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Diodes and AC

The Circuit

An ac voltage applied to a diode and resistor.

When we apply an ac voltage to a diode as shown in the figure to the right, we must also include at least a resistor in series with that diode. Otherwise, the diode will fail (and quite possibly explode) as soon as power is applied to the circuit. The reason for this is that an ideal diode has zero resistance when forward biased, and will therefore allow infinite current to flow. However, a real diode always has some internal resistance when conducting current. This resistance is small enough to allow a large current to flow, but high enough so that the heat it dissipates due to the high current flow (remember, P = I²R) is sufficient to melt or even vaporize the semiconductor material of which the diode is made.

Therefore, we include a series resistance to limit the amount of current flowing through the circuit whenever the diode is conducting. In practical circuits, other components may provide the necessary current limiting function, and often do. but in all cases, something must be present to protect the diode from excessive current flow.

Voltage and Current

A semiconductor diode has the interesting property of being able to conduct electrical current in one direction through itself, but not in the other direction. To see why and how it works this way, take a look at the structure of the PN junction. On this page, we are concerned more with how such a diode behaves when an ac voltage is applied to it.

Voltage and current for an ideal diode/

In the graph to the right, the red curve represents the voltage across the diode, while the blue curve represents the current through the diode. As you can see, during the positive half-cycle of the applied ac, the voltage across the diode is the full applied ac voltage, while the circuit current is zero. That zero current leaves no voltage drop across the resistor, which is why all of the applied voltage appears across the diode.

During the negative half-cycle, current does flow through the diode (electrons move from top to bottom through the diode, travelling in a clockwise direction around the loop). During this time, an ideal diode would have no voltage drop across itself, and only the resistor would limit the amount of current flowing through the circuit. Note that this behavior is not dependent on frequency; an ideal diode simply conducts current in one direction only, regardless of frequency.

Of course, real-world diodes aren't ideal, and will drop a fraction of a volt across themselves. In some types of circuits this is negligible; in others, this voltage drop must be included in mathematical calculations about circuit behavior and performance. In addition, a real-world diode exhibits a small amount of capacitance between its terminals, especially when reverse biased. In any case, however, the basic behavior of the diode remains the same.

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