im trying to figure out how this works.

i have taken this circuit from the LM317 data sheet and just deleted/changed some of the details, i tried to make this work but i dont think the transistors are switching on, as the 78L12ACZ was starting to get warm.

in the end i want to have: Vin = 20-30V, Vout = 12V, to supply a maximum of 1A of Current

The 78L12ACZ has a maximum output current of 100mA.

The BC556 (PNP) has a max current of 100mA (Ic) and a gain of 800

The TIP120 has a max current of 5A and a gain of 1000.

before using 20-30v, i tried this on a

**18V battery supply**, the voltage regulator started to get warm.

what values i chose and how a calculated the values.

Ra resistor was 70ohms ( wanted the 78L12ACZ to have a maxium current flow of around 10mA, the voltage drop of across the resistor would be 0.7v)

Rb resistor was 13.8Mohms ( base of BC556 restricted to 1.25uV with a voltage drop of 17.3V to give a gain of 800, 1mA)

The 500ohm, i wasnt sure about, so i left it as it is.

Im not 100% sure on the full working of this circuit, my understanding is:

the Ra resistor will delevop a voltage drop of 0.7V when the 78L12ACZ starts to draw 10mA, this will give the base of the

BC556 the 0.7v drop and so Rb will have the rest of the voltage (17.3V), to restrict the current to 1.25uA the 13.8M resistor is used (17.3/1.25uA = 13.8M), As the base current comes out of a PNP transistor. The BC556 will amplify the 1.25uA to 1mA (gain of 800) and so the TIP 120 will amplify the 1mA to 1A (gain of 1000).

typing this out i have neglected the 500ohm resistor, if 1mA is flowing then, 1mA * 500ohm = 0.5V, not enought for the TIP120, is this my only mistake?

can some one point out any mistakes i have made and help me to understand how to get circuit working correcly.