Rectification and smoothing
Inside a phone charger
- Your phone needs steady DC, but the wall gives alternating AC.
- A charger fixes this in two steps: rectify, then smooth.
- Both steps use simple components.
Diodes rectify
- A diode conducts in one direction only.
- Rectification uses diodes to turn alternating voltage into a one-way (DC-like) voltage.
A diode conducts current:
This one-way behaviour is what lets diodes rectify a.c. into d.c.
Half-wave vs full-wave
- Half-wave (one diode): keeps only the positive halves, with flat gaps. Wasteful.
- Full-wave (a four-diode bridge): uses every half-cycle → continuous humps at twice the frequency.
Half-wave rectification uses ____ diode.
A single diode passes the positive half-cycles and blocks the negative ones (leaving gaps).
How many diodes are in a bridge (full-wave) rectifier?
Four diodes, arranged so the load current always flows the same way whichever input terminal is positive.
A full-wave rectified output repeats at twice the frequency of the a.c. input.
Both half-cycles become positive humps, so the output ripples at double the input frequency.
Smoothing
- Add a capacitor in parallel with the load.
- It charges to the peak, then discharges through the load between peaks, holding the voltage near the top — leaving only a small ripple.
To smooth a rectified output, you connect a capacitor:
In parallel, it charges to the peak and then discharges through the load between peaks, holding the voltage up.
Reducing the ripple
- A larger capacitor, a larger load resistor, or a higher frequency all reduce the ripple.
- In short, a large $RC$ compared with the time between peaks gives a smoother output.
Select all the changes that reduce the ripple.
A bigger $RC$ (or less time between peaks) makes the capacitor discharge less between peaks — smaller ripple. A smaller capacitor does the opposite.
You've got it
- rectification uses diodes; full-wave (bridge, 4 diodes) uses every half-cycle
- smoothing: a capacitor across the load holds the voltage near the peak
- larger $C$, larger $R$, or higher frequency → less ripple