I am trying to design a current limiter or my LM317 based power supply using this schema.
The power supply must be able to provide a maximum of 1.5A@30V. I made some calculations and I would like to understand if I am on the right way. Goal is to find suitable values for the resistors and the right BJTs.
1) Q1: Assuming the power supply is working at its limit, on Q1 we will have a Ic=1.5A and Vce=Vcc-Vq2be=30-0.7=29.3V. A suitable BJT able to support this values could be the D44H8.
2) Value of R1: The D44H8 has a worst case current gain of 10. So, on Q1, if Ic=1.5A the Ib=1.5/10=0.15A. Being near the limit of 1.5A, we can assume Vq2be=VRsen=0.7V, thus: VR1 = 30-0.7-0.7=28.6. This lead to R1 = 28.6/0.15=190 Ohm
3) Value of Rsen: On Q1: Ic=1.5, Ib= 0.15, Ie=1.65 Voltage drop across Rsen is circa 0.7. Then: Rsen=0.7/1.65=0.42 Ohm
4) Q2: Q2 must be able to support at least 30-0.7-0.7=28.6V on its collector, but much smaller current. I choose a BJT in the BC337 family, which will guarantee 45Vce and a max collector current of 800mA
Keeping R1 constant and varying Rsen I can set different maximum current values on the load.
Am I more or less right?