Question

Which component is used to reduce ripples in a DC power supply?

• A. Resistor
• B. Filter
• C. Transformer
• D. Diode

Hint:

To remember the sequence of components in a standard linear DC power supply, use the block diagram order: \[ \text{Transformer} \rightarrow \text{Rectifier (Diodes)} \rightarrow \text{Filter (Capacitor/Inductor)} \rightarrow \text{Regulator} \] Filters remove the AC ripples, while Diodes convert AC to pulsating DC.

Answer 1

The Correct Option is B

Concept: In a regulated DC power supply system, alternating current (AC) from the mains is converted into stable direct current (DC). This conversion process involves several sequential stages:
• Step-down Transformer: Reduces the high-voltage AC mains to a lower, manageable AC voltage level.
• Rectifier (Diodes): Converts the bidirectional AC voltage into a unidirectional pulsating DC voltage. This pulsating DC contains both a desired DC component and unwanted AC variations known as ripples.
• Filter Circuit: Smooths out the pulsations by bypassing or blocking the AC ripple components, allowing only a steady DC voltage to pass to the load.
• Voltage Regulator: Maintains a constant output voltage regardless of variations in input voltage or load current. The primary measure of the effectiveness of a filter is the ripple factor ($\gamma$), which is defined as the ratio of the root-mean-square (RMS) value of the AC component of the output voltage to the DC component of the output voltage: \[ \gamma = \frac{V_{ac(rms)}}{V_{dc}} \] An ideal filter circuit reduces $\gamma$ as close to zero as possible.

Step 1: Understanding the nature of Rectifier Output
When an AC signal passes through a rectifier (whether half-wave or full-wave), the resulting output waveform is unidirectional but not smooth. It fluctuates between zero and peak values at a frequency equal to or double the line frequency. This fluctuating behavior means the waveform consists of a pure DC value superimposed with a series of high-frequency AC harmonics (ripples).

Step 2: Role and Working Mechanism of a Filter Circuit
To eliminate these unwanted AC ripples, a filter circuit containing reactive components like capacitors ($C$) and/or inductors ($L$) is connected after the rectifier:
• Capacitor Filter: Connected in parallel with the load. A capacitor offers low reactance to high-frequency AC signals ($X_C = \frac{1}{2\pi f C}$) and infinite reactance to DC ($f = 0$). Therefore, it shunts the AC ripple components to the ground while forcing the DC component to flow through the load.
• Inductor Filter: Connected in series with the load. An inductor offers high reactance to AC ($X_L = 2\pi f L$) and zero resistance to DC. Thus, it blocks the AC ripples from reaching the load. Consequently, the component designed specifically to suppress and minimize these ripples is the Filter.

Step 3: Analysis of Incorrect Options
To ensure absolute clarity, let us examine why the other choices are incorrect for this specific function:
• Resistor: A resistor simply opposes the flow of current uniformly regardless of frequency ($R$ is independent of frequency). It cannot differentiate between AC ripples and DC components, and using it alone causes significant power loss without filtering ripples.
• Transformer: A transformer works purely on the principle of mutual electromagnetic induction to step up or step down AC voltage levels. It cannot rectify AC to DC or filter out ripples.
• Diode: A diode acts as a unidirectional electronic switch that permits current to flow in only one direction. It is the core component used to construct a rectifier to change AC into pulsating DC, but it does not remove the resulting ripples.