This voltage regulator circuit diagram can deliver up to 3A at 12V output voltage. it can replace the regulator IC type 7812. The circuit can be employed on occasions when a current of more that 3A is demanded for regulator. IC regulators of such high current rating are pretty hard to find.

The transformer T1 steps down mains voltage, to 12rms & the rectifier bridge D1 rectifies it to produce a DC voltage. The C1 filters the rectifier output  and produces a  DC level. The series pass transistor Q1 (2N 3055) is biased by resistor R1 (680?). Since zener diode D1 is under breakdown region the voltage across it will be 12V. So the total output voltage will be steady 12.7 V(theoretically). That is the zener voltage plus base emitter voltage of Q1.Here transistor Q1 will conduct the excess current required .

If 12V zener is not available ,use the nearest value.The transformer T1 can be as 230V primary;15V/5A secondary  step down transformer. The capacitors must be rated at least 25V. By changing the value of the Zener diode, different output voltages can be obtained from the circuit.

12-v-current-regulator-circuit

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