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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:
- 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:
- 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:
- 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.
