Production of Automatic Charger for Emergency Light

Emergency hand-held lights powered by 6V maintenance-free batteries are widely used in rural areas. The charger used is a transformer step-down, single diode half-wave rectifier device, and the charging time is difficult to master. Since battery exhaustion or overcharging often shortens the battery life, I designed and made a simple automatic charging device that can effectively extend the battery life.

How it works

The circuit is shown in Figure 1. GB is the battery to be charged. When it is not used, the charger plugs X1 and X2 are inserted into the charging socket of the portable lamp. Press the start button SB, and the 220V AC mains is added to the primary of the power transformer T1 through the fuse Fu and SB. The secondary outputs two AC voltages: AC 12V is half-wave rectified by VD1 and smoothed by C2 to turn on VT. Relay K1 is energized and K1-1 is closed. At this time, after releasing SB, the charger still works. In the figure, R2 is the bias resistor of VT; C1 is the step-down capacitor, and LED1 (red) is the mains power indicator tube; after the start is completed, the AC 9V voltage is rectified by VD2 to charge the battery GB, and LED2 (green) is the charging indicator tube. As the charging process proceeds, the voltage across the battery continues to rise. When it reaches the limit voltage value (about 7.5V), the voltage regulator tube VD3 reversely breaks down and turns on, the light-emitting diodes in the ① and ② pins of the optocoupler 4N25 light up, and the photoelectric tubes in the ④ and ⑤ pins turn on after receiving the light. The VT base is pulled down (about 0V) and loses the forward bias, so VT is cut off, K1 is released to disconnect K1-1, the charger stops charging, and the charging indicator tube LED2 goes out.

Component selection and production

T1 can use a power transformer with a 10VA or so and a secondary output of 9V and 12V. If the original transformer (secondary output 9V) is used, the working voltage of relay K1 should also be 9V. Relays should be 12V relays such as 4088, 4089, and JRX-13F. VT uses low-power transistors such as 9013, 9014, and 8050, requiring a small penetration current and good switching characteristics, β = 100 ~ 1500. Optocoupler ICs can use 4N23, 4N25, 4N33 and other models. The withstand voltage of C1 must not be less than 450V, and the capacity should be 220nF ~ 470nF. The larger the capacity, the brighter LED1. When debugging, first disconnect VD2, add a 7.5V DC voltage to both ends of X1 and X2, and adjust the potentiometer RP from large to small to make the potential of 4N250 ⑤ foot suddenly drop (K1 is released).