This circuit was found in a 1967 General Electric manual. It is intended for the control of high voltage Nixie pipes used at the time. SCRs may be equivalent.
This circuit indicates whether the voltage to be measured is higher or lower than the zener diode...
Obtained from an English publication, this circuit actually needs a current limiting resistor,...
Constant current sources are used in applications such as battery chargers, electrolytic tanks, etc....