Many devices need direct current, but the mains delivers alternating current. Diodes turn it into one-way current, and a capacitor smooths the result into something close to steady d.c.
A diode passes current one way only. A single diode gives half-wave rectification (only the positive halves pass); a bridge of four diodes gives full-wave rectification (the negative halves are flipped up). A smoothing capacitor across the load charges to the peak and discharges slowly between peaks, reducing the ripple.
Switch between half-wave and full-wave rectification and watch the output. Then switch on the smoothing capacitor and see the dips fill in, leaving only a small ripple on a nearly steady d.c. output.
Three ideas carry most of the marks.
The smoothing capacitor goes in parallel with the load, not in series. A larger capacitance (or load resistance) gives less ripple, because the time constant RC is longer. The ripple of a full-wave rectifier is at twice the supply frequency (two bumps per cycle), while a half-wave output ripples at the supply frequency.
Four quick checks on rectification and smoothing. Each correct answer earns XP and lights this skill on your star map.
A single diode in series with a resistor and an a.c. supply produces:
A bridge rectifier that gives full-wave rectification contains:
To reduce the ripple in a rectifier output, a smoothing capacitor is connected:
A full-wave rectifier is supplied from 50 Hz mains. The ripple in its output has a frequency of:
When asked to sketch, draw the rectified output touching zero between bumps without a capacitor, and a sawtooth-like ripple sitting near the peak with one. State the direction of the change clearly: more capacitance (or a higher load resistance) means a longer time constant and so a smaller ripple.
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