Diode, Half and Full Wave Rectification or AC to DC Conversion

We learned about the function of diodes from the previous Diode Introduction article. As we know, a diode is an electronic device that allows the charge or electricity of a certain religion to go only in a certain direction.

PN junction and biasing

The diode basically has two sections. If we divide it into two halves, there are some positive charges on one side and some negative charges on the other side. If we add the negative end of the battery to the positively charged end of the diode and the positive end of the battery to the negative end of the diode as shown in the first figure below, the negative end of the battery will be negatively charged to the battery and the negative end of the battery will be negative to the battery. As a result, the layer between the two ends of the diode will increase and current will not flow. But if we add the positive end of the battery with the positively charged end of the diode and the negative end of the battery with the negatively charged end of the diode as shown in the second figure, the positive and negative charges will accumulate in the battery. As a result, these charges will start coming towards each other and the width of the layer between the positive and negative charges of the diode will continue to decrease.

Thus at some point its amplitude will disappear and electricity will flow through the diode. We call it forward bias as current can flow in it. And if not, I call that connection reverse bias.

We call the end of the diode that has a positive charge P and the end that has a negative charge N. However, when drawing a circuit, we present this PN junction as an image.

If the positive end of the battery is connected to the P end of the diode and the negative end of the battery is connected to the N end of the diode, then electricity can flow. Which we call forward bias. And if the negative end of the battery is connected to the P end of the diode and the positive end of the battery is connected to the N end of the diode, electricity cannot flow. Which we call reverse bias. With this knowledge we can easily convert AC current to DC. But before that you need to know about AC and DC current.

AC and DC current

We know that any electronic connection has at least two connections. One of which we know as positive and the other as negative. But this does not happen in the case of AC current. In the case of AC current, the two wires alternately become positive and negative. Sometimes one is positive and the other is negative, and sometimes one is negative and the other is positive. That's the way it goes.

But this does not happen in the case of DC current. Here one is always positive and the other is always negative. Change does not happen. In real life we ​​need both currents. So sometimes AC needs to be DC, again DC needs to be AC. AC current is better than DC for such low dissipation power transmission and we are using it. Although modern scientists say high voltage DC transport is better than AC.

AC currents move in a positive and negative direction like the waves in an image. We call it a full wave to cross a completely positive and a negative path. We will direct this AC wave in two ways.

  • Half-wave Rectification.
  • Full wave Rectification.

Half-wave Rectification

Since the S source is an AC voltage source, if the P wire of this wave in the figure is positive, then the Q wire will be negative. But since a full wave has both positives and negatives, once in a full wave the P wire will be positive, the Q wire will be negative and once again the Q wire will be positive and the P wire will be negative.


Notice in the figure, when only the P wire is positive and the Q wire is negative then the diode is in the forward bias and passing that part of the wave. But when the P wire is negative and the Q wire is positive then the diode is in reverse bias and cannot pass that part of the wave. So we are always getting positive current from point A and always negative current from point B. Then we get DC from AC. But since we get only half of each full wave, we call it half-wave unidirectional. Again this is but pure DC. Although the positive and negative edges are different, the voltage is getting higher and lower. That's why we call it pulsating DC.

Full wave Rectification

Now if we connect the circuit as shown below, when P end is positive and Q end is negative then diode D1 and D2 are in forward bias between the four diodes. As a result current is flowing and getting positive current at the A end of the resistor and negative current at the B end. Similarly when P end is negative and Q end is positive then d3 and D4 of the four diodes are in forward bias. As a result electricity is flowing and again we are getting positive current at the A end of the resistor and negative current at the B end.

So for both sides of the full wave we get positive current at the end of A and negative current at the end of B. That means we are getting pulsating DC voltage.

Thus using diodes we can convert AC current to DC. However, the DC we got is not a pure DC. We got pulsating DC. In the next article we will make this pulsating DC a pure DC.


Edited By
Jeion Ahmed
B.Sc. in Electrical & Electronic Engineering (EEE)
Chittagong University of Engineering & Technology (CUET)

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