Circuit on the right is the control circuit that uses sourcing and sinking transistors to keep COMMON regulated at +12V.
The positive rail (#POS) is from 24V to #COMM which gives a p.d. of +12V and the negative rail (#NEG) is from 0V to #COMM which gives a p.d. of -12V.
Zener diodes are used to set a fixed reference voltage for the transistor activations.
#Lpos is the dummy load on the +12V rail.
#Lneg is the dummy load on the -12V rail.
