“In Electronic circuits powered by mains voltage, the input AC voltage must be converted into a DC voltage with sufficient stability. The easiest way to rectify AC voltage is to use a conventional semiconductor diode, a passive nonlinear electronic component whose property is to allow current to flow in one direction and the other. Figure 1 shows the schematic of a half-wave rectifier circuit, while Figure 2 shows a full-wave rectifier using a center-tapped transformer. Resistor RL simulates the presence of an output load, while VM represents the maximum voltage across each secondary winding of the transformer.
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Author: Editorial Staff
In electronic circuits powered by mains voltage, the input AC voltage must be converted into a DC voltage with sufficient stability. The easiest way to rectify AC voltage is to use a conventional semiconductor diode, a passive nonlinear electronic component whose property is to allow current to flow in one direction and the other. Figure 1 shows the schematic of a half-wave rectifier circuit, while Figure 2 shows a full-wave rectifier using a center-tapped transformer. Resistor RL simulates the presence of an output load, while VM represents the maximum voltage across each secondary winding of the transformer.
Figure 1: Basic half-wave rectifier circuit schematic
Figure 2: Basic full-wave rectifier circuit schematic
In the two configurations just shown, the peak voltage across the load is approximately equal to the peak voltage supplied by the transformer secondary winding. In the case of half-wave rectifiers in particular, the VCCDC output voltage is given by the following formula, where VMAX represents the peak value of the AC input voltage:
On the other hand, for a full-wave rectifier, the VCC voltage is given by the following equation, where VMAX now represents the peak value of each of the secondary windings of the two transformers:
Reduce ripple
For most applications, the output voltage produced by the above circuit has excessively high ripple. Conversely, for very simple applications (such as powering lights or controlling small motors) this is acceptable. By adding a filter capacitor after the rectifier diode, the output voltage waveform will be significantly improved, resulting in a significant reduction in ripple. The circuit in Figure 3 uses a center-tapped transformer and two rectifier diodes, while the circuit in Figure 4 uses a conventional transformer, which in a conventional bridge configuration has only one secondary winding and four rectifier diodes. Two schematics are commonly used to obtain DC voltage starting from an AC source.
Figure 3: Full-wave rectifier with center-tapped transformer
Figure 4: Full-wave rectifier with bridge rectifier diodes
output waveform
Figure 5 shows the effect of adding a filter capacitor to the half-wave rectifier circuit of Figure 1: As we can see, the output voltage is more regular and has a smoother trend. In the bCc section, with a linear trend, it is the filter capacitor that provides the charging current. As the current increases, the slope of this section becomes steeper and thus determines the location of point c on the positive half-wave. The lower the point c, the longer the diode conducts (corresponding to the period of cCd) and therefore the more ripple in the output voltage. During the period Cd associated with section c, the capacitor is charged. If the connected load requires high current, the capacitor will discharge very quickly, increasing the ripple. Therefore, for circuits requiring high power levels, solutions based on full-wave rectifiers are preferred.
Figure 5: Output waveform of a full-wave rectifier with filter capacitors
If the current drawn by the load is zero, the DC output voltage is equal to the peak value of the rectified AC voltage.
The maximum voltage ripple in a full-wave rectifier depends not only on the capacity of the filter capacitor, but also on the ripple frequency and load current:
where ILOAD(A) is the DC current absorbed by the load, f(Hz) is the ripple frequency, and C(farad) is the capacity of the filter capacitor.
