I needed a simple LED indicator showing that a particular voltage level had been reached—specifically, that a capacitor had charged to a particular potential. I was convinced that there must be a way that a programmable zener diode, such as the TL431, could be used but I found no simple circuits.
My first thought was just to connect a series current limiting resistor and LED to the cathode so that the LED would light when the reference potential was reached through a divider from my monitored potential (Fig. 1). The shunt regulator would go low and light the LED.
However, the LED showed a preliminary glow prior to the reference potential being reached due to a small initial current draw by the TL431. The datasheet confirms this current, which is less than 0.5 mA but sufficient to light a high-brightness LED to an unacceptable degree. The indication was not “sharp” enough for me.
I realized that I could provide a separate path, from a higher potential, to supply this “leakage” current so that the point at which the LED lit up was sharp and clear (Fig. 2).
With the circuit values shown, R2 will drop a bit less than 5 V before D3 turns on. Prior to turn on, D2 remains back-biased and no current flows through the LED (D1). 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. The potential at the TL431 cathode will drop as low as a volt or two. The TL431 can sink upwards of 100 mA and lights the LED very well. The TL431 will turn on when the sensed potential reaches:
VSENSE = VREF(R3 + R4)/R4
which in this example is about 7.5 V.