A small and easy-to-make fast charger

1. Circuit Principle Introduction

This small fast charger can charge two No. 5 or No. 7 batteries at the same time. It is equipped with a charging indicator light to indicate the charging status. When the battery is low, the indicator light is brighter. As the power is continuously replenished, the brightness of the indicator light continues to decrease.

When charging is complete, the indicator light goes out. The charger has an automatic adjustment function of starting with a high current fast charge and then a low current slow charge, which has a good protection effect on the rechargeable battery.

The following figure is the schematic diagram of this charger. The power supply is connected from the ACl and AC2 terminals after the transformer steps down the voltage. After full-wave rectification and capacitor filtering, the power supply is provided to the charger. The power supply provides base current to the two charging switch transistors through R1. CWl always keeps the base voltage of the two transistors at about 2.4V. When the charging battery is connected, the charging circuit is connected. Taking DCl as an example, since the emitter voltage of VTl is determined by the battery voltage, the base voltage is kept at a stable value. When the battery voltage is low, the voltage applied to the VTl emitter junction is large, the collector charging current is large, and the brightness of the light-emitting diode is also large. As the battery power is continuously replenished, the battery voltage continues to increase, the voltage applied to the VTl emitter junction continues to decrease, and the charging current continues to decrease. When the current is sufficient, when the VTl emitter potential is lower than the base potential by 0.5V, VTl is cut off, charging is completed, and the charging indicator light goes out. Even if you forget to remove the battery at this time, the rechargeable battery will not be damaged due to overcharging. The charging situation of DC2 is basically the same as the above process. Therefore, this charger can automatically adjust the charging current according to the battery power during the entire charging process.

2. Installation Instructions

1. When installing diodes and transistors, be sure to pay attention to the polarity and do not insert them in reverse. Strictly follow the markings on the circuit board:

2. Ceramic capacitors and resistors have no direction. So just install them according to the specific location;

3. Pay attention to the polarity when installing the electrolytic capacitor. Before cutting the pins, the longer one of the two pins of the capacitor is positive and the shorter one is negative. Pay special attention to this when installing.

4. When installing the positive electrode of the battery, tin it first, and then tin the circuit board. The installation direction is opposite to other components, and the protruding part should be on the welding surface of the circuit board. When welding, be careful not to burn your hands because of the large area of ​​this device. When actually installing, it is best to clamp the metal part with pliers to position it, and then weld it. Since the metal part has a large area and dissipates heat quickly, it is best to use a 60W electric soldering iron for welding if conditions permit. This way, the welding time is shorter and it is not easy to damage the circuit board;

5. When installing several higher components, all of them should be installed horizontally, otherwise they will press against the outer shell when installed in the box.

3. Debugging instructions

This kit can be successfully installed at once as long as it is installed correctly. Turn on the power and measure the voltage across CWl. Normally, it is 2.4V. Take a low-power No. 7 rechargeable battery and put it into the charging station. You can see that the charging indicator light is brighter. After more than an hour of fast charging, the brightness of the indicator light will continue to decrease. The specific time varies depending on the performance of the battery. Finally, the charging indicator light will go out. At this time, actually remove the battery for measurement. The voltage should not be lower than 1.3V.

This charger production kit is highly practical and has a high cost-effectiveness as an express charger for household batteries.