Rectification (CIE A Level Physics)

Rectification

• Rectification is defined as:

The process of converting alternating current  and voltage into direct current  and voltage

• Rectification is used in electronic equipment which requires a direct current
  • For example, mains voltage must be rectified from the alternating voltage produced at power stations

Types of Rectification

• There are two types of rectification:
  • Half-wave rectification
  • Full-wave rectification

• For half-wave rectification:
  • The graph of the output voltage V_out against time is a sine curve with the positive cycles and a flat line (V_out = 0) on the negative cycle
  • This is because the diode only conducts in the positive direction

• For full-wave rectification:
  • The graph of the output voltage V_out against time is a sine curve where the positive cycles and the negative cycles are both curved ‘bumps’

Half and full wave rectification

The difference between the graphs of full-wave and half-wave rectification

• Half-wave rectification consists of a single diode
  • An alternating input voltage is connected to a circuit with a load resistor and diode in series

• The diode will only conduct during the positive cycles of the input alternating voltage
  • Hence there is only current in the load resistor during these positive cycles

• The output voltage V_out across the resistor will fluctuate against against time in the same way as the input alternating voltage except there are no negative cycles

Circuit diagram for half-wave rectification

Half-wave rectification requires a single diode and the graph is represented by only the positive cycles

• This type of rectification means half of the time the voltage is zero
  • So, the power available from a half-wave rectified supply is reduced

• Full-wave rectification requires a bridge rectifier circuit
  • This consists of four diodes connected across an input alternating voltage supply

• The output voltage V_out is taken across a load resistor
• During the positive cycles of the input voltage, one terminal if the voltage supply is positive and the other negative
  • Two diodes opposite each other that are in forward bias will conduct
  • The other two in reverse bias will not conduct
  • A current will flow in the load resistor with the positive terminal at the top of the resistor

• During the negative cycles of the input voltage, the positive and negative terminals of the input alternating voltage supply will swap
  • The two diodes that were forward bias will now be in reverse bias and not conduct
  • The other two in reverse bias will now be in forward bias and will conduct
  • The current in the load resistor will still flow in the same direction as before

Circuit diagram for full-wave rectification

When A is positive and B is negative, diodes 2 and 3 will conduct and 1 and 4 will not. When A is negative and B is positive, diodes 1 and 4 will conduct and diodes 2 and 3 will not. The current in the load resistor R will flow downwards

• In both the positive and negative cycles, the current in the load resistor is the same
• Each diode pair is the same as in half-wave rectification
  • Since there are two pairs, this equates to full-wave rectification overall

• The main advantage of full-wave rectification compared to half-wave rectification is that there is more power available
  • Therefore, a greater power is supplied on every half cycle