Rectifiers and Filters
A rectifier is a device which converts alternating current (AC) to direct or unidirectional current (DC)
Half wave rectifier
It consists of a transformer, the primary of which is fed by signal voltage. The end A of the secondary is connected to the p type while the end B is connected to n-type of the diode (D) through a load RL. The d.c. output is drawn from across the load resistance.
For the first half of the a.c. cycle, let A is +ve and B is -ve, then the p-n junction diode is in forward bias. Hence the diode conducts large conventional current flows across the load RL from C to D. The variation in the current corresponds to the variation in the magnitude of the voltage during this half cycle.
For the next half cycle, A is -ve and B is +ve, then the p-n junction diode is in reverse bias. Hence the diode does not conduct, and no current flows across Load RL.
Thus we find that the output voltage is unidirectional consisting of only half the cycle, hence it is called half wave rectifier.
Full wave rectifiers
In this case two p-n junction diodes D1 and D2 across the secondary AB of a transformer. A load resistance RL is connected between the centre tapping (T) point and the n-sides of the two diodes. The output is drawn from across the load resistance RL.
Working principle –
During the first half of a.c. cycle, let A is +ve and B is -ve. The . Then D1 diode is in forward bias but D2 diode is in reverse bias. Hence diode D1 conducts and a large conventional current flows across RL from C to D but diode D2 does not conduct . The variation in the current corresponds to the variation in the magnitude in the voltage.
During the next half cycle of a.c. signal, A is -ve and B is +ve cycle. Then D1 diode is in reverse bias but D2 diode is in forward bias.Hence D1 doesn’t conduct but diode D2 conducts and a large conventional current flows across RL from C to D i.e. Same direction as the first half cycle.
Thus output consists of continuous pulses giving full wave rectification.
***Remember the table-
A bridge rectifier circuit acts as a full wave rectifier without the use of a centre tapped transformers.
It consists of four diodes in two pairs D1 , D3 and D2, D4 connected in the form of a bridge. The a.c. signal to be rectified is applied to the diagonally opposite corners of the bridge through a transformer . The load resistance RL is connected between the other ends of the bridge.
Working principle :
During the positive half cycle of a.c. let A is +ve and B is -ve, then diodes D1, and D3 are in forward bias but diodes D2 and D4 are in reverse bias and do not conduct. Since D1 and D3 diode conduct, a large conventional current flows along the dotted line from C to D across the load.
During the negative half cycle of a.c. A is -ve and B is +ve, then diodes D2 and D4 are in forward biased , hence conduct and a conventional current flows (not shown) across the load RL from C to D i.e. same direction as before. Hence full wave rectification occur.
What is filter ?
A filter is a device which removes the a.c. component of rectifier output and it allows the d.c. component to reach the load.
What is the necessity of using filter in a rectifier ?
The output of a full wave rectifier is unidirectional and continues but it is not steady and has a pulsating character.
In most of the electrical and electronic circuits a steady direct current is required. It is therefore, necessary to reduce the ripples of a rectifier output, hence filter is necessary.
Full wave rectifier (Bridge rectifier) with π filter
A pi filter consists of two capacitors and an inductor are connected in the form of Greek letter π, hence it is called “π” filter.
The inductor (L) offers high resistance to a.c. component (XL = ωL) and offers low resistance to d.c. components.
For d.c. f → 0
For a.c. f → high.
But a capacitor (C) offers high resistance to d.c. component (Xc = 1/ωC) and offers low resistance to a.c. component.
Hence most of a.c. components are quenched across L but easily pass through a and the rest a.c. component and all the d.c. component after passing through L when reach to capacitor C2, the a.c. components can easily pass but d.c. components are quenched’. Hence we get a steady current through load (RL).
It can be consists of a capacitor filter and a LC section filter.
Ripple factor of a capacitor filter,
Ripple factor of a L section filter
So, Ripple factor of a π section filter,
Obviously, a high load resistance called bleeder resistance helps in reducing ripple.
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