Resistor R1 only enhances temperature stability and can be eliminated if desired by connecting pins 5 and 6 of IC-1 together. You may use a front panel mounted 1K potentiometer for this purpose if desired. The only adjustment required is setting R9 to the desired output voltage of anywhere between 10 and 14 volts. Because this circuit samples the collector current of the pass transistors, thermal runaway is not a problem in this circuit making it a much more reliable power supply. The fault in that system is that the HFE of the pass transistors actually increases when the transistors heat up and risks a thermal runaway condition causing a possible failure of the pass transistors. To reduce costs, most commercial units rely on the HFE of the pass transistors to determine the current limiting. The LM723 limits the current when the voltage drop across R5 approaches. Resistor R9 allows the fine tuning of the voltage to exactly 13.8 volts and the resistor network formed by resistors R4 through R7 controls the current limiting. The circuit uses 3 pass transistors which must be heat sinked. Just like other commercial units, this circuit uses the LM723 IC which gives us excellent voltage regulation. The circuit even has a current limiting feature which is a more reliable system than most commercial units have. The circuit in Figure 1 will give us 10 amps (12 amps surge) with performance that equals or exceeds any commercial unit.

Of course rolling our own gives us the ability to customize the circuit and make it even more reliable than commercial units. It should be noted that there is no real cost advantage to building your own power supply unless you can get a large power transformer and heat sink for a super low price. The advantage to rolling your own power supply is that it teaches us how they work and makes it easier to troubleshoot and repair other power supply units in the shack. The output voltage is scheduled using the voltage divider (1K/560) to ensure that 1.7 volts can be ascribed to the 3904 base once the output is 5 volts.Sometimes amateurs like to home-brew their power supplies instead of purchasing one off the shelf at any of the major ham radio retail dealers.

This exact same current runs via the emitter resistor of the 2N3904 which will create around a 1 volt drop over the 62 ohm resistor as well as 1.7 volts on the base. The LM317 constantly works using a voltage variation of 1.2 between the output and adjustment terminals and demands a minimal load of 10mA, hence a 75 ohm resistor has been picked that will probably bring (1.2/75 = 16mA). The LM317 will supply more than 1 amp of current to push the bases of the external power transistors and with an approximate gain of TEN the combo ought to provide 15 amps or even more. The LM317 takes a few added volts on the input side, as well the emitter/base drop from the 3055s, and also no matter what is shed along the (0.1 ohm) equalling resistors (1volt at 10 amps), therefore a different transformer and rectifier/filter circuit is employed which adds some volts more than the output voltage. To get decent efficiency the voltage with the collectors of the a pair of parallel 2N3055 pass transistors needs to be near to the output voltage.

2n3055 pass transistor schematic