I am making a simple 1.25v to 12v adjustable voltage regulator capable of 3 amps. The input is 18v DC. I came across a circuit (below) but I have a question about the resistor between the base and emitter of the PNP pass transistor.
As I see it, the 317T will be supplying the power to the load until the current through the 33 ohm resistor exceeds 21mA (assuming a .7 Vbe on the PNP). At that point the voltage drop across the 33 ohm resistor will be enough to turn the transistor on and do the work. However, I happen to have a TIP42 PNP transistor with the following specs: Ic=6A, Ib=2A, hfe (when Ic=3A) is 15 minimum. Assuming the min current gain of 15, that would make the maximum current allowed from the pass transistor to be 318mA correct? (21mA x 15) This is not even close to 3A.
So, I was planning to use a 3 ohm resistor in place of the 33 ohm resistor. That value would require 233mA of current to flow through it before it develops enough of a voltage drop (.7v) to turn on the pass transistor. Then, 233mA times the current gain of 15 would allow for 3.5A from the pass transistor.
I would like to know if I am correct in my assumptions of how the PNP pass transistor is biased and being used. Also, are the adjustment resistors correct for the output voltages I am looking for?
It's been a while since I've worked with electronics and want to be sure I'm calculating correctly.
I've already taken into account the maximum heat dissipated across the 317T and the pass transistor and have heat sinked them accordingly, and the device tabs have been insulated from the heatsink. The 3 ohm resistor is a bit overkill at 5W, but it's what I have. The adjustment resistors are a 1/4W 470 ohm, and a 5k pot in parallel with 33k ohm for a 4k ohm pot.
**broken link removed**
Thanks in advance for your assistance, and sorry if this is a simple question...