How is a steady DC supply produced from the AC mains, and how is it regulated and made safe?
Power supplies and mains: the transformer, rectification (half-wave and full-wave bridge), smoothing with a reservoir capacitor, voltage regulation, and electrical safety including fuses and earthing.
A focused answer to WJEC A-Level Electronics power supplies and mains, covering the transformer, half-wave and full-wave bridge rectification, smoothing with a reservoir capacitor, voltage regulation with a Zener or regulator, and electrical safety with fuses and earthing.
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What this dot point is asking
Almost every electronic system needs a steady DC supply, usually derived from the AC mains, and Component 2 expects you to understand how and to do so safely. WJEC expects you to describe the transformer, half-wave and full-wave bridge rectification, smoothing with a reservoir capacitor, voltage regulation, and electrical safety including fuses and earthing. The four-stage power-supply description and the half-wave versus full-wave comparison are reliable, high-mark exam content.
The answer
The four stages of a power supply
Rectification
Full-wave is preferred because it makes better use of the input and is easier to smooth.
Smoothing
Regulation and safety
Examples in context
- Example 1. A bench DC supply
- A laboratory supply uses a transformer, a bridge rectifier, a large reservoir capacitor and an IC voltage regulator to give a steady, adjustable DC from the mains. The regulator holds the set voltage even as you change the load, which is why the reading does not sag when you connect a circuit.
- Example 2. A phone charger
- A small charger steps the mains down, rectifies and smooths it, and regulates to 5 V. Modern chargers add switching electronics for efficiency, but the same four functions (transform, rectify, smooth, regulate) are present, ending in a stable 5 V output.
- Example 3. Why mains equipment is earthed and fused
- A metal-cased appliance has its case earthed so that if a live wire touches the case, a large fault current flows to earth and blows the fuse, disconnecting the supply before anyone can get a shock. The fuse and earth work together as the core mains safety system.
Try this
Q1. State the function of the reservoir (smoothing) capacitor in a DC power supply. [2 marks]
- Cue. It charges to the peak on each rectified pulse and discharges through the load between pulses, filling the gaps to reduce the ripple and give steadier DC.
Q2. Explain why a full-wave rectifier output is easier to smooth than a half-wave output. [2 marks]
- Cue. Full-wave uses both half-cycles, so pulses come twice as often with no gaps; the reservoir capacitor discharges less between them, giving smaller ripple for the same capacitor.
Exam-style practice questions
Practice questions written in the style of WJEC exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
WJEC Eduqas 20206 marksDescribe the stages of a DC power supply that converts the AC mains into a smooth, regulated DC output, and explain the function of each stage.Show worked answer →
A DC power supply has four stages.
Transformer: steps the high AC mains voltage down to a lower AC voltage and isolates the circuit from the mains.
Rectifier: converts the AC to DC. A full-wave bridge of four diodes uses both halves of the AC cycle, giving a pulsating DC output.
Smoothing: a large reservoir capacitor across the output charges on each peak and discharges between peaks, filling in the gaps to give a much steadier DC with only a small ripple.
Regulation: a Zener diode or voltage regulator holds the output at a fixed voltage despite changes in load current or input voltage, giving a stable, well-defined DC.
Markers reward all four stages (transformer, rectifier, smoothing capacitor, regulator) each with its correct function.
WJEC Eduqas 20184 marksExplain the difference between half-wave and full-wave rectification, and state why a full-wave rectifier is easier to smooth.Show worked answer →
Half-wave rectification uses a single diode that conducts on only one half of each AC cycle, so the output is a series of pulses with gaps where the other half-cycle is blocked.
Full-wave rectification (a bridge of four diodes) uses both halves of each cycle, inverting the negative half so the output is a series of pulses with no gaps, at twice the frequency.
A full-wave output is easier to smooth because the pulses come twice as often and there are no long gaps, so the reservoir capacitor discharges less between peaks, giving smaller ripple for the same capacitor.
Markers reward one half-cycle used (half-wave) versus both (full-wave), and the smaller ripple of the full-wave output making it easier to smooth.
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Sources & how we know this
- WJEC Eduqas GCE A-level Electronics specification — WJEC Eduqas (2017)