Explanation of Experiment 8
Diode Rectifier Circuits
Diode Rectification:
A rectifier is an electrical device that converts alternating current (AC) to direct current (DC), a
process known as rectification. A device which performs the opposite function (converting DC to AC)
is known as an inverter.
When only one diode is used to rectify AC (by blocking the negative or positive portion of the
waveform), the difference between the term diode and the term rectifier is merely one of usage,
i.e., the term rectifier describes a diode that is being used to convert AC to DC. Almost all rectifiers
comprise a number of diodes in a specific arrangement for more efficiently converting AC to DC than
is possible with only one diode.
1- Half-wave rectification:
In half wave rectification, either the positive or negative half of the AC wave is passed, while
the other half is blocked. Because only one half of the input waveform reaches the output, it
is very inefficient if used for power transfer. Half-wave rectification can be achieved with a
single diode in a one-phase supply, or with three diodes in a three-phase supply.
A- Forward biased diode rectification:
B- Reverse biased diode rectification:
The output DC voltage of a half wave rectifier can be calculated with the following two ideal
equations.
2- Full-wave rectification:
A full-wave rectifier converts the whole of the input waveform to one of constant polarity
(positive or negative) at its output. Full-wave rectification converts both polarities of the
input waveform to DC (direct current), and is more efficient. However, in a circuit with a
non-centre tapped transformer, four diodes are required instead of the one needed for halfwave rectification. Four rectifiers arranged this way are called a diode bridge or bridge
rectifier:
The average and root-mean-square output voltages of an ideal full wave rectifier can be
calculated as:
Where:
VDC,VAVG - the DC or average output voltage,
VP - the peak value of half wave,
VRMS - the root-mean-square value of output voltage.
π = ~ 3.14159
e = ~ 2.71828
Rectifier output smoothing:
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While half-wave and full-wave rectification suffice to deliver a form of DC output, neither produces
constant-voltage DC. In order to produce steady DC from a rectified AC supply, a smoothing circuit or
filter], is required. In its simplest form this can be just a reservoir capacitor or smoothing capacitor,
placed at the DC output of the rectifier. There will still remain an amount of AC ripple voltage where
the voltage is not completely smoothed.
A half-wave rectifier will only give one peak per cycle and for this and other reasons is only used in
very small power supplies. A full wave rectifier achieves two peaks per cycle and this is the best that
can be done with single-phase input.
Time –Domain Ripple
Ripple voltage is commonly expressed as the peak-to-peak value. This is largely because peak-topeak is both easier to measure on an oscilloscope and is simpler to calculate theoretically. Filter
circuits intended for the reduction of ripple are usually called smoothing circuits.
The simplest scenario in ac to dc conversion is a rectifier without any smoothing circuitry at all. The
ripple voltage is very large in this situation; the peak-to-peak ripple voltage is equal to the peak ac
voltage. A more common arrangement is to allow the rectifier to work into a large smoothing
capacitor which acts as a reservoir. After a peak in output voltage the capacitor (C) supplies the
current to the load (R) and continues to do so until the capacitor voltage has fallen to the value of
the now rising next half-cycle of rectified voltage. At that point the rectifiers turn on again and
deliver current to the reservoir until peak voltage is again reached. If the time constant, CR, is large
in comparison to the period of the ac waveform, then a reasonable accurate approximation can be
made by assuming that the capacitor voltage falls linearly. A further useful assumption can be made
if the ripple is small compared to the dc voltage. In this case the phase angle through which the
rectifiers conduct will be small and it can be assumed that the capacitor is discharging all the way
from one peak to the next with little loss of accuracy.
With the above assumptions the peak-to-peak
ripple voltage can be calculated as:
For a full-wave rectifier:
For a half-wave rectification:
where
VP-P is the peak-to-peak ripple voltage
I is the current in the circuit
f is the frequency of the ac power
C is the capacitance
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Ripple factor (γ) may be defined as the ratio of the input root mean square (RMS) value of the ripple
voltage to the absolute value of the dc component of the output voltage, usually expressed as a
percentage.
For the RMS value of the ripple voltage, the calculation is more involved as the shape of the ripple
waveform has a bearing on the result. Assuming a saw tooth waveform is a similar assumption to the
ones above and yields the result:
γ is the ripple factor
R is the resistance of the load
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