Solar Charger

This simple circuit uses the energy from solar cells to charge a lead-acid battery. The circuit consists of an oscillator, a DC booster, and a voltage regulator.

IC1 (40106B) has 6 Schmitt triggers, among which IC1B acts as an oscillator. The oscillation frequency is determined by the values ​​of R1 and C3. Since the oscillation frequency cannot exceed the maximum value of the ripple frequency on the output capacitor C2, the oscillation frequency is required to be lower than 100kHz.

IC1A acts as a buffer, so that the output end of the oscillator gets a lighter constant load. It plays an isolating role and ensures the frequency stability. The power supply of IC1 can be directly connected to the rechargeable battery at the output end. T2 acts as a switch to control the charging of the battery by the solar cell. When the buffer IC1A outputs a high level, the transistor T2 is turned on, and the collector current passes through the inductor L1 to store the magnetic field energy. And a reverse voltage VL1 is established (positive on the left and negative on the right in the figure). When the buffer IC1A outputs a low level, the transistor T2 is cut off, the polarity of VL1 becomes reversed, and it is superimposed with the solar cell voltage. The current flows into the load (capacitor C2 and battery BT1) through the inductor L1 and the diode D1. The capacitor C2 and the battery BT1 are charged. In a stable state, the output voltage will be higher than the input voltage. When the transistor 12 is turned on again, the process repeats.

The parallel regulator is formed with transistor T1 as the center to prevent the battery from being damaged by overcharging. And to ensure the stability of the output voltage. Once the output voltage rises due to overcharging and exceeds the rated value, the voltage regulator D2 breaks down and turns on the transistor T1. The current of the solar cell enters the ground through T1 and the low resistance R3, and the battery will not be overcharged. When the battery voltage is lower than the voltage regulation value, the voltage regulator D2 is blocked, T1 is turned off, and the charging process returns to normal.

Be careful not to measure the voltage directly at the output terminal when no load is connected, because the pulse current will damage the voltmeter.