Full Wave Rectifier
The Full Wave Rectifier
In the previous Power Diodes
tutorial we discussed ways of reducing the ripple or voltage variations on a direct DC voltage by connecting capacitors
across the load resistance. While this method may be suitable for low power applications it is unsuitable to applications
which need a "steady and smooth" DC supply voltage. One method to improve on this is to use every half-cycle of the input
voltage instead of every other half-cycle. The circuit which allows us to do this is called a Full Wave Rectifier.
Like the half wave circuit, a full wave rectifier circuit produces an output voltage or current which is
purely DC or has some specified DC component. Full wave rectifiers have some fundamental advantages over their half wave rectifier
counterparts. The average (DC) output voltage is higher than for half wave, the output of the full wave rectifier has much
less ripple than that of the half wave rectifier producing a smoother output waveform.
In a Full Wave Rectifier circuit two diodes are now used, one for each half of the cycle.
A multiple winding transformer
is used whose secondary winding is split equally into two halves with a common centre tapped connection,
(C). This configuration results in each diode conducting in turn when its anode terminal is positive
with respect to the transformer centre point C producing an output during both half-cycles, twice
that for the half wave rectifier so it is 100% efficient as shown below.
Full Wave Rectifier Circuit
The full wave rectifier circuit consists of two power diodes connected to a single load
resistance (RL) with each diode taking it in turn to supply current to the load. When
point A of the transformer is positive with respect to point C, diode
D1 conducts in the forward direction as indicated by the arrows.
When point B is positive (in the negative half of the cycle) with respect to
point C, diode D2 conducts in the forward direction and the
current flowing through resistor R is in the same direction for both half-cycles. As the output
voltage across the resistor R is the phasor sum of the two waveforms combined, this type of full
wave rectifier circuit is also known as a "bi-phase" circuit.
As the spaces between each half-wave developed by each diode is now being filled in by the other diode
the average DC output voltage across the load resistor is now double that of the single half-wave rectifier circuit and
is about 0.637Vmax of the peak voltage, assuming no losses.
Where: VMAX is the maximum peak AC voltage in one half of the
secondary winding and one of the diodes, and VRMS is the corresponding rms value.
The peak voltage of the output waveform is the same as before for the half-wave rectifier provided
each half of the transformer windings have the same rms voltage value. To obtain a different DC voltage output different
transformer ratios can be used. The main disadvantage of this type of full wave rectifier circuit is that a larger
transformer for a given power output is required with two separate but identical secondary windings making this type
of full wave rectifying circuit costly compared to the "Full Wave Bridge Rectifier" circuit equivalent.
The Full Wave Bridge Rectifier
Another type of circuit that produces the same output waveform as the full wave rectifier circuit above,
is that of the Full Wave Bridge Rectifier. This type of single phase rectifier uses four individual
rectifying diodes connected in a closed loop "bridge" configuration to produce the desired output. The main advantage of this
bridge circuit is that it does not require a special centre tapped transformer, thereby reducing its size and cost. The single
secondary winding is connected to one side of the diode bridge network and the load to the other side as shown below.
The Diode Bridge Rectifier
The four diodes labelled D1 to D4
are arranged in "series pairs" with only two diodes conducting current during each half cycle. During the positive half
cycle of the supply, diodes D1 and D2 conduct in series while diodes
D3 and D4 are reverse biased and the current flows through the load as shown below.
The Positive Half-cycle
During the negative half cycle of the supply, diodes D3 and D4
conduct in series, but diodes D1 and D2 switch "OFF" as they are now reverse biased.
The current flowing through the load is the same direction as before.
The Negative Half-cycle
As the current flowing through the load is unidirectional, so the voltage developed across the load
is also unidirectional the same as for the previous two diode full-wave rectifier, therefore the average DC voltage across
the load is 0.637Vmax. However in reality, during each half cycle the current flows through
two diodes instead of just one so the amplitude of the output voltage is two voltage drops ( 2 x 0.7 = 1.4V ) less than the input
VMAX amplitude. The ripple frequency is now twice the supply frequency (e.g. 100Hz for a 50Hz supply)
Typical Bridge Rectifier
Although we can use four individual power diodes to make a full wave bridge rectifier, pre-made bridge
rectifier components are available "off-the-shelf" in a range of different voltage and current sizes that can be soldered
directly into a PCB circuit board or be connected by spade connectors.
The image to the right shows a typical single phase bridge rectifier with one corner cut off. This
cut-off corner indicates that the terminal nearest to the corner is the positive or +ve output
terminal or lead with the opposite (diagonal) lead being the negative or -ve output lead. The
other two connecting leads are for the input alternating voltage from a transformer secondary winding.
The Smoothing Capacitor
We saw in the previous section that the single phase half-wave rectifier produces an output wave every
half cycle and that it was not practical to use this type of circuit to produce a steady DC supply. The full-wave
bridge rectifier however, gives us a greater mean DC value (0.637 Vmax) with less superimposed ripple while the output
waveform is twice that of the frequency of the input supply frequency. We can therefore increase its average DC output
level even higher by connecting a suitable smoothing capacitor across the output of the bridge circuit as shown 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. Generally for DC power supply circuits the smoothing capacitor is an Aluminium Electrolytic type that
has a capacitance value of 100uF or more with repeated DC voltage pulses from the rectifier charging up the capacitor
to peak voltage. However, their are two important parameters to consider when choosing a suitable smoothing capacitor
and these are its Working Voltage, which must be higher than the no-load output value of the rectifier and its
Capacitance Value, which determines the amount of ripple that will appear superimposed on top of the DC voltage.
Too low a capacitance value and the capacitor has little effect on the output waveform. But if the
smoothing capacitor is sufficiently large enough (parallel capacitors can be used) and the load current is not 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 is not only
determined by the value of the smoothing capacitor but by the frequency and load current, and is calculated as:
Bridge Rectifier Ripple Voltage
Where: I is the DC load current in amps, ƒ
is the frequency of the ripple or twice the input frequency in Hertz, and C is the capacitance
The main advantages of a full-wave bridge rectifier is that it has a smaller AC ripple value for a
given load and a smaller reservoir or smoothing capacitor than an equivalent half-wave rectifier. Therefore, the fundamental
frequency of the ripple voltage is twice that of the AC supply frequency (100Hz) where for the half-wave rectifier it
is exactly equal to the supply frequency (50Hz).
The amount of ripple voltage that is superimposed on top of the DC supply voltage by the diodes can be
virtually eliminated by adding a much improved π-filter (pi-filter) to the output terminals
of the bridge rectifier. This type of low-pass filter consists of two smoothing capacitors, usually of the same value and
a choke or inductance across them to introduce a high impedance path to the alternating ripple component.
Another more practical and cheaper alternative is to use an off the shelf 3-terminal voltage regulator
IC, such as a LM78xx (where "xx" stands for the output voltage rating) for a positive output voltage
or its inverse equivalent the LM79xx for a negative output voltage which can reduce the ripple by
more than 70dB (Datasheet) while delivering a constant output current of over 1 amp.
In the next tutorial about diodes, we will look at the
Zener Diode which takes advantage of
its reverse breakdown voltage characteristic to produce a constant and fixed output voltage across itself.