Environmental Engineering Reference
In-Depth Information
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Supply
Load
Figure 4.22
Circuit of a single -phase diode bridge rectifi er
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Figure 4.23
Waveforms in a diode bridge rectifi er connected to a purely resistive load
4.5.3 Diode Bridge Rectifi er
In Figure 4.22 alternating current power is fed from the supply to the diode junctions. It is
rectifi ed by the four diodes that form the bridge, and the resulting DC power is supplied to
the load on the right. Thus, the circuit converts the AC power to DC. Assuming, for simplic-
ity, that the load is purely resistive, the resulting waveforms are shown in Figure 4.23.
The voltage supplied to the load is DC, in that it is always positive, but it is far from smooth
because it contains a considerable
ripple
. Very few practical loads would tolerate a supply
like this except those, e.g. heating resistors, that would be equally happy on an AC supply.
For this reason, it is common to add a smoothing capacitor as shown in Figure 4.24.
The waveforms in Figure 4.25 show that, as before, the diode bridge is rectifying AC power
to DC. Now, however, the voltage supplied to the load
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is almost constant; a bigger capacitor
would reduce the ripple even further. The capacitor acts as an energy store so that when the
output voltage from the rectifi er is lower than the voltage across the capacitor, energy is sup-
plied to the load from the capacitor. When the output voltage from the rectifi er is larger than
the capacitor voltage the capacitor is charged from the supply. This charging effect is provided
