In normal conditions I would have 30V (and -30 and the othre rail) and 25 mA to 50 mA. These power rails will be used to make the symetric supply of an OpAmp. It's rated for 10 to 45 volts for its +vcc pin (and -10 to -45 and the -vcc pin). A great plus would be to have a rail openned if a fault is detected on the other one.
The idea is: when the voltage "In" dropes, the current in Rsense will drop too and thus the BJT will pass in "OFF" mode because the current in its base is too low. Is my thinking correct?
I'm looking for a solution that can be implemented on a postiive voltage rail and on a negative one for a symetric voltage supply. If you have a better idea (that works for postive and negative voltages). It doesn"t have to be only with BJTs. I'm all ears!
EDIT 20/04: so I did what Math Keeps Me Busy suggested. And it works:
I was wondering if it was possible to have a more snappy response when the 10V are reached?
The goal of this thing is to make a split (+/-30V) supply for an OpAmp. The OpAmp is rated for 10 V to 45 V on +Vcc pin and -45 to -10 V on -Vcc pin. It has to be able to handle votlages has high as +/-60V. Q1 is a pre regulator. LM317 is the voltage regualtor. Q2 is for current regualtion in case of a short. Q6 and Q5 are for the Undervoltage Lockout (if that's the correct name). The 1200 Ohm resistor acts like a 25mA load.
EDIT: just to clarify. In the end, I don't need this UVLO for my circuit. I missunderstood the opamp's specs. Voltage under 10 V isn't going to damage it. But the solution proposed by Math Keep Me Busy does work.