182 Electrical Power Systems Technology
Single-phase Rectification Systems
The simplest system for converting AC to DC is single-phase rectifica-
tion. A single-phase rectifier changes AC to pulsating DC. The most com-
mon and economical method is the use of low-cost, silicon, semiconductor
rectifiers.
Single-phase Half-wave Rectification
A single-phase half-wave rectifier circuit, such as shown in Figure 7-20,
converts an AC source voltage into a pulsating direct current. Let us as-
sume that during the positive alternation of the AC cycle, the anode of
the diode is positive (Figure 7-20). The diode will then conduct, since it is
forward biased and the pn junction is low-resistant. The positive half cycle
of the alternating current will then appear across the load device (repre-
sented by a resistor). When the negative portion of the AC alternation is
input to the circuit, the anode of the diode becomes negative. The diode
is now reverse biased, and no significant current will flow through the load
device (Figure 7-20B). Therefore, there will be no voltage across the load.
The input and resulting output waveforms of the half-wave rectifier circuit
are shown in Figure 7-20C. The pulsating direct current of the output has
an average DC level. The average value of the pulsating DC produced by
single-phase half-wave rectification is expressed as:
Vd = 0.318 × Vmax
where:
Vdc = the average value of rectified voltage, and
Vmax = the peak (maximum) value of applied AC voltage.
Sample Problem:
Given: a single-phase, half-wave rectifier has 15 volts (rms) applied
to its input.
Find: DC output voltage of the rectifier.
Solution:
Vdc = 0.318 × Vmax
= 0.318 × (15 V × 1.41)
Vdc = 6.72 volts
Certain diode ratings should be considered for half-wave rectifier
circuits. The maximum forward current (Imax) the largest current that can is
flow through the diode while it is forward biased, without damaging the