The full wave rectifier is used to transform from AC power sources to DC power supplies by connecting the power diodes to each other with a specific scheme.
In previous Power Diodes training, we discussed ways to reduce fluctuations or voltage changes in DC voltage directly by connecting softening capacitors along load resistance.
Although this method is suitable for low-power applications, it is not suitable for applications that need a "constant and uniform" DC supply voltage.One method of improving this is to use each half cycle of the input voltage instead of every other half loop.The circuit that allows us to do this is called the Full Wave Rectifier.
Like a half-wave circuit, a full wave rectifier circuit produces an output voltage or current that is completely DC or has a specific DC component.Full wave rectifiers have some basic advantages over their half-wave rectifier counterparts.The average (DC) output voltage is higher than half a wave, the output of the full wave rectifier has much less fluctuation than that of the half-wave rectifier, which produces a smoother output waveform.
In one Full Wave Rectifier circuit, two diodes are used, one for each half of the cycle.A multi-winding transformer is used, the secondary winding is divided into two halves on par with a common center gear connection (C).This configuration is 100% efficient, as shown below, by producing an output during both half cycles, when the anode terminal of each diode is positive relative to transformer center point C.
The full wave rectifier circuit consists of two power diodes connected to a single load resistance (R L), and each diode takes it in turn to provide current to the load.If point A is positive relative to point C, the arrow-direction current starts flowing from the D1 diode.
When point B is positive relative to point C, the D2 diode begins to carry forward current. Since the output voltage on resistance R is the phaser sum of the two waveforms combined, this type of full wave rectifier circuit is also known as a "two-phase" circuit, i.e. bi-phase.
Since the gap between each half wave developed by each diode is filled by the other diode, the average DC output voltage during load resistance is now almost twice that of a single half-wave rectifier circuit, assuming there is no loss, peak voltage is a maximum of 0.637V.
Where: V MAX is the maximum peak value in one half of the secondary winding, and V RMS is the rms value.
The peak voltage of the output waveform is the same as before for the half-wave rectifier, provided that each half of the transformer windings has the same rms voltage value.Different transformer ratios can be used to achieve a different DC voltage output.
The main disadvantage of this type of full wave rectifier circuit is that a larger transformer needs two separate but identical secondary windings for a given power output, making this type of full wave rectifier circuit costly compared to the "Full Wave Bridge Rectifier" circuit equivalent.
Full Wave Bridge Rectifier
Another type of circuit that produces the same output waveform as the full wave rectifier circuit above is the Full Wave Bridge Rectifier circuit.This type of single-phase rectifier uses four separate rectifier diodes connected in the closed loop "bridge" configuration to produce the desired output.
The main advantage of this bridge circuit is that it does not require a special center gear transformer, thereby reducing its size and cost.Single secondary winding, the load is connected to one side of the diode bridge network and the load to the other side, as shown below. Often many devices, adapters, circuits are used.
The four diodes labeled D1 to D4 are arranged in "serial pairs", through which only two diodes will transmit current during each half cycle.During the positive half cycle of feeding, the D1 and D2 diodes move in series, while the D3 and D4 diodes are inverted and the current flows over the load, as shown below.
During the negative half cycle of feeding, the D3 and D4 diodes work serially, but the D1 and D2 diodes now switch to the "OFF" state because they are inverted.The current passing through the load is in the same direction as before.
Since the current flowing along the load is one-way, the voltage developed throughout the load is also one-way, in the same way as the previous two diode full wave rectifiers, so the average DC voltage during the load is a maximum of 0.637V.
But in reality, during each half cycle, the current flows through two diodes instead of just one diode, so the amplitude of the output voltage is less than two voltage drops (2*0.7 = 1.4V) than the input V MAX amplitude.The surge frequency is now twice the feed frequency (for example, 100 Hz for 50 Hz feed or 120 Hz for 60 Hz feed.)
Although we can use four separate power diodes to make a full wave bridge rectifier, pre-made bridge rectifier components are available "ready for use" in a range of different voltage and current sizes that can be soldered directly to a PCB circuit board. or can be connected with shovel connectors.
The image at the top shows a typical single-phase bridge rectifier with one corner cut off.This cutting corner indicates that the terminal closest to the corner is positive or +and the output terminal or end, and the opposite (diagonal) tip is negative or -and output end.The other two connection cables are for alternative voltage input from a transformer secondary winding.
In the previous section, we found that the single-phase half-wave rectifier produces an output wave every half cycle, and it is impractical to use this type of circuit to produce a fixed DC source.However, the full wave bridge rectifier gives us a larger average DC value (0.637 Vmax) with less overlayed fluctuation, while the output waveform is twice the frequency of the input feed frequency.
We can reduce the AC variation of the straightened output by using softening capacitors to filter the output waveform while improving the average DC output of the rectifier.Softening or reservoir that connects parallel to the load along the output of the full wave bridge rectifier circuitcapacitors further increase the average DC output level because they act like a storage device, as shown below.
The softening capacitor converts the full wavelength output of the rectifier to a smoother DC output voltage.
5uF Softening Condenser
The blue drawing in the waveform shows the result of using a 5.0 uF softening capacitor along the rectifier output.Previously, the load voltage followed the corrected output waveform up to zero volts.Here 5uFthe capacitor output is loaded into the peak voltage of the DC pulse, but when it drops from peak voltage to zero volts, due to the RC time constant of the circuitthe capacitor can not discharge that fast.
Thisresults in the capacitor dropping to about 3.6 volts, in this example, the capacitor maintains the voltage throughout the load resistance until recharged at the next positive inclination of the DC pulse.In other words, the capacitor only has time to ejaculate briefly before the next DC pulse recharges it to peak value.Therefore, the DC voltage applied to load resistance drops only a small amount.However, smoothing as shownwe can improve this situation even further by increasing the value of its capacitor.
50uF Softening Condenser
Here we increased the value of the softening capacitor tenfold from 5uF to 50uF, which reduced the fluctuation and increased the minimum discharge voltage to 7.9 volts from the previous 3.6 volts.However, we chose a load of 1kΩ to achieve these values in the graph, but as the load impedance decreases, the load current increases, causing the capacitor to discharge faster between charging pulses.
The effect of providing a heavy load with a single softening or reservoir capacitor can be reduced by using a larger capacitor that stores more energy and discharges less between charging pulses.Usually for DC power supply circuits, the softening capacitor is an Aluminum Electrolytic type with a capacitance value of 100 uF or more with repeated DC voltage pulses from the redresser charging the capacitor to peak voltage.
However, there are two important parameters to consider when choosing an appropriate softening capacitor, and these are the Capacitance Value, which determines the Operating Voltage and the amount of fluctuation that will occur, which should be higher than the loadless output value of the rectifier.
It has a very low capacitance value and very little effect on the output waveform of the capacitor.But if the softening capacitor is large enough (parallel capacitors can be used) and the load current is not too large, the output voltage will be almost as smooth as the pure DC.As a general rule, we want to have a surge voltage of less than 100mV from top to top.
The maximum current surge voltage for a Full Wave Rectifier circuit is determined not only by the value of the softening capacitor, but also by frequency and load current and calculated as follows:
Where: I is the DC load current in amps, ε is the frequency of fluctuation and is twice the input frequency in Hertz, and C is capacitance in Persian.
The main advantages of a full wave bridge rectifier are that it has a smaller AC surge value for a given load and a reservoir or softening capacitor smaller than an equivalent half-wave rectifier.Therefore, the basic frequency of the surge voltage is exactly equal to the feed frequency (50Hz) for the half-wave rectifier, while it is twice the AC feed frequency (100Hz).
The amount of surge voltage placed on top of the DC supply voltage by diodes can be almost eliminated by adding a much more advanced π filter (pi filter) to the bridge rectifier's output terminals.This type of low-pass filter consists of two softening capacitors with the same value and an inductanc between them, usually to provide a high impedance path to the alternative surge component.
In the next tutorial on diodes, we will look at Zener Diode, which uses the reverse fault voltage feature to produce a constant output voltage in itself.