Making a solar cell phone charger circuit diagram

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Here’s how it works:

Transistor VT1 is a switching power supply tube, which forms a self-excited oscillation circuit with T1, R1, R3, C2, etc. After adding the input power, the current flows to the base of VT1 through the starting resistor R1, causing VT1 to conduct.

After VT1 is turned on, the input DC voltage is added to the primary coil Np of the transformer, and its collector current Ic increases linearly in Np. The feedback coil Nb generates an induced voltage of 3 positive and 4 negative, so that VT1 has a positive base and a negative emitter. Positive feedback voltage, this voltage injects base current into VT1 through C2 and R3 to further increase the collector current of VT1. Positive feedback generates an avalanche process, causing VT1 to saturate and conduct. During the saturated conduction period of VT1, T1 stores magnetic energy through the primary coil Np.

At the same time, the induced voltage charges C2. As the charging voltage of C2 increases, the base potential of VT1 gradually becomes lower. When the base current change of VT1 cannot satisfy its continued saturation, VT1 exits the saturation zone and enters the amplification zone.

After VT1 enters the amplified state, its collector current decreases from the maximum value before the amplified state, and an induced voltage of 3 negative and 4 positive is generated in the feedback coil Nb, which reduces the base current of VT1, and its collector current decreases accordingly. The feedback avalanche process occurred again, and VT1 was cut off quickly.

After VT1 is cut off, the energy stored in transformer T1 is provided to the load. The 5 negative and 6 positive voltage generated by the secondary coil Ns is rectified and filtered by the diode VD1, and then a DC voltage is obtained on C3 to charge the mobile phone battery.

When VT1 is cut off, the DC power supply input voltage and the 3 negative and 4 positive voltages induced by Nb reversely charge C2 through R1 and R3, gradually increasing the base potential of VT1, causing it to re-conduct, flip again and reach the saturation state. The circuit oscillates repeatedly in this way.

R5, R6, VD2, VT2, etc. form a voltage limiting circuit to protect the battery from overcharging. Taking a 3.6V mobile phone battery as an example, its charging limit voltage is 4.2V. During the charging process of the battery, the battery voltage gradually rises. When the charging voltage is greater than 4.2V, the voltage regulator diode VD2 begins to conduct after being divided by R5 and R6, causing VT2 to conduct. The shunting effect of VT2 reduces the base voltage of VT1. pole current, thereby reducing the collector current Ic of VT1, thereby limiting the output voltage. At this time, the circuit stops charging the battery with high current and uses a small current to maintain the battery voltage at 4.2V.

Component selection, installation and debugging

VT1 requires Icm>0.5A, hEF is 50-100, 2SC2500, 2SC1008, etc. can be used, and VD1 is a Zener diode with a stable voltage value of 3V.

The high-frequency transformer T1 must be homemade, using an E16 ferrite core. Np is wound with 26 turns of φ0.21 enameled wire, Nb is wound with 8 turns of φ0.21 enameled wire, and Ns is wound with 15 turns of φ0.41 enameled wire. When winding, pay attention to the starting ends of each coil so as not to cause the circuit to not vibrate or the output voltage to be abnormal. During assembly, a layer of plastic film with a thickness of approximately 0.03mm is placed between the two magnetic cores to serve as the core air gap.

The solar panel uses four silicon solar panels with an area of ​​6cm×6cm. Its no-load output voltage is 4V, and when the operating current is 40mA, the output voltage is 3V. Since the working efficiency of the DC converter increases with the increase of the input voltage, four solar panels are used in series. At this time, the input voltage of the circuit is 12V. Readers can decide the number and connection methods to use based on the specifications of the solar panels you can buy.

The printed circuit board is as shown below, the size is 45×26mm2.

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After the installation is completed, connect the solar panel and place it in the sun. The output voltage of the circuit is about 4.2V when no-load. When the no-load output voltage is higher than 4.2V, the resistance of R5 can be appropriately reduced, and vice versa. The resistance of R5. The operating current of the circuit is related to the intensity of sunlight. Normally it is about 40mA. At this time, the charging current is about 85mA.