Phase Control Circuit

  
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The most elementary application is a half-wave control circuit. The thyristor is acting both as a power control device and as a rectifier, providing variable power to the load !luring the positive half cycle and no power to the load during the negative half cycle. The circuit is designed to be a two terminal control which can be inserted in place of a switch. If full-wave power· is desired as the upper extreme of this control, a switch can be added which will short circuit the SCR when R r is turned to its maximum power position.
Phase Control Circuit - schematic

Full-wave control might be realized by the addition of a bridge rectifier, a pulse transformer, and by changing the thyristor from an SCR to a TRIAC, shown in Fig 50-9b. In this circuit, R 81 is not necessary since the pulse transformer isolates the thyristor gate from the steady-state UJT current. Occasionally, a circuit is required to provide constant output voltage regardless of line voltage changes. Adding potentiometer Pl to the circuits will provide an approximate solution to this problem. The potentiometer is adjusted to pro· vide reasonably constant output over the desired range of line voltage.





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