Just like the half-wave rectifier, a full-wave rectifier circuit generates an output-voltage or current which is absolutely DC or has some specific DC components. Full-wave rectifiers have some basic advantages & merits over the half-wave rectifiers. The average-dc output voltage is higher than for half-wave, the output of the full-wave rectifier has less ripple than that of half-wave rectifier generating a smoother output wave-form. A circuit diagram of full wave rectifier with capacitor filter will be shown later.
Now two diodes are utilized in a full-wave rectifier circuit, one for each half of the cycle. Multiple winding transformers are used whose secondary winding is split equally in all proportions into 2 halves with a common-center tapped connection, (C). Such a configuration as here results in each diode that is conducting in turn when its anode terminal is +ve with respect to the transformer center point C generating an output during both half-cycles.
Full Wave Rectifier Circuit
The full-wave rectifier circuit constitutes 2 power diodes connected to a load-resistance (Single RL) with the each diode taking it in turn to provide current to load. Whenever, point A of transformer is +ve w.r.t. the point C, diode D1 conducts in forward direction as shown with the help of arrows.
When point B is +ve (in the negative half of cycle) w.r.t. the point C, diode D2 conducts in forward direction & the current flowing through resistor R is in same direction for both half cycles. As the output voltage across the resistor R is the phasor sum of the 2 wave-forms combined & this type of full-wave rectifier circuit is also called as a “bi phase” circuit.
As the spaces between each half wave developed by the each diode is now filled in by other diodes, the average DC output voltage across the load resistor is now double that of single half wave rectifier circuit & is about 0.637Vmax of the peak-voltage, assuming no losses.
Where: VMAX is the max. peak value in one-half of the secondary winding & VRMS is the ‘rms’ value. The peak-voltage of the output wave-form is the same as before for half wave rectifier given each half of trans-former windings have the same rms voltage value. To obtain a different DC-voltage output different trans-former ratios can be utilized. The main demerits of this type of full-wave rectifier circuit is that a larger trans-former for a given power output is required with two separate but identical secondary windings making this type of full-wave rectifying circuit is costly as compared to the “Full-Wave Bridge Rectifier” circuit equivalent.
The Full-Wave Bridge Rectifier
The type of circuit that gives the same output wave-form as the full-wave rectifier circuit given above is the Full-Wave Bridge Rectifier. This single-phase type of rectifier utilizes 4 individual rectifying diodes interfaced in a closed-loop ‘bridge’ configuration to generate the required output. The main benefit of this circuit is that it doesn’t need a special center-tapped trans-former reducing its size & cost. The single-secondary winding is interfaced with one side of the diode-bridge network & the load to the other side.
The Diode Bridge Rectifier
The 4 diodes named D1 to D4 are organized in “series pairs” with only 2 diodes conducting current during each half cycle. During the +ve half-cycle of supply, diodes D1 & D2 operate in series while diodes D3 & D4 are biased reversely & the current flows through the load as below.
The Positive Half-cycle
During the -ve half-cycle of supply, diodes D3 & D4 operate in series, but diodes D1 & D2 switch OFF as they are now biased reversely. The current flowing through the load is the same direction as before.
The Negative Half-cycle
Since, the current moving through the load is uni-directional so, the voltage developed across the load is also uni-directional the same as for the recently explained two diode full-wave rectifier. Therefore, the average DC-voltage across the load is about 0.637Vmax. However, in reality the current flows through two diodes instead of only one during each half-cycle so, the amplitude of output-voltage is 2 voltage drops ( 2 x 0.7 = 1.4V ) that is less than the input VMAX amplitude. The ripple-frequency is now 2 times the supply-frequency (for example 100Hz for a 50Hz supply or 120Hz for a 60Hz supply.)
Although, we can make use of 4 individual power diodes to make a full-wave bridge rectifier, premade bridge rectifier components are available ‘off the shelf’ in a range of different voltage & current sizes which can be soldered directly into a PCB circuit board or be interfaced by spade connectors.
The image above shows a typical single-phase bridge rectifier with 1 corner cut-off. This cut off corner indicates that the terminal that is nearest to the corner is +ve or positive output terminal or is lead with the opposite (diagonal) lead being -ve or negative output lead. The other 2 connecting leads are for the input alternating-voltage from a trans-former secondary winding.
The Smoothing Capacitor
In the previous section it is clear that the single-phase half wave rectifier produces an output wave every half-cycle & it wasn’t practical to bring this type of circuit into play to generate a stream line DC supply. The full wave bridge rectifier then provides us a greater mean DC value (0.637 Vmax) with the less superimposed ripple while output wave-form is 2 times that of the input supply’s frequency. Therefore, we can increase its average DC-output level even higher by connecting a suitable smoothing capacitor across the output of the bridge-circuit as below.
Full wave Rectifier with Smoothing Capacitor
The smoothing capacitor converts the full wave rippled output of the rectifier into a smooth DC-output voltage. Commonly, for DC power-supply circuits the smoothing capacitor is an ‘Aluminium Electrolytic’ type that has a capacitance value of 100 uF or more with repeated DC voltage pulses from the rectifier charging up the capacitor to peak-voltage.
However, their are two important para-meters to consider when choosing a suitable smoothing capacitor & these are its working voltage, which should be higher than the no load output value of rectifier & its capacitance value, which tells the amount of ripple that will appear superimposed on top of the DC-voltage.
A capacitance & the capacitor having too low value has little effect on the output wave-form. But if the smoothing capacitor is larger enough (parallel capacitors can be used) & the load current isn’t too large, the output voltage will be almost as smooth as pure DC. As a general rule of thumb, we are looking to have a ripple voltage of less than 100mV peak-to-peak.
The maximum ripple-voltage present for a Full-Wave Rectifier circuit isn’t only determined by the value of the smoothing capacitor but also by the frequency & load current, & is calculated as given blow:
Bridge Rectifier Ripple Voltage
Where: I is the DC load current in amperes, ƒ is the frequency of the ripple or two times the input frequency in Hertz, & C is the capacitance in Farads (F).
The main merits of a full-wave bridge rectifier is that it has a smaller AC ripple value for a given load & a smaller reservoir or smoothing capacitor than an equivalent half wave rectifier. Therefore, the fundamental frequency of the ripple-voltage is 2 times the AC supply’s frequency (100Hz) where for the half wave rectifier it is exactly equal to the supply’s frequency (50Hz).
Read>>Circuit Diagram Of High Power Audio Amplifier
The amount of ripple-voltage that is superimposed on top of the DC-supply’s voltage by the diodes can be virtually eliminated by adding an improved π-filter (pi filter) to the bridge rectifier’s output terminals. This type of low-pass filter consists of two smoothing capacitors, commonly of the same value & a choke or inductance across them to introduce a high impedance path to the alternating ripple component.
Another more practical & cheaper alternative is to utilize an off-the-shelf (3-terminal) voltage regulator IC, such as a LM78xx (where ‘xx’ represents the output voltage rating) for a +ve output-voltage or its inverse equivalent the LM79xx for a -ve output-voltage which can deprive the ripple by more than 70dB (Datasheet) while delivering a constant output-current of over 1 amp.
