This new rectifier circuit is made from four diodes in a configuration called a bridge rectifier. What this does is that on any given half of the input cycle, only two diodes are conducting. They get connected in such a way that the two conducting diodes will route the incoming current on the same direction, rectifying the current.
Let's take a closer look at what is going on in this circuit.
On the positive half of the wave, D1 is forward biased (positive applied to anode) and conducts current, and D4 is reverse biased (positive applied to cathode) and blocks current flow. D2 is also reverse biased, since it is connected to the positive voltage on its anode and transformer negative, which is our ground, that has a 0v potential.
The return path of the current is wired to D3 and D4 anodes. Since D4's cathode is in a higher voltage it will not conduct. D3 anode is not on a higher voltage, in fact it is in the 0v potential of the transformers negative, so it conducts.
On the negative half, the polarities have reversed, and now D2 is forward biased, D1 reverse biased (cathode now connected to a more positive voltage than its anode) and current ends up flowing in the same direction as it did in the positive half of the wave.
On the return path, D3 is blocked by the positive at its anode and doesn't conduct, but D4 does, completing the path to transformer ground.
As you can see, on both halves of the input signal the current ends up flowing in the same direction, and so it is said to be rectified.
This bridge has applications outside rectifying ac current, it is also useful in protecting against wrong connections on the power supply terminal, like connecting a battery in reverse, since no matter which way the input is connected, the current will always flow in the same direction.
