Power Supply Card of Float Charger 150A
Figure 1. The region of interest.


On power-up all transistors are off. \$ V_c \$ = 0.
As the input voltage rises \$ V_a \$ will rise and TR1 will begin to turn on. \$ V_b \$ will rise 0.7 of a volt "behind" \$ V_a \$ because TR1 is wired as an emitter-follower.
Similarly \$ V_c \$, the output voltage, will follow 0.7 V behind \$ V_b \$. Note that R7 limits the current in TR1 to a safe value.
As the output voltage rises R9 and ZD1 will form a voltage regulator holding TR2's emitter at the voltage determined by ZD1. Note that there is no junction at 'e' on the schematic. It is a cross-over.
As the output voltage rises further \$ V_f \$ rises and TR2 starts to turn on. This steals the bias for TR1 and holds \$ V_a \$ slightly above the Zener voltage - maybe a volt or so.


The output voltage is now stable. If it tends to rise TR2 turns on more pulling it down. If the voltage falls due to increasing load TR2 turns off a little allowing more bias into TR1 turning on the big fellow some more to raise the output voltage.