• Modern electronic devices like CD players, computers, television receivers etc requires DC supply. The power we get from wall outlets is AC. Have you ever wondered how we convert AC to DC? In this section, we shall discuss the construction and working of half wave rectifiers which converts AC into pulsating DC.
  • Circuit diagram of half wave rectifiers along with input and output waveform is discussed.
  • In addition to the underlying theory about half wave rectifiers, we shall also see the applications of half wave rectifiers.

The block diagram of half wave rectifier along with the input and output waveform is shown in the figure below.

Block diagram of half wave rectifirer

 

It is clear from the block diagram of half wave rectifier that the output is not a pure DC but a pulsating DC. The reason will be clear as we discuss half wave rectifier circuit. As you can see from the block diagram of half wave rectifier, the negative portion of the sine wave is completely clipped-off. Only positive cycle is allowed to pass through, with the negative cycle completely eliminated. In other words, if we measure the output across some load resistor, then we can conclude that the current is flowing through the load for the positive half cycle, whereas the current is blocked when the input goes negative. Thus we can use the property of diode to conduct current in ONLY one direction to construct a half wave rectifier. A half wave rectifier circuit using a diode is shown in the figure below.

Half wave rectifier circuit

The circuit of half wave rectifier only consists of a diode and a load resistor. The voltage is measured across the load resistor. To understand the working of half wave rectifier, we analyse the circuit in two parts.

  1. First we analyse the circuit for the positive values of input.
  2. We analyse the circuit when the input voltage goes negative.

When the input signal is positive, the anode of the diode is at a higher potential than the cathode. This makes the diode forward biased. We know from the discussion on diode biasing that a forward biased diode acts as closed switch. Thus a forward biased diode allows the current to pass through. As the diode allows the current to pass through, this current flows through the load resistor and constitutes output voltage. This is shown graphically in the figure below.

Half wave rectifier, diode acts as closed switch.

Now as soon as the input voltage goes negative, the anode of the diode is at lower potential than the cathode. This makes the diode reverse biased. A reverse biased diode acts as an open switch. Hence the current stops flowing in the circuit. As there is no current flowing through the diode and load resistor, the output voltage becomes zero. This is shown in the figure below.

Half wave rectifier, diode acts as open switch.

Thus the output of half wave rectifier is a pulsating DC. Although half wave rectifier doesn’t eliminate the AC components of the signal, the average value of signal is definitely non-zero.