TL431A (TO-92) is an Adjustable Precision Zener (Shunt Regulator).It’s output voltage can be set to any value between 2.5V and 36V with the use of two external resistors – acting as a voltage divider (or alternatively a single trim-pot can be used instead) With a very sharp turn-on characteristics, the TL431 is an excellent alternative to zener diodes in many applications.
this low-cost IC is a type of a Linear Regulator that affords a nice variety of applications. (examples below)
A Simple Battery Monitor with TL431
this simple circuit with LED can give a visual indication of the state of charge.
Let’s say we have a 12V battery, and we want an LED to be on whenever the battery bank voltage is above 12.0V.
the R2 can be replaced in part by a variable resistor so that the full range of useful low voltage warning values is available. Should you wish to calculate the resistor values precisely for various output voltages please refer to the formula in the datasheet.
This circuit can be easily adapted for any battery voltages between 2.5 and 36v with minor adjustments.
in this case the LED will turn ON after the battery voltage drops below a designated value:
Simple LED “Battery-Low” warning indicator.The cutoff point is set by the ratio of R2 and R3.
6V is appropriate for a 7 cell Nicad or 2 cell Li-ion pack,
7.2V for an 8 cell Nicad,
and 9V for a 3 cell Li-ion battery.
Resistors R2 and R3 should be 1% tolerance for accuracy.
Alternatively, a variable resistor could be used to adjust the cutoff voltage. LED1 should be Green or Blue or White, as these will be in an OFF-state at 2,5v IC Reference voltage. Alternatively two Red or Orange LEDs could be used in series.
Also our FLASHING Red LED could work well in this circuit.
Simple Constant-Current Driver ( Current Source )
In this example most common low-gain NPN transistor can be used in tandem with TL431 to produce a simple Constant-Current regulator (or Current-Source).
The Current Source is connected in series with the load (LED)
The Current limit value is set by the R2 and is defined by the formula :
I ~ U1 (U -U LED V1) / R1.
Maximum supply voltage is 40v
Please keep in mind the limitations of the transistor -Max. current & Dissipating power…