21.2 Rectification and Smoothing

2026 Syllabus Objectives

By the end of these notes, you should be able to:

  1. Distinguish graphically between half-wave and full-wave rectification
  2. Explain the use of a single diode for half-wave rectification of an alternating current
  3. Explain the use of four diodes (bridge rectifier) for full-wave rectification of an alternating current
  4. Analyse the effect of a single capacitor in smoothing, including the effect of the values of capacitance and load resistance

1. Background: What is Rectification?

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Many electronic devices — phones, laptops, TVs — need a direct current (d.c.) supply to work. This means the current must always flow in the same direction.

However, the mains electricity supply provides alternating current (a.c.), where the current constantly switches direction, producing a sine wave pattern.

Rectification is the process of converting an a.c. supply into a d.c. supply — turning the back-and-forth wave into something that always flows one way.

To do this, we use diodes.

2. The Diode — The Key Component

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A diode is an electronic component that allows current to flow in one direction only.

  • It has two terminals: the anode (positive end) and the cathode (negative end).
  • When the anode is more positive than the cathode, the diode is forward-biased — it acts like a wire with zero resistance, so current flows freely.
  • When the cathode is more positive than the anode, the diode is reverse-biased — it acts like a broken wire with infinite resistance, so no current flows at all.

Think of a diode like a one-way gate: it opens for current going one way, and slams shut for current going the other way.

3. Half-Wave Rectification

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What it is

Half-wave rectification converts a.c. to d.c. using a single diode. It only allows one half of the a.c. cycle to reach the output.

The Circuit

The circuit consists of:

  • An a.c. supply
  • A single diode connected in series
  • A load resistor (the device being powered), connected in parallel with the output

How it works — step by step

Positive half-cycle:

  • The top of the a.c. supply becomes positive.
  • The diode is forward-biased (current can flow).
  • Current flows through the diode and through the load resistor.
  • There is a voltage across the resistor — the output is "on".

Negative half-cycle:

  • The top of the a.c. supply becomes negative.
  • The diode is now reverse-biased (current cannot flow).
  • No current flows through the circuit at all.
  • The output voltage is zero — the output is "off".

The output graph

The input is a full sine wave (positive humps and negative humps alternating).

The output of half-wave rectification looks like this:

  • Positive humps appear exactly as in the input.
  • Where the negative humps would be, the graph is flat at zero.
  • The pattern repeats: hump, flat, hump, flat…

This is called a pulsating d.c. — it always stays at zero or above (never negative), but it is not steady.

⚠️ Disadvantage: During every negative half-cycle, the output is zero. This means roughly 50% of the available power is wasted. The supply is being "thrown away" for half the time.

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