When the potential at the reference junction of D3 reaches the reference, about 2.495 V, D3 begins to function as a shunt regulator sinking current at its cathode and turning on the LED through D2.

This time it’s zener diodes. Page three or four of every ‘beginners guide to electronics’ covers a diode as, “a component that only allows current to flow in one direction.” ...

The targeted nature of this process offers a far greater measure of control over the zener breakdown voltage (VZ), and greatly reduces other variables that can occur during the process. Most zener ...