Shunt regulator monitors battery voltage

Vladimir Rentyuk

EDN

A TL431 shunt regulator is a perfect choice for many applications. You can use it as a comparator with hysteresis by taking advantage of its inner voltage reference along with few additional components. You can use this comparator with hysteresis, like a Schmitt trigger, as a simple battery monitor (Figure 1). You calculate the threshold voltage, V_T+, of this comparator as

 

where V_REF, the internal reference voltage of shunt-regulator TL431, is 2.5 V.

Figure 1.	A shunt regulator and associated circuitry function as a Schmitt trigger, lighting LED1 when the battery is fully charged.

When the battery voltage is higher than the threshold voltage, the cathode voltage of the TL431 is at its low level of approximately 2 V, and transistor Q₁ turns on, lighting LED₁. You calculate the release voltage, V_T–, of the trigger as

 

When the battery voltage is less than the release voltage, the cathode voltage of the TL431 goes to its high level – to the battery voltage. Transistor Q₁ turns off, and LED₁ does not shine. LED₁ turns on again when the battery voltage, after recharging, exceeds the threshold voltage.