This power supply circuit has a three-pin voltage regulator IC along with a buffer transistor. This high current 78XX regulator is developed because in its natural form these regulators cannot produce more than 1 Amp current.
But once it is added with a transistor its current capacity increases to great levels as high as 5 amp or even 10 amp.
How the Circuit Works
In this whole setting or combination, when the current surpasses over 200 mA, the buffer transistor will take over the task and allow currents up to 5 A.
The 78xx Integrated Circuit allows a range of voltages that are 5 V, 6 V, 8 V, 9V, 12V and up to 24 V. If you choose the required regulator in the series, the circuit can be adapted for any voltage. The only required rule here is that the transformer output voltage is supposed to be 4 V greater than the stabilized output voltage.
The transistor T1 and resistor R1 are omitted in any situation where a current of only 1 A or less is required. Though, R1 is retainable in the circuit to safeguard IC and functions as a “bleed” resistor. In such a case though, the rating is increases from 0.5W to 5W.
Even though the 78XX regulator is protected internally against overheating, the demand on this protection is not beneficial in reality. For a stable design, two measures are taken in the circuit shown: The buffer transistor always has a more apt current rating. And the second is that the current does not exceed 300 mA other than when there is a short circuit on the output.
Short Circuit protection
Both the buffer transistor and the high current voltage regulator can survive short circuits to the maximum of output current of 5 A provided the heatsink used is sufficiently large. As the voltage regulator limits the amount base drive current to transistor T1, the actual amount of short-current circuit gets limited.
Capacitor C1 smoothens any AC ripple that might occur, however, its value should be modifies to cope with the maximum flow of current possible. For reference, a current flow of 1A or less, a value of 1000 u F are sufficient, but for a current of 5 A the value needs to get elevated to 4700 u F.