The hallmark of a rectifier diode is that it passes current in only one direction. This makes it useful for changing ac to dc. Generally speaking, when the cathode is negative with respect to the anode, current flows; when the cathode is positive relative to the anode, there is no current. The constraints on this behavior are the forward breakover and avalanche voltages, as you learned about in previous topics.
At A, a half-wave rectifier circuit. At B, the output of the circuit shown at A when an ac sine wave is applied to the input.
Examine the circuit shown at A in above figure. Suppose a 60-Hz ac sine wave is applied to the input. During half the cycle, the diode conducts, and during the other half, it doesn’t. This cuts off half of every cycle. Depending on which way the diode is hooked up, either the positive half or the negative half of the ac cycle will be removed. Drawing B in above figure shows a graph of the output of the circuit at A. Remember that electrons flow from negative to positive, against the arrow in the diode symbol.

The circuit and wave diagram of above figure show a half-wave rectifier circuit. This is the simplest possible rectifier. That’s its chief advantage over other, more complicated rectifier circuits. You’ll learn about the various types of rectifier diodes and circuits in previous topics.