Diodes are also used in AC circuits where it is desired to convert AC voltages to DC voltages.
Half Wave Rectifier : 
In the half-wave rectifier circuit, the diode is forward-biased during the positive half-cycle from 0 to 𝜋 of the input voltage Vin and so current flows through the diode and resistor where it develops an output voltage drop Vout = Vin - VD . For instance, if the maximum value of Vin is 10 volts, the maximum value of Vout will be Vout = 10 - 0.7 = 9.3 V The diode is reverse-biased during the negative half-cycle from 𝜋 to 2𝜋 so no current flows, and thus Vout = 0.
Full Wave Rectifier :
We can see that during the positive half cycle conventional current flows from the voltage source to Point A, then to Point B, it goes through the resistor from Point B to Point C, and
through Point D returns to the negative terminal of the voltage source. During the negative half cycle
the lower terminal of the voltage source becomes the positive terminal, current flows from Point D
to Point B, it goes through the resistor from Point B to Point C, and through Point A returns to the
upper (now negative) terminal of the voltage source. We observe that during both the positive and
negative half-cycles the current enters the right terminal of the resistor, and thus is the same for
both half-cycles as shown in the output waveform. The output voltages from the half and full wave rectifiers are often called pulsating DC voltages.
Peak Rectifier Circuits :
Above figure shows the input Vin and
output Vout waveforms and we observe that the value of is approximately equal to the peak
of the input sinewave . We have assumed that the diode is ideal and thus as is applied and
reaches its positive peak value, the voltage across the capacitor assumes the same value.
However, when starts decreasing, the diode becomes reverse-biased and the voltage across
the capacitor remains constant since there is no path to discharge.
Clipper Circuits :
Clipper (or limiter) circuits consist of diodes, resistors, and sometimes DC sources to clip or limit the output to a certain level. Clipper circuits are used in applications where it is necessary to limit the input to another circuit so that the latter would not be damaged. The input-output characteristics of clipper circuits are typically those of the forward-biased and reverse-biased diode characteristics except that the output is clipped to a a certain level. As shown a clipper circuit and its input-output characteristics where the diode does not conduct for Vin < 0.6 V and therefore Vin = Vout. Diode conducts for Vin >= 0.7 V and thus Vout = 0.7 V.
DC Restorer Circuit :
Here , Diode conducts only when Vin < 0 and thus the capacitor charges to the negative peak of the input which for this example is -5 V , and the capacitor voltage is Vc = 5 V with polarity as shown in above figure. We observe that there is no path for the capacitor to discharge and therefore Vc = 5V always. Since Vout = Vin + Vc , when Vin = -5 V, Vout = -5+5 = 0 V. and the output has shifted upwards to zero volts as shown in output waveform. When the input voltage is positive, Vin = 5 V and Vout = 5 + 5 = 10 V.
Voltage Doubler Circuits :
DC restorer circuit consisting of the capacitor C1 and diode D1 and a peak
rectifier consisting of C2 and diode D2 . During the positive half-cycle of the input waveform
diode D1 conducts, the peak value Vp of the input Vin appears across the capacitor C1 of and
thus VD1 = 0 . During the negative half-cycle of the input waveform diode D2 conducts, capacitor C2 is being charged to a voltage which is the sum of Vin and VC1 and thus the output voltage is Vout = Vin + VC1 = -Vin - Vin = 2Vin. Voltage triplers and voltage quadruplers can also be formed by adding more diodes and capacitor to
a voltage doubler.