Automatic charger made with LM3914

Working Principle Working Process

This charging circuit has a discharge function. After the discharge is completed, it automatically switches to the charging state. After charging to about 90%, it automatically enters a small current to continue charging until full. It also has a battery power indication function. The voltage values ​​indicated by LED1~LED10 are shown in the attached table.

How it works

The circuit is shown in Figure 1. LM3914 is a level and voltage LED display driver integrated circuit that can work in the dot/line display state. Its internal block diagram is shown in Figure 2. LM3914 contains an adjustable reference voltage source and 10 voltage comparators. Ten 1kΩ precision resistors are connected in series to form a voltage divider, which provides a comparison reference for each voltage comparator. The required reference voltage can be provided to the 6th pin (high input end of the resistor divider) and the 4th pin (low input end of the resistor divider) through an external circuit. The resistor divider divides the voltage between the 6th and 4th pins into ten sections and compares it with the input voltage. The corresponding voltage comparator outputs a low level (in the dot display state; if it is set to the bar display state, the comparator that is lower than the input voltage outputs a low level), driving the corresponding LED to emit light. The included adjustable reference voltage source is adjusted by the 8th pin and output by the 7th pin. In Figure 1, the external resistors R1l and R2 on the 7th and 8th pins can control the voltage of the 7th pin, the output end of the reference voltage source, and its value = 1.25 (1+Rl/R2). This charging and discharging circuit uses the voltage output by its reference voltage source as the termination voltage for controlling charging, and its value is 1.25 (1 + 120 / 1000) = L4V. The external resistor R3 at the 4th foot controls the lighting start voltage of the LED1 at the 1st foot, that is, the termination voltage of the discharge, and its value = 1.4 (21.5 + 1) / (10 + 21.5) = 1lV. The printed board is shown in Figure 3. In order to facilitate self-production, all transistors are designed in TO-220 packaging.

Working process:
Turn on the power, K2 is closed, and the rechargeable battery is connected. The battery voltage is input from the ⑤ foot to the voltage comparator for comparison. At this time, if the battery voltage is lower than 1.4V and the thyristor SCR is not triggered, V2 and V4 are cut off, the high level of the ⑩ foot cuts off V1 through R6, V3 and V5 are turned on, and the power charges the battery through K2, R12, and V5. Because the LM3914 of this circuit works in the dot display state, only the corresponding voltage comparator outputs a low level, and the corresponding LED (one of the following LEDl0) lights up, indicating the battery charging capacity at this time. When the battery charging voltage rises to 1.4V, the ⑩ foot outputs a low level, V1 and V2 are turned on, V3 and V5 are cut off, and charging stops. If the battery needs to be discharged, press the discharge button K, the thyristor SCR is turned on, V2 and V4 are turned on, V2 turns off V3 and V5, the battery is discharged through R11 and V4, and the driving current of V2 and V4 passes through R7, R9, SCR, and R4. LED11 is lit, indicating that the battery is discharging, and the battery voltage makes the corresponding LED light up, indicating the battery discharge capacity at this time. When the battery is discharged to 1.0V, the ① foot outputs a low level, the thyristor SCR exits conduction and becomes cut off, V2 and V4 are also cut off, V3 and V5 are turned on, and the battery is automatically charged. Resistors R4 and R5 are pull-down resistors, and their function is to make the potential of ① and ⑩ feet lower when they are in a low level state, making the discharge and charging termination actions more reliable. LED0 is a power indicator. If you want to display the battery capacity when it is unloaded, you need to disconnect K2.

In actual use, when the battery charging voltage just rises to the charging limit voltage (1.4V), the 10th voltage comparator output terminal ⑩ pin outputs a low level, and the charging is stopped through the action of V5, V2, V1, and V3. However, the battery voltage during charging (especially when charging with a large current) is not the actual voltage of the battery. After charging stops, the battery voltage will drop a little, so charging starts again. In this process, charging stops, the voltage drops, and charging stops again and again... At this time, the charging current begins to drop. The closer the battery voltage is to 1.4V, the smaller the charging current is, and finally it becomes a trickle charge (about a few mA). The battery voltage is stable at 1.4V, it will not be overcharged, and it is very safe.

Component selection
SCR uses a small unidirectional thyristor below 1A. If the SCR cannot remain on after pressing the K button, a resistor of about 2k1) can be connected to its cathode to increase its load and increase the on-current. By selecting different R1 resistance values, this circuit can charge and discharge multiple rechargeable batteries. The charging and discharging currents are determined by R12 and R11 respectively. In this circuit, the charging current is about 120mA and the discharging current is about 100mA.