Summary of Shunt regulator monitors battery voltage
### Summary The article describes using a TL431 shunt regulator as a battery monitor with hysteresis, functioning like a Schmitt trigger. It explains how to calculate the threshold and release voltages using internal references and external resistors. The circuit controls an LED via a transistor: the LED lights when battery voltage exceeds the upper threshold and turns off below the lower release voltage, providing a simple visual indicator for battery status.
Parts used in the Battery Monitor:
- TL431 shunt regulator
- Resistor R1
- Resistor R2
- Resistor R3
- Transistor Q1
- LED1
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, VT+, of this comparator as VT+=VREF×(1+R1/R3), where VREF, the internal reference voltage of shunt-regulator TL431, is 2.5V.
When the battery voltage is higher than the threshold voltage, the cathode voltage of the TL431 is at its low level of approximately 2V, and transistor Q1 turns on, lighting LED1. You calculate the release voltage, VT–, of the trigger as VT–=VREF×(1+R1×R2/(R1+R2) ×1/R3).
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.
For more detail: Shunt regulator monitors battery voltage
- What is the function of the TL431 in this project?
The TL431 acts as a comparator with hysteresis or a Schmitt trigger to monitor battery voltage. - How do you calculate the threshold voltage VT+?
The formula is VT+=VREF×(1+R1/R3), where VREF is 2.5V. - When does LED1 turn on?
LED1 turns on when the battery voltage is higher than the threshold voltage VT+. - What happens to the cathode voltage when the battery is high?
The cathode voltage drops to a low level of approximately 2V. - How is the release voltage VT– calculated?
It is calculated using the formula VT–=VREF×(1+R1×R2/(R1+R2) ×1/R3). - What occurs when the battery voltage falls below the release voltage?
The cathode voltage rises to the battery voltage, turning off transistor Q1 and extinguishing LED1. - Can this circuit be used as a simple battery monitor?
Yes, the article confirms it can be used as a simple battery monitor with hysteresis.
