External control of dc input voltage can be obtained by the use of a______.

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Chopper

Explanation: A DC-to-DC converter, commonly known as a chopper, is a static power electronics device used to step up or step down a fixed DC input voltage to a variable DC output voltage. It achieves this by rapidly switching a semiconductor device (like a MOSFET or IGBT) on and off, effectively controlling the average output voltage.

Working Principle: The chopper operates on the principle of pulse-width modulation (PWM). By varying the duty cycle (k), which is the ratio of the "on" time to the total switching period, the average output voltage ($V_{out}$) can be regulated independently of the input source ($V_{in}$). The relationship is defined as $V_{out} = k 	imes V_{in}$, where $k = \frac{T_{on}}{T}$.

Key Formulas: $V_{out} = k \cdot V_{in}$ — Average output voltage relationship; $k = \frac{T_{on}}{T_{on} + T_{off}}$ — Duty cycle definition

Key Points: Choppers perform DC-to-DC conversion, acting as the DC equivalent of an AC transformer.; They are widely used in variable speed drives and power supply units.; Efficiency is very high due to semiconductor switching instead of resistive control.

Advantages: High efficiency; Compact size and low weight; Fast response time

Disadvantages: Generates electromagnetic interference (EMI); Requires complex control circuitry for high-frequency operation

Applications: Electric traction systems; Battery-operated electric vehicles; Regenerative braking systems

Additional Info: A chopper is a high-speed semiconductor switch that turns on/off to modulate the average voltage.. Transformers are AC devices based on electromagnetic induction and cannot work on steady DC.

Answer: B is correct — A chopper is a static power electronics circuit used to convert fixed DC input into variable DC output voltage.

Exam Tip: Always remember: Transformers operate on the principle of Faraday's Law of Induction, which requires changing magnetic flux, making them unsuitable for steady DC without a conversion stage.

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