Rectification (Cambridge International A-Level Physics)

Rectification Graphs

• 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

Spec point — 21.2.2

• 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 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

• 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

Worked Example A bridge rectifier consists of four ideal diodes A, B, C and D as connected in the figure shown below:

An alternating supply is applied between the terminal X and Y.

State which diodes are conducting when terminal X of the supply is positive.

Answer:

• Draw the path of the current direction with diodes in forward bias
• Remember that conventional current flow is from positive to negative and only travels through the paths with diodes in forward bias

• Therefore, the answer is: diodes A and C