Here is another adjustable, stabilized power supply unit which is based on LM317T. An additional PNP transistor is used to boost up power performance. The unit is able to supply up to 3A, and output voltage is continuously adjustable between 1.2 -25V.
The circuit is extremely simple and has proven reliable. However, there isn’t any current limiting section and there is no short circuit protection. Only the LM317T is internally protected.
The circuit is an AC to DC linear converter, which absorbs power from a conventional transformer (not showed in the schematic). Maximum AC voltage at the input is about 28V rms, and you must not exceed this value. Ideally, a transformer with secondary winding voltage of 24V and nominal current capacity of about 3-4A, will be sufficient.
Both the transistor and the regulator must mount onto a large thermal radiator. As in any linear regulator, keeping voltage difference between input and output as low as possible allows obtaining lower power losses and better power efficiency.
On the prototype, we use a BD240 transistor. However you may also use BD244, BD282, BDX54 or another equivalent type. There are two different positions for placing the transistor on the pcb. This is because different transistor types have different pinouts. Thus, depending on the transistor you choose to use, you should select the appropriate slot to place it. To make sure you chose the right one, check the transistor information booklet.
The Printed Circuit Board for the 1-25V/ 3A Power Supply Unit