IB DP Physics: HL复习笔记11.2.8 Rectification

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
  • 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’21.1-Rectification-graphs

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 time in the same way as the input alternating voltage except there are no negative cycles21.1-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
  • 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 in 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

21.1-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, 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.

21.1-WE-Full-wave-rectification-question-image

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

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

  • Draw 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

21.1-WE-Full-wave-rectification-solution-image

  • Therefore, the answer is: diodes A and C

Exam Tip

Being able to reproduce the diode bridge correctly and explain the cycles of full-wave rectification are important physics concepts that may occur during an examination. It is worth learning to draw and explain full-wave rectification accurately

 

 

 

 

 

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