How is the a.c. mains turned into the smooth d.c. that electronic circuits need?
Power supplies: rectifying a.c. to d.c., smoothing with a reservoir capacitor, the idea of ripple, and stabilising the output voltage.
An Eduqas GCSE Electronics answer on power supplies: rectifying a.c. to d.c. with diodes, smoothing the output with a reservoir capacitor, the idea of ripple and how a larger capacitor reduces it, and stabilising the output voltage with a regulator or Zener diode.
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What this dot point is asking
Eduqas wants you to describe how the a.c. mains is turned into the steady d.c. that electronic circuits need: rectifying a.c. to d.c. with diodes, smoothing with a reservoir capacitor, the idea of ripple, and stabilising the output voltage. This is the power subsystem that every other circuit depends on.
The answer
Rectifying a.c. to d.c.
Smoothing with a reservoir capacitor
Ripple
Stabilising the output voltage
Examples in context
Almost every project in the course needs a power subsystem, so this topic underpins the non-exam assessment: a sensing-and-switching system, a logic circuit or a microcontroller all need a clean d.c. supply built by transforming, rectifying, smoothing and regulating the mains (or by regulating a battery). The smoothing capacitor reuses the charge-and-discharge idea of the time constant, and the rectifier diodes reuse the forward-and-reverse-bias behaviour of the diode topic, tying the power supply to the rest of the course.
Try this
Q1. State, in order, the four stages of a mains d.c. power supply. [2 marks]
- Cue. Transform (step down), rectify, smooth, regulate.
Q2. State how the smoothing capacitor is connected relative to the load. [1 mark]
- Cue. In parallel with the load.
Q3. State one change that reduces the ripple on a smoothed supply. [1 mark]
- Cue. A larger smoothing capacitor (or full-wave rectification, or a smaller load current).
Exam-style practice questions
Practice questions written in the style of WJEC Eduqas exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Eduqas 20195 marksDescribe the stages by which the a.c. mains is converted into a steady low-voltage d.c. supply for an electronic circuit, naming the component responsible for each stage.Show worked answer →
Stages (up to 5 marks): a transformer steps the mains a.c. down to a lower a.c. voltage. A rectifier (one diode for half-wave, or four in a bridge for full-wave) converts the a.c. to a pulsing d.c. that flows one way only. A reservoir (smoothing) capacitor in parallel with the load charges on each peak and discharges between peaks, filling the gaps so the output is much smoother (it has only a small ripple). A regulator (or Zener diode) then holds the output at a constant voltage despite changes in load or input.
Markers reward the four stages in order (transform, rectify, smooth, regulate) with the correct component named for each.
Eduqas 20224 marksExplain what is meant by ripple on a smoothed d.c. supply, and state two changes that would reduce it.Show worked answer →
Ripple (up to 2 marks): ripple is the small remaining variation (rise and fall) in the d.c. output voltage; it occurs because the reservoir capacitor discharges a little between the peaks of the rectified waveform before being topped up again.
Reducing it (up to 2 marks): use a larger smoothing capacitor (stores more charge, so it discharges less between peaks); use full-wave rather than half-wave rectification (the capacitor is topped up twice as often); or draw less current from the supply (the capacitor discharges more slowly). Any two.
Markers reward the definition (residual variation from capacitor discharge between peaks) and two valid ways to reduce it.
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Sources & how we know this
- WJEC Eduqas GCSE (9-1) Electronics specification (C490) — WJEC Eduqas (2017)