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Rectification (CIE A Level Physics)

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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 Vout against time is a sine curve with the positive cycles and a flat line (Vout = 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 Vout against time is a sine curve where the positive cycles and the negative cycles are both curved ‘bumps’

Half and full wave rectification

Rectification graphs, downloadable AS & A Level Physics revision notes

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

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 Vout 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, downloadable AS & A Level Physics revision notes

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 Vout 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

Full-wave rectification, downloadable AS & A Level Physics revision notes

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:WE Full wave rectification question image, downloadable AS & A Level Physics revision notes

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

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Ashika

Author: Ashika

Expertise: Physics Project Lead

Ashika graduated with a first-class Physics degree from Manchester University and, having worked as a software engineer, focused on Physics education, creating engaging content to help students across all levels. Now an experienced GCSE and A Level Physics and Maths tutor, Ashika helps to grow and improve our Physics resources.