Friday, December 25, 2015

Full wave rectifier; Full wave bridge rectifier

Full Wave Rectifier:-
Full Wave Rectifier are used to convert AC into DC. Average DC output of full wave rectifier is more than half wave rectifier also ripples are lower in DC in full wave rectifier in comparison to half wave rectifier.

Full wave rectifier circuit consists of 2 no. of diodes one for each half cycle.
Full wave rectifier circuit is as shown below:-

Full wave rectifier

In above circuit secondary winding is center tapped. In this circuit each diode starts conducting when anode terminal is positive with respect to transformer center point. This will leads to conduct both diodes alternately and produces DC in both half cycles of AC.
In circuit shown above you can see that there are two diode connected to single resistive load.In Transformer when point 1 get positive with respect to center point 3 diode 1 starts conducting and direction of flow is shown as in figure.

Now when point 2 get positive with respect to center point 3 than diode 2 starts conducting in forward direction and direction of conduction is  as shown in figure. You can see that direction of flow of current through load that is R is same. As direction is same in Resistor this means it act as full wave rectifier.
Output waveform of Full wave rectifier is shown below:-

Full wave rectifier Output waveform

From Waveform we can see that there are 2 Vmax in every alternating current full cycle. From above we can get:-

Output Voltage of Full wave rectifier is= 2 Vmax  = 0.637 Vmax= 0.9 V rms
Where Vmax is the maximum peak value in one half of secondary winding
Vrms is the RMS value

There are some disadvantages associates with full wave rectifier one of the main disadvantages is that full wave rectifiers requires large rating transformer is required for given output which makes it very costly. To Overcome this problem full wave Bridge rectifier are used as this doesn’t require center tapping of transformer which will leads to reduction in cost and size of Transformer required.

Full wave Bridge Rectifier
There are four diodes used in this circuit and circuit diagram of full wave bridge rectifier is as shown below:-

Full wave Bridge Rectifier

There are four diodes connected to one load and 2 no.s diodes conduct in each half cycle. In the circuit diode 1 and 2 conduct in positive half cycle and 3 and 4 conduct in negative half cycle.

To minimize ripples in the output waveform smoothing capacitors are used at Output of the rectifier. This capacitor is used to smoothen the DC output . By connecting capacitor at rectifier output average DC output level will be increased.

Smoothing capacitor:-

Circuit diagram for the same is shown below with capacitor connected in circuit.

Full wave rectifier with Smoothing capacitor

The capacitor converts the rippled output of full wave rectifier to smooth DC output voltage. Capacitor used for smoothing DC output is usually of rating 100 microfarad. Capacitors used for this purpose are aluminum electrolytic type. Repeated DC voltage pulses from the rectifier charging up the capacitor to peak voltage.
While selecting capacitor of smoothing DC voltage following factors must be taken into consideration:-
(a)   Working voltage, working voltage should be higher than the no-load output value of rectifier
(b)   Capacitance value, this is used to determine the ripples which will superimpose on DC output voltage.
Capacitor value should be large enough to smoothen out the DC voltage as too low capacitance will have little effect on output voltage waveform.
Resultant waveform with Smoothing capacitor is as shown below:-

Full wave rectifier Output Waveform with smoothing capacitor

From above you will see how smoothing capacitor will increase DC output of full-wave bridge rectifier.