Some circuits require the input voltage to be current-limited. This can be achieved by a simple circuit using two transistors and two resistors.
In this example we have an input voltage VBB_BURN of 59 Volts, that has to be limited to 100 mA before it goes to the socket (VBB_SOCKET). Initially, the transistor Q2 on the right blocks and current flows through the transistor Q1, which is in saturation mode.
The voltage UEB is around 0.7 Volts giving 58.3 Volts on the upper point of R49. The base current is therefore 59V / (R48 + R49).
Resistor R48 is the sense-resistor. If the voltage across it reaches around 0,7 Volts, Q2 will turn on which brings the base potential of Q1 to the same level as of the emitter. This stops the current through Q1 and the voltage across R48 will decrease again, until Q2 opens. This way, the circuit regulates itself at
I = 0.7 V / R48, flowing to VBB_SOCKET (at maximum).
Note: It is important that Q1 has a heat-sink, because if it goes out of saturation mode, it dissipates power, depending on the transistor type.