the full-wave rectifier
The half-wave rectifier throws away half the AC. The full-wave rectifier keeps both halves by flipping the negative one right-side up, so the output pulses twice as often and never rests as long at zero. The classic version uses a centre-tapped transformer and two diodes.
A centre-tapped transformer gives two equal AC voltages that are mirror images — when the top end is positive, the bottom end is negative, both measured from the centre tap (which becomes the output's ground). Two diodes, one from each end, each conduct only on their positive half and feed the load through a common point; the centre tap returns the current. So on every half-cycle one diode is delivering an upward hump. The output is a continuous run of humps at twice the line frequency.
Compared with a bridge, the centre-tap version has only ONE diode drop in the path (not two), so it wastes a little less voltage — handy at low voltages. The price is a more expensive, centre-tapped transformer that uses only half its winding at a time. Both approaches need a smoothing capacitor to turn the humps into near-DC.
From a centre-tapped 12 V-0-12 V transformer (each half ~17 V peak), each diode delivers humps peaking at ~16.3 V at twice the mains frequency (100 Hz from a 50 Hz line), with no negative gaps.
Centre-tap full-wave: two diodes, one drop, humps at twice the line frequency.
The centre tap, not the transformer's outer ends, is your DC reference here. Wiring it like a bridge by mistake either shorts a half-winding or rectifies only half the wave.