21.2 Rectification and smoothing
- Syllabus
- 9702–2028–2029
- Topic
- 21.2
- Level
- A2
Learning objectives
21.2.1Distinguish graphically between half-wave and full-wave rectification• distinguish graphically between half-wave and full-wave rectification21.2.2The use of a single diode for the half-wave rectification of an alternating• explain the use of a single diode for the half-wave rectification of an alternating current21.2.3The use of four diodes (bridge rectifier) for the full-wave rectification of• explain the use of four diodes (bridge rectifier) for the full-wave rectification of an alternating current21.2.4The effect of a single capacitor in smoothing• analyse the effect of a single capacitor in smoothing, including the effect of the values of capacitance and the load resistance
Half-wave and full-wave rectification differ in which half-cycles reach the load
Half-wave rectification passes one polarity half-cycle; full-wave rectification flips both half-cycles to the same load polarity.
Read the output graph: half-wave has gaps each alternate half-cycle, while full-wave pulses occur twice per input cycle and have higher ripple frequency.
A bridge rectifier uses four diodes to produce full-wave output without a centre-tapped transformer.
Rectification does not by itself make perfectly steady DC; smoothing capacitors reduce ripple after the diode stage.
A single diode performs half-wave rectification by passing one polarity half-cycle
A diode conducts mainly in forward bias and blocks reverse bias, so one diode passes only one half-cycle of an AC input.
Read diode orientation and output polarity, then identify the gaps in the load voltage waveform.
A positive half-wave output has pulses separated by zero-voltage intervals during the negative input half-cycle.
A diode does not convert AC directly into smooth DC; the output is pulsating and needs smoothing.
A four-diode bridge routes both AC half-cycles through the load in one direction
A bridge has four diodes in a diamond. Connect the AC source to one pair of opposite corners and the load to the other opposite pair. Both diodes meeting the positive load corner point toward it; both connected from the negative load corner point away from it.
| Input half-cycle | Conducting path | Load result |
|---|---|---|
| first AC terminal positive | one diagonal pair of diodes conducts | current crosses load from its fixed + side to fixed - side |
| second AC terminal positive | the other diagonal pair conducts | current crosses load in the same direction |
Both input halves become same-polarity output pulses, so the rectified output frequency is twice the input frequency. Each conducting route contains two forward-biased diodes.
To check a drawn bridge, trace conventional current from either AC corner through one diode, the load in the chosen direction, and a second diode back to the other AC corner. Repeat after swapping which AC corner is positive.
Only two opposite-path diodes conduct on each half-cycle, not all four. The bridge rectifies but does not itself remove ripple; a smoothing stage is separate.
A capacitor across the load smooths rectified voltage by charging and discharging
Connect the smoothing capacitor in parallel with the load resistor, with correct polarity for a polarised capacitor. Its voltage is therefore the output voltage.
| Part of rectified cycle | Capacitor action | Output shape |
|---|---|---|
| rising input exceeds capacitor voltage near a peak | diode conducts; capacitor charges rapidly toward peak | rapid rise/recharge |
| input falls below capacitor voltage | diode turns off; capacitor discharges through load | curved exponential fall |
| next peak arrives | diode conducts and recharges capacitor | ripple repeats above zero |
Betweenpeaks,V≈Vpeake(−t/(RloadC));dischargetimescaleτ=RloadC.
| Change | Effect on discharge/ripple |
|---|---|
| increase C | larger τ, slower fall, smaller ripple, higher minimum output |
| increase R_load (lighter load) | larger τ, smaller load current, smaller ripple, higher minimum output |
| use full-wave rather than half-wave at same input f | peaks arrive twice as often, less discharge time, smaller ripple |
Decreasing C or R_load produces faster decay, a lower minimum and larger peak-to-peak ripple. Very large C reduces ripple but causes large short charging-current pulses near peaks.
Smoothing does not create perfectly constant voltage and the capacitor must not be placed in series with the load. Increasing load resistance reduces ripple; increasing load current does the opposite.