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First exams 2025

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Smoothing (CIE A Level Physics)

Revision Note

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Katie M

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Katie M

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Smoothing

  • In rectification, to produce a steady direct current  or voltage from an alternating current  or voltage, a smoothing capacitor is necessary
  • Smoothing is defined as:

The reduction in the variation of the output voltage or current

  • This works in the following ways:
    • A single capacitor with capacitance C is connected in parallel with a load resistor of resistance R
    • The capacitor charges up from the input voltage and maintains the voltage at a high level
    • It discharges gradually through the resistor when the rectified voltage drops but the voltage then rises again and the capacitor charges up again

Smoothing capacitor circuit diagram

Smoothing circuit and graph, downloadable AS & A Level Physics revision notes

A smoothing capacitor connected in parallel with the load resistor. The capacitor charges as the output voltage increases and discharges as it decreases

  • The resulting graph of a smoothed output voltage Vout or output current, Iout against time is a ‘ripple’ shape

A smoothed out current-time graph

Smoothing current graph, downloadable AS & A Level Physics revision notes

A smooth, rectified current graph creates a ‘rippling’ shape against time

  • The amount of smoothing is controlled by the capacitance C of the capacitor and the resistance R of the load resistor
    • The less the rippling effect, the smoother the rectified current and voltage output

  • The slower the capacitor discharges, the more the smoothing that occurs ie. smaller ripples
  • This can be achieved by using:
    • A capacitor with greater capacitance C
    • A resistance with larger resistor R

  • Recall that the product RC is the time constant τ of a resistor
  • This means that the time constant of the capacitor must be greater than the time interval between the adjacent peaks of the output signal

Worked example

The graph below shows the output voltage from a half-wave rectifier. The load resistor has a resistance of 2.6 kΩ. A student wishes to smooth the output voltage by placing a capacitor in parallel across the load resistor.

WE Smoothing question image, downloadable AS & A Level Physics revision notes

Suggest if a capacitor of 60 pF or 800 µF would be suitable for this task.

Answer:

Step 1: Calculate the time constant with the 60 pF capacitor

  • Resistance, R = 2.6 kΩ = 2.6 × 103 Ω

Time space constant space equals space tau italic space italic equals italic space R C

tau space equals space open parentheses 2.6 space cross times space 10 cubed close parentheses space cross times space open parentheses 60 space cross times space 10 to the power of negative 12 end exponent close parentheses space equals space 1.56 space cross times space 10 to the power of negative 7 end exponent space equals space 156 space ns

Step 2: Compare the time constant of 60 pF capacitor with the interval between adjacent peaks of the output signal

  • The time interval between adjacent peaks is 80 ms
  • The time constant of 156 ns is too small and the 60 pF capacitor will discharge far too quickly
  • There would be no smoothing of the output voltages
    • Therefore, the 60 pF capacitor is not suitable

Step 3: Calculate the time constant with the 800 µF capacitor

tau space equals space open parentheses 2.6 space cross times space 10 cubed close parentheses space cross times space open parentheses 800 space cross times space 10 to the power of negative 6 end exponent close parentheses space equals space 2.08 space straight s

Step 4: Compare the time constant of 800 µF capacitor with the interval between adjacent peaks of the output signal

  • The time constant of 2.08 s is much larger than 80 ms
  • The capacitor will not discharge completely between the positive cycles of the half-wave rectified signal
  • Therefore, the 800 µF capacitor would be suitable for the smoothing task

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Katie M

Author: Katie M

Expertise: Physics

Katie has always been passionate about the sciences, and completed a degree in Astrophysics at Sheffield University. She decided that she wanted to inspire other young people, so moved to Bristol to complete a PGCE in Secondary Science. She particularly loves creating fun and absorbing materials to help students achieve their exam potential.