Rectifiers Theory with Circuit Diagrams

Rectifiers are defined as electronic or electrical devices which are used to convert alternating current of a.c. into direct current or d.c.

Rectifiers Theory

Current is defined as flow of electrons through conductor. An alternating current is one in which direction of flow of electrons or current flow reverses periodically. Whereas direct current is one in which direction of flow of electrons or current flow remains same i.e., current flows in one direction. A.C. has certain cycles per second which is called frequency and it is measured in Hertz (Hz). Each cycle has two half portions one of which is positive and another half is negative. In a.c. supply, as current flows in both directions periodically, therefore, polarity of terminals of a.c. supply also changes periodically i.e., a terminal which remains positive during first half cycle becomes negative during another half cycle, whereas in d.c. supply, polarity of output terminals does not change.

Rectifiers allow flow of electrons in one direction and block electrons flow in reverse direction. This is achieved by diodes. Diodes are electronic or electrical devices made from semiconductors and conduct only in one direction i.e., in forward biased condition. Diodes have  two terminal one is Anode and another is Cathode. Diode is said to be forward biased when its Anode is connected to positive terminal of battery and Cathode is connected to negative terminal of battery. A diode is said to be reverse biased when its Anode is connected to negative of battery and Cathode is connected to positive terminal of battery. Diodes conduct when they are forward biased and they do not conduct when they are reverse biased.


As we can see from above figure that a cycle of alternating current has both positive and negative portions and as we discussed above that diodes conduct when they are forward biased, so diodes conduct and allow a.c. to pass during half cycle when they are forward biased and block a.c. during next half cycle when they are reverse biased.


It may be noted that d.c. voltage shown in above figure is ideal d.c. voltage which has constant magnitude. Output of rectifiers is pulsating d.c. i.e., voltage with changing magnitude but fixed polarity. Capacitors may be introduced in the output circuits of rectifiers to convert pulsating output d.c. voltage into ripple d.c. voltage. A ripple d.c. voltage is one which has slight variations in magnitude and the variation is such that the voltage changes little above and down  periodically to a given straight line d.c. voltage.

CLASSIFICATION OF RESISTORS

     1.  Half Wave Rectifiers,
     2.  Full Wave Rectifiers,

     Full Wave rectifiers can be further classified as ;

          a)  Center Tapped Full Wave Rectifiers,
          b)  Bridge Rectifiers.

Have Wave Rectifier

A half wave rectifier blocks one half portion or cycle of alternating current and converts a.c. voltage into pulsating d.c. voltage using only another half portion or cycle of alternating current during each cycle of a.c. A simplest half wave rectifier circuit consists of a single P-N junction diode. A load is connected in series with diode. A simplest half wave rectifier circuit and wave forms of input a.c. and converted output d.c. is shown in figure below ;

WORKING OF HALF WAVE RECTIFIER

There are two portions of alternating current one is positive and another is negative portion and thus there are two switching states of diode, one is forward biased when diode conducts and another is reverse biased when it does not conduct and stops current to pass on due to which circuit remains open. During first half cycle, diode remains forward biased when it conducts and voltage is observed across the load as shown in figure above. During second half cycle, diode remains reverse biased when it does not conduct and no voltage is observed across the load.

CENTER TAPPED FULL WAVE RECTIFIER

A full wave rectifier converts a.c. voltage into pulsating d.c. voltage using both half portions of alternating current during each cycle of a.c. This is achieved using two diodes and two secondary coils of a transformer. A transformer known as center tapped transformer that has three secondary output leads may be used. One output lead  at center becomes common and two diodes are connected to two remaining leads. During first half cycle, one diode remains forward biased and during another half cycle another diode remains forward biased. In this way, we get pulsating output d.c. voltage during a complete cycle of a.c. A simplest full wave rectifier circuit consists of two P-N junction diodes. A simplest full wave rectifier circuit and wave forms of input a.c. voltage and converted output d.c. voltage is shown in figure below ;

WORKING OF FULL WAVE RECTIFIER

During first half cycle diode D1 becomes forward biased and diode D2 remains reverse biased. D1 then conducts, D2 does not conduct and central lead remains at zero, therefore, we observe voltage across load. During second half cycle diode D2 becomes forward  biased and diode D1 becomes reverse biased. D2 then conducts, D1 does not conduct, therefore, we observe voltage across load. In full wave rectifier voltage is observed during whole cycle of input a.c. unlike half wave rectifier in which voltage is observed across load during only positive half cycle of alternating current. As output of diodes D1 & D2 are common and both the diodes conduct one by one consecutively during positive half & negative half portions of each cycle of input alternating current, therefore, voltage is observed during whole cycle of input a.c.

FULL WAVE BRIDGE RECTIFIER

A full wave bridge rectifier also known as simply bridge rectifier converts a.c. voltage into pulsating d.c. voltage using both half negative & positive portions of alternating current. This is achieved using four diodes connected in bridge arrangement so that there becomes two inputs and two outputs. No center tapped transformer is required in bridge rectifier unlike simple two diodes full wave rectifier, however, the wave form of full wave bridge rectifier is same to that of simple two diodes full wave rectifier. During first half cycle, diodes D2 & D4 are forward biased and during another half cycle, diodes D1 & D3 are forward biased. In this way, we get pulsating output d.c. voltage during each complete cycle of input alternating current. A simplest full wave bridge rectifier circuit consists of four P-N junction diodes. A simplest full wave bridge rectifier circuit and wave forms of input a.c. voltage and converted output d.c. voltage is shown in figures below ;

WORKING OF FULL WAVE BRIDGE RECTIFIER

During first half cycle, diodes D2 & D4 becomes forward biased whereas diodes D1 & D3 becomes reverse biased. D2 & D4  then conduct and D1 & D3 do not conduct, therefore, voltage is observed across load. During second half cycle, diodes D1 & D3 becomes forward biased whereas diodes D2 & D4 becomes reverse biased. D1 &  D3 then conducts and D2 & D4 do not conduct, therefore, voltage is observed across load. In bridge rectifier, voltage is observed during whole cycle of input a.c. voltage unlike half wave rectifier in which voltage is only observed across load during only positive half cycle of a.c.  As output of diodes D2 & D3 are cathodes and they are common, so this output becomes positive terminal because they become forward biased when positive signal is applied to their anodes which are the inputs. Similarly as output of diodes D1 & D4 anodes and they are common, so this output becomes negative terminal because they become forward biased when negative signal is applied to their cathodes which are the inputs. Diodes D1 & D3 collectively conduct and diodes D2 & D4 collectively conduct one by one consecutively during negative half and positive half portions of each cycle of input alternating current, therefore, in this way voltage is observed during whole cycle.