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Hi could any one explain how this circuit works... is the transistor here used to sink any extra voltage that may appear at the output... i.e if there is any extra voltage the op-amp turns on the transistor to sink the current... or does it have any voltage regulation function...
This is the adjustable shunt voltage regulator. The necessary output voltage is obtained by adjusting the resistor in parallel with zener.
The output voltage is compared with voltage reference by the op amp, which in turn will give the higher voltage at TIP31 transistor base when the output voltage goes higher than reference due to any reason (and vice versa when output voltage goes low). Transistor will conduct the higher current that makes the dropout voltage on R1 higher. The result is output voltage is maintained constantly.
3 diodes in transistor base circuit to make sure the transistor will be shutdown entirely when the opamp output goes lower than about 1.5V.
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