CBSE NOTES CLASS 12 PHYSICS
CHAPTER 14 SEMICONDUCTORS
Application of junction diode as a rectifier
The device which restricts the current (or voltage) to only one direction is called rectifier.
A p-n junction diode can be utilized as a rectifier.
If an alternating voltage is applied across a diode the current flows only in that part of the cycle when the diode is forward biased.
The rectifier circuit, which rectifies only the half-wave, is called a half-wave rectifier.
The secondary of a transformer supplies the desired ac voltage across terminals A and B. When the voltage at A is positive, the diode is forward biased and it conducts. When A is negative, the diode is reverse-biased and it does not conduct. The reverse saturation current of a diode is negligible and can be considered equal to zero for practical purposes.
The reverse breakdown voltage of the diode must be sufficiently higher than the peak ac voltage at the secondary of the transformer to protect the diode from reverse breakdown.
The circuit using two diodes as in the figure gives output rectified voltage for both the positive as well as negative half of the ac cycle. Hence, it is known as full-wave rectifier. The secondary of the transformer is provided with a centre tapping and so it is called centre-tap transformer.
Here the p-side of the two diodes is connected to the ends of the secondary of the transformer. The n-sides of the diodes are connected together and the output is taken between this common point of diodes and the midpoint of the secondary of the transformer.
Each diode rectifies only for half the cycle, but the two do so for alternate cycles. Thus, the output between their common terminals and the centre tap of the transformer becomes a full-wave rectifier output.
The rectified voltage is in the form of pulses of the shape of half sinusoids and does not have a steady value. To get steady dc output a capacitor is connected across the output terminals (parallel to the load RL) or an inductor in series with RL. This arrangement is called filter.
Role of capacitor in the filter
- When the voltage across the capacitor is rising, it gets charged. If there is no external load, it remains charged to the peak voltage of the rectified output.
- When there is a load, it gets discharged through the load and the voltage across it begins to fall. In the next half-cycle of rectified output it again gets charged to the peak value.
- The rate of fall of the voltage across the capacitor depends upon the inverse product of capacitor C and the effective resistance RL used in the circuit and is called the time constant.