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.