One of the most common uses for rectifier diodes in electronics is to convert household alternating current into direct current that can be used as an alternative to batteries. The rectifier circuit, which is typically made from a set of cleverly interlocked diodes, converts alternating current to direct current.
In household current, the voltage swings from positive to negative in cycles that repeat 60 times per second. If you place a diode in series with an alternating current voltage, you eliminate the negative side of the voltage cycle, so you end up with just positive voltage.
If you look at the waveform of the voltage coming out of this rectifier diode, you'll see that it consists of intervals that alternate between a short increase of voltage and periods of no voltage at all. This is a form of direct current because it consists entirely of positive voltage. However, it pulsates: first it's on, then it's off, then it's on again, and so on.
Overall, voltage rectified by a single diode is off half of the time. So although the positive voltage reaches the same peak level as the input voltage, the average level of the rectified voltage is only half the level of the input voltage. This type of rectifier circuit is sometimes called a half-wave rectifier because it passes along only half of the incoming alternating current waveform.
A better type of rectifier circuit uses four rectifier diodes, in a special circuit called a bridge rectifier.
Look at how this rectifier works on both sides of the alternating current input signal:
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In the first half of the AC cycle, D2 and D4 conduct because they're forward biased. Positive voltage is on the anode of D2 and negative voltage is on the cathode of D4. Thus, these two diodes work together to pass the first half of the signal through.
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In the second half of the AC cycle, D1 and D3 conduct because they're forward biased: Positive voltage is on the anode of D1, and negative voltage is on the cathode of D3.
The net effect of the bridge rectifier is that both halves of the AC sine wave are allowed to pass through, but the negative half of the wave is inverted so that it becomes positive.