Full wave rectifier with filter
The device that converts Alternating Current (AC) into Direct Current (DC) is referred to as rectifier.
In half wave rectifier, the conversion of Alternating Current (AC) into Direct Current (DC) is not efficient. Half wave rectifier allows either positive half cycle or negative half cycle of the input AC signal and the remaining half cycle is blocked. As a result, a large power is wasted. Also, the output Direct Current (DC) produced by the half wave rectifier contains large ripples. This ripple voltage fluctuates with respect to time. So it is not suitable for practical applications.
To overcome these problems, we use filters at the output. Even though we use filters at the output, the DC signal obtained at the output is not a pure DC. Furthermore, the power loss is high in half wave rectifier. Therefore, to reduce the power loss and reduce the ripples at the output, we go for another type of rectifier known as full wave rectifier.
As the name suggests, the full wave rectifier rectifies both positive and negative half cycles of the input AC signal.
Even though the full wave rectifier rectify both positive and negative half cycles, the DC signal obtained at the output still contains some ripples. To reduce these ripples at the output, we use a filter.
The filter is an electronic device that converts the pulsating Direct Current into pure Direct Current.
The filter is made up of a combination of electronic components such as resistors, capacitors, and inductors. The property of inductor is that it allows the DC components and blocks the AC components. The property of a capacitor is that it allows the AC components and blocks the DC components.
In this tutorial, a center tapped full wave rectifier with a filter made up of capacitor and resistor is explained. The filter made up of capacitor and resistor is known as capacitor filter.
In the circuit diagram, the capacitor C is placed across the load resistor RL.
The working of the full wave rectifier with filter is almost similar to that of the half wave rectifier with filter. The only difference is that in the half wave rectifier only one half cycle (either positive or negative) of the input AC current will charge the capacitor but the remaining half cycle will not charge the capacitor. But in full wave rectifier, both positive and negative half cycles of the input AC current will charge the capacitor.
The main duty of the capacitor filter is to short the ripples to the ground and blocks the pure DC (DC components), so that it flows through the alternate path and reaches output load resistor RL.
When input AC voltage is applied, during the positive half cycle, the diode D1 is forward biased and allows electric current whereas the diode D2 is reverse biased and blocks electric current. On the other hand, during the negative half cycle the diode D2 is forward biased (allows electric current) and the diode D1 is reverse biased (blocks electric current).
During the positive half cycle, the diode (D1) current reaches the filter and charges the capacitor. However, the charging of the capacitor happens only when the applied AC voltage is greater than the capacitor voltage.
Initially, the capacitor is uncharged. That means no voltage exists between the plates of the capacitor. So when the voltage is turned on, the charging of the capacitor happens immediately.
During this conduction period, the capacitor charges to the maximum value of the input supply voltage. The capacitor stores a maximum charge exactly at the quarter positive half cycle in the waveform. At this point, the supply voltage is equal to the capacitor voltage.
When the AC voltage starts decreasing and becomes less than the capacitor voltage, then the capacitor starts slowly discharging.
The discharging of the capacitor is very slow as compared to the charging of the capacitor. So the capacitor does not get enough time to completely discharged. Before the complete discharge of the capacitor happens, the charging again takes place. So only half or more than half of the capacitor charge get discharged.
When the input AC supply voltage reaches the negative half cycle, the diode D1 is reverse biased (blocks electric current) whereas the diode D2 is forward biased (allows electric current).
During the negative half cycle, the diode (D2) current reaches the filter and charges the capacitor. However, the charging of the capacitor happens only when the applied AC voltage is greater than the capacitor voltage.
The capacitor is not completely uncharged, so the charging of the capacitor does not happens immediately. When the supply voltage becomes greater than the capacitor voltage, the capacitor again starts charging.
In both positive and negative half cycles, the current flows in the same direction across the load resistor RL. So we get either complete positive half cycles or negative half cycles. In our case, they are complete positive half cycles.
How exactly the capacitor filter removes the ripples in the signal
The pulsating Direct Current (DC) produced by the full wave rectifier contains both AC and DC components.
We know that the capacitor allows the AC components and blocks the DC components of the current. When the DC current that contains both DC components and AC components reaches the filter, the DC components experience a high resistance from the capacitor whereas the AC components experience a low resistance from the capacitor.
Electric current always prefers to flow through a low resistance path. So the AC components will flow through the capacitor whereas the DC components are blocked by the capacitor. Therefore, they find an alternate path and reach the output load resistor RL. The flow of AC components through the capacitor is nothing but the charging of a capacitor.
Thus, the filter converts the pulsating DC into pure DC.
