Full-wave rectification
Activity 11.2
You will need: a bicycle dynamo or laboratory AC generator, four 1N4001 diodes, a centre-zero galvanometer, a 100 Ω rheostat, solder, a soldering iron and wires
- Solder the four diodes into a bridge, with anodes and cathodes the right way round.
- Connect the rheostat and the centre-zero galvanometer to the bridge as shown.
- Connect the dynamo to X and Y and turn it slowly. Watch the galvanometer, and use the rheostat if the deflection is too large.
The observation
The galvanometer deflects one way only: the alternating current has become a direct current.
With four diodes arranged as a bridge, both halves of the alternating current flow the same way through the load. With a battery and an LED (figure 11.12): when X is positive, D1 and D3 are forward biased and D2 and D4 reverse biased; turn the battery round and D2 and D4 conduct instead — and the LED lights both times, with current through it from P to Q.
Figure to be added
| t (cycles) | 0.25 | 0.75 | 1.25 | 1.75 |
|---|---|---|---|---|
| V | 1 | 1 | 1 | 1 |
Full-wave rectification
Both halves of the alternating current are made to flow the same way through the load: full-wave rectification.
Exercise 11.2
Explain what the galvanometer showed in activity 11.2, and sketch how its current varies with time.
The answer
In each half-cycle a different pair of diodes conducts, but both pairs send current through the galvanometer the same way. So it deflects one way only, rising and falling twice in every cycle — the full-wave graph above.