TL431 battery charger

Charging a battery beyond the specified battery voltage will significantly reduce its life expectancy.

Here we introduce a simple charging circuit, which uses a three-terminal adjustable shunt voltage regulator chip TL431 (IC1) as the main regulating device. When the voltage of the non-inverting input pin (REF) is lower than its internal reference voltage (2.5V), the TL431 cathode K outputs a high potential, keeping the relay in the release state and charging continues. on the contrary. When the battery voltage exceeds the threshold voltage of the circuit. When the REF voltage exceeds 2.5V. TL431 cathode K outputs low potential. The relay is closed, the charging circuit is disconnected, and charging stops. TL431 plus several external components form a charging control circuit. Because TL431 provides a reference voltage internally. Therefore, the TL431 comparator has a hysteresis effect, that is, there is a difference between the upper and lower limits of the comparator.

The threshold voltage of the circuit is the upper limit of the charging voltage. This upper limit voltage can be calculated according to the following formula: VT=Vref×(1+R1/R2), where Vref is the internal reference voltage of TL431, which is approximately 2.5V.

When the battery voltage rises to exceed the threshold voltage, relay RL1 is closed, its contact N/C disconnects the charging circuit, and the indicator LED1 flashes. At the same time, R3 provides a hysteresis path to control the release voltage Vr of the relay. Vr can be calculated by the following formula: Vr=Vref×(1+R1||R3/R2). In the formula, R1||R3 is the parallel value of R1 and R3, that is, (R1xR3)/(R1+R3).

When the battery voltage is lower than the release voltage, the cathode (K) voltage of TL431 is close to the battery voltage. The relay remains in the released state. Charging continues and LED1 does not flash. Until the recharged battery voltage exceeds the threshold voltage Vt, LED1 flashes again and charging stops. R5 is used to limit the charging current.

The power supply of the charger is provided by the 220V AC power supply through the step-down transformer X1 and the D1~D4 rectifier bridge.

The circuit can be mounted on an ordinary PCB board and enclosed in an appropriate box. Use alligator clips to connect the battery when charging.