first 7 Power Supply Design & Repair flashcards as text

  1. What is the purpose of a snubber circuit across a switching transistor in an SMPS?

    Answer: Suppress voltage spikes caused by transformer leakage inductance

    Snubber circuits (RC or RCD networks) clamp the voltage transient that occurs when the switching transistor turns off and the transformer leakage inductance rings.

  2. Which measurement tool is best suited for evaluating the ESR of electrolytic capacitors in a power supply?

    Answer: ESR meter or LCR meter with ESR function

    An ESR meter measures equivalent series resistance of capacitors in-circuit or out, which a standard DMM capacitance function cannot accurately determine.

  3. A 78xx series three-terminal regulator requires a minimum input-to-output voltage differential to regulate properly. This is called the:

    Answer: Dropout voltage

    Dropout voltage is the minimum voltage difference between input and output required for a linear regulator to maintain regulation; for 78xx regulators it is typically 2–3 V.

  4. When testing a power supply under load and the output voltage drops significantly, which parameter is being evaluated?

    Answer: Load regulation

    Load regulation measures how much the output voltage changes as load current varies from minimum to maximum.

  5. What is the primary safety hazard when working on the primary side of a plugged-in switching power supply?

    Answer: High-voltage AC/DC present even when power switch is off

    Large filter capacitors on the primary side retain dangerous voltages (300–400 V DC) even after the unit is unplugged and the power switch is off.

  6. In a flyback converter, energy is stored in the transformer core during which switch state?

    Answer: When the switch is ON

    In a flyback converter, energy is stored in the transformer's gapped core while the primary switch is ON, then transferred to the secondary when the switch turns OFF.

  7. A power supply produces a loud high-pitched whine under load. What is the most likely cause?

    Answer: Loose or saturating transformer/inductor core

    Magnetostriction in transformer or inductor cores, especially when a core is saturating or mounting is loose, causes audible noise at the switching frequency.