Bridge rectifier with two thyristors and two diodes

A single-phase bridge can be made using two thyristors and two diodes. Figure 4 shows an example of such a circuit.

This circuit offers the same control as four-thyristor bridge. In addition, the output voltage never becomes negative, even when the load is inductive. This is because the two diodes provide a free-wheeling path: they conduct while the energy stored in the inductor is released. This circuit operates as the circuit of Figure 3, yet has one less component and requires only two thyristors.

Figure 5 shows another bridge with two thyristors and a free-wheeling diode. This circuit could operate without the free-wheeling diode. In this case, the free-wheeling path would change every half-cycle of the ac source voltage. Q₁ and D₁ would free-wheeling during part of one half-cycle, and Q₂ and D₂ during part of the second half-cycle.

Figure 4: Bridge rectifier with two thyristors on common ac line.

The free-wheeling D₃ is necessary, however, to ensure that the circuit be able to turn off an inductive load. Without this diode, when the gate pulses are stopped, the current may never drop to zero and one thyristor may continue to conduct. The free-wheeling diode also relieves the thyristors from free-wheeling duty, allowing the use of lower power thyristors.

Figure 5: Bridge rectifier using two common-cathode thyristors.

One advantage of the circuit in Figure 5 is that the firing control circuit can be simplified since the cathodes of the two thyristors are at a common potential. Both the circuit of Figure 4 and that of Figure 5 are of lower cost than a four-thyristor bridge of comparable power, nad both allow control of power from 0 to 100%. Unlike the four-thyristor bridge, however, these bridge cannot be used in the inverter mode, which is explained in the next section.

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