OK, so how do I calculate the resistor value for figure 23 on page 25 of this voltage regulator (L78S05C): http://www.st.com/internet/com/TECHNICAL_RESOURCES/TECHNICAL_LITERATURE/DATASHEET/CD00000449.pdf
Rsc = Vbe / Isc, where Isc = 500 ma. Unless I am mistaken, that is 2.4 Ohms.
The transistor listed in the datasheet is no longer produced (or I cannot find it), so I am having to find an equivalent part. Maybe I made a mistake on the transistor?
That is right, but you wouldn't use a darlington transistor for Q2 - just a conventional PNP transistor. You can use any PNP transistor as long as it can handle the absolute maximim current of the L78SXX family of regulators.
A non-darlington transistor will have a V
be of about 0.7V, so that means a 1.2 ohm resistor at 500mA.
Richard
Edit: The current limit in Fig 23 only limits the current in the bypass transistor, not the regulator. So it is a bit odd setting the current limit to 0.5A in the bypass transistor when the regulator can still pass 2A.
Looks like the datasheet does not set a maximum current, just a typical short circuit current of 3A. So I would pick a PNP transistor for Q2 with that will saturate to less then 0.5V at 6A with a base current of less then 0.3A.