What is a rectifier?
A rectifier is a circuit that turns alternating current into direct current. Alternating current is convenient to generate and transmit, but most electronics need a steady one-direction supply — so somewhere in between, a rectifier has to change one into the other. The component that does the work is the diode.
Why four diodes?
A single diode can only conduct on the half of the cycle when the current is flowing its way, so it throws away half the supply. A bridge rectifier uses four diodes arranged in a ring so that both halves are used: when the current reverses, the other two diodes pick it up and send it to the load the same way as before. The result is a smoother, more useful output.
Reading the diagram
The circle on the left is the AC source, wired across the left and right corners of the diamond. The four triangles are the diodes D1 to D4. The top corner is the positive rail and the bottom corner the negative rail, feeding the load on the right. The inset panel compares the two-sided input wave with the one-sided output.
1. The source keeps changing direction
The AC source pushes current one way, then the other, over and over. Left to itself this current would drive the load first forwards, then backwards — useless for anything that needs a steady direction.
2. A diode is a one-way valve
A diode conducts in only one direction: current flows easily from its anode to its cathode, but is blocked the other way. That single property, repeated four times, is all a bridge rectifier needs.
3. On the first half, D1 and D3 conduct
Say the left input is positive. Current leaves the source, passes through diode D1 up to the positive rail, goes through the load to the negative rail, and returns through D3 back to the right input. D2 and D4 are reverse-biased and stay shut.
4. On the second half, D2 and D4 take over
Now the source reverses and the right input is positive. Current enters through D2 to the same positive rail, crosses the load the same way, and returns through D4 to the left input. The conducting pair has swapped, but the load sees current in the same direction as before.
5. The output is one-direction, with ripple
Because both halves are steered the same way, the output never reverses. It is not yet flat, though: it rises and falls in a series of humps, so a smoothing capacitor is normally added next to fill the dips and make the supply steady.
What is a diode?
A diode is a component that lets current flow in one direction and blocks it in the other. It behaves like a one-way valve for electricity, so it can be used to sort an alternating current into a current that only ever flows one way.
What is a rectifier?
A rectifier is a circuit that converts alternating current into direct current. The simplest versions use diodes; a single diode gives half-wave rectification, while four diodes in a bridge give full-wave rectification, using both halves of the cycle.
Why use four diodes instead of one?
One diode conducts only on half of each cycle, so half the time no power is delivered and the output is very uneven. A four-diode bridge uses both halves — whichever way the current flows, a pair of diodes routes it through the load the same way, so the output is far smoother.
Can a diode make current flow backwards?
No. A normal diode blocks current in its reverse direction, so you can never get current to flow the wrong way through it. That one-way property is the whole reason a rectifier works, and it is also what protects circuits from being connected the wrong way round.
Is the rectified output steady DC?
Not yet. The output is a series of humps that rise and fall — a pulsed, one-direction current. To make it steady, a smoothing capacitor is added next, which stores charge on the peaks and releases it into the dips.