Rectification and smoothing Cambridge International AS & A Level Physics revision
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In plain words
Phones, laptops and almost everything electronic need current that flows one way, but the mains flows both ways. Turning alternating current into direct current is called rectification, and the tool for the job is the diode: a one-way valve for current.
4 things to know
- Half-wave rectification: a single diode in series with the load lets current through during only one half of each cycle. The output is a row of humps with gaps between them.
- Full-wave rectification: four diodes in a bridge. In each half-cycle a different pair of diodes conducts, steering the current through the load in the same direction each time. The output has a hump for every half-cycle, with no gaps.
- Smoothing: a capacitor is connected in parallel with the load. It charges up to the peak and then discharges slowly through the load between peaks, filling in the dips. The output is nearly steady, with a small ripple.
- The ripple is smaller with a larger capacitance or a larger load resistance, because the capacitor then discharges more slowly.
Tips and tricks
- Full-wave rectification gives twice as many humps each second as the supply frequency: 100 a second from a 50 Hz supply.
- In a bridge rectifier, two of the four diodes conduct at any one time.
It lands in your notebook with its questions as flashcards.
Rectification and smoothing: 5 questions and answers
These are the quiz’s questions. Do the quiz first, then come back here for the ones that got you.
How many diodes are there in a bridge rectifier?
Two conduct in each half-cycle.
What does half-wave rectification need?
It simply blocks one half of each cycle.
How is a smoothing capacitor connected?
It can then keep the p.d. across the load up between peaks.
The capacitance of a smoothing capacitor is increased. What happens to the ripple?
A larger capacitor discharges more slowly.
A full-wave rectifier is run from a 50 Hz supply. How many pulses a second does it give?
One for every half-cycle.
Quiz
5 questions
Tap an answer and you’ll see straight away whether it’s right, and why.
Worksheet
4 questions, 8 marks. Write your answers on paper, then check them.
Rectification and smoothing
Cambridge International AS & A Level Physics 9702 · 8 marks · papermunch.org
Name ______________________________ Date ______________
Describe the output of a half-wave rectifier.[2]
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The current flows in one direction only, during alternate half-cycles. The output is a series of humps, each half a cycle long, separated by gaps of the same length.
Explain how a capacitor smooths the output of a rectifier.[3]
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The capacitor is connected in parallel with the load. As the rectified p.d. rises, the capacitor charges to the peak value. As the p.d. falls, the capacitor discharges through the load, keeping current flowing and the p.d. high until the next peak recharges it.
State and explain what happens to the ripple if the smoothing capacitor is swapped for one of larger capacitance.[2]
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The ripple gets smaller. The time constant RC is larger, so the capacitor loses less of its charge between peaks.
A 50 Hz supply is full-wave rectified. State the number of humps in the output each second.[1]
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100.
Answers: Rectification and smoothing
- 1. The current flows in one direction only, during alternate half-cycles. The output is a series of humps, each half a cycle long, separated by gaps of the same length.
- 2. The capacitor is connected in parallel with the load. As the rectified p.d. rises, the capacitor charges to the peak value. As the p.d. falls, the capacitor discharges through the load, keeping current flowing and the p.d. high until the next peak recharges it.
- 3. The ripple gets smaller. The time constant RC is larger, so the capacitor loses less of its charge between peaks.
- 4. 100.