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Explain with a proper diagram how an ac signal can be converted into dc ( pulsating)signal with output frequency as double than the input frequency using pn junction diode. Give its input and output waveforms. 

Answer

A p-n junction diode is formed by combining a p-type and a n-type semiconductor.

  • A p-n junction diode allows current to pass only when it is forward biased
  • This property is used to rectify alternating voltages and the circuit used for this purpose is called a rectifier .
  • AC current becomes DC when it stops changing directions in a set period of time. That is, DC current has a constant direction .
  • A rectifier that produces an output signal with frequency as double than the input frequency is known as a full wave rectifier

The diagram is as given below:

Full wave rectifier-01.jpg

 

  • The input is given across the primary coil .
  • The secondary coil of the transformer receives the signal
  • Output is taken across resistor R L

(a) In the positive cycle of the input AC waveform:

POSITIVE CYCLE OF THE INPUT AC WAVEFORM.jpg

  • Diode D1 is forward biased - it acts as a short circuit
  • Diode D2 is reversed biased - it acts as an open circuit 

Hence, current flows through R L and is observed as output.

 

(b) In the negative cycle of the input AC waveform:

NEGATIVE CYCLE OF THE INPUT AC WAVEFORM.jpg

  • Diode D1 is reversed biased - it acts as a open circuit
  • Diode D2 is forward biased - it acts as an short circuit 

Hence, current flows through  R L and is observed as output.

You may notice that the direction of current in the first case is clockwise, while the second case is anti-clockwise.

  • The current produced is DC because the direction of the current in the load resistor in both cases is the same
  • Since, the no. of cycles per second has doubled , a full wave rectifier produces an output with double the input frequency.

FULL WAVE RECTIFIER- DC OUTPUT.jpg

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Davneet Singh

Davneet Singh has done his B.Tech from Indian Institute of Technology, Kanpur. He has been teaching from the past 12 years. He provides courses for Maths, Science, Social Science, Physics, Chemistry, Computer Science at Teachoo.