CET Power Supply Design & Repair 3

Question 1

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

Accepted Answer

Suppress voltage spikes caused by transformer leakage inductance

Answer

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

Question 2

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

Accepted Answer

ESR meter or LCR meter with ESR function

Answer

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

Question 3

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

Accepted Answer

Dropout voltage

Answer

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.

Question 4

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

Accepted Answer

Load regulation

Answer

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

Question 5

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

Accepted Answer

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

Answer

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.

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CET Power Supply Design & Repair 3