Rectifiers: AC → DC
Your gadgets run on DC but the wall supplies AC. Diodes to the rescue — this lesson is the inside of every charger you own.
Builds on: 3.1 Diodes & LEDs2.3 Capacitors & the RC Time Constant5.1 Alternating Current
The one-way valve meets the wiggle
You already own the key component: the diode (Lesson 3.1) passes current one way only. Feed it AC and it simply deletes the negative half of every cycle. That’s a half-wave rectifier: one diode, output that pulses in one direction — but half the energy is thrown away and the gaps are huge.
The bridge: use both halves
Four diodes in a diamond — the bridge rectifier — steer both halves of the cycle the same way through the load. When the AC swings positive, one diagonal pair conducts; when negative, the other pair. The output is the absolute value of the input (minus two diode drops ≈ 1.4 V), pulsing at twice the mains frequency.
Smoothing: the reservoir
Pulsing DC isn’t good enough for electronics — so add a big capacitor across the output (Lesson 2.3, now at industrial scale). It charges to each peak, then supplies the load while the input dips, sagging only slightly until the next peak refills it. The leftover wobble is called ripple, and the fight against it is a pure RC story: bigger C or lighter load (bigger R) → bigger τ = RC relative to the 10 ms between peaks → smaller ripple.
The smoothing capacitors in real supplies are large and can hold a charge long after unplugging (Lesson 2.3’s warning, now with teeth). Never poke inside a mains power supply — build your low-voltage skills first; the concepts transfer exactly.
⚡ Lab — Charger Anatomy
A 9 V-peak, 50 Hz source through your choice of rectifier.
- Compare half-wave and bridge: count the humps per 3 cycles.
- Add the capacitor and watch the sawtooth ripple appear. Grow C — ripple shrinks.
- Now lower the load R (heavier load). Why does ripple grow? Think τ = RC.