CBSE Class 12 · 2008 · SemiconductorsShort Answer

The ac supply is applied to the primary of a centre-tapped transformer; the two secondary ends connect to diodes $D_1$ and $D_2$, and the load $R_L$ is taken between the centre tap and the junction of the two diode outputs.
Working: during the half-cycle when the upper end is positive, $D_1$ is forward biased and conducts while $D_2$ is reverse biased; during the next half-cycle the lower end is positive, so $D_2$ conducts and $D_1$ is off.
Functions of the diodes: each diode conducts for one half-cycle only, and because of the centre-tap arrangement both drive current through $R_L$ in the same direction.
Hence the output is a unidirectional pulsating voltage with a hump for every half-cycle of the input.
$\boxed{D_1\text{and}{D}_2\text{conduct alternately, same load direction.}}$