Simple Threshold Switch For An LED Indicator

Texas Instruments TL431

Robert H. Russell

Electronic Design

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.

Connecting a series current-limiting resistor and LED to a cathode caused a preliminary glow prior to the reference potential being reached. Though that current is enough for the LED indication, it didn't have the desired sharpness.
Figure 1. Connecting a series current-limiting resistor and LED to a
cathode caused a preliminary glow prior to the reference
potential being reached. Though that current is enough for
the LED indication, it didn't have the desired sharpness.

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).

Connecting a series current-limiting resistor and LED to a cathode caused a preliminary glow prior to the reference potential being reached. Though that current is enough for the LED indication, it didn't have the desired sharpness.
Figure 2. Adding a separate path from a higher potential delivered
a sharp and clear LED indication.

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:

which in this example is about 7.5 V.

Materials on the topic

  1. Datasheet Texas Instruments TL431

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