Working principle
Due to significant changes in sunlight and the relatively high internal resistance of solar cells during use, the output voltage is unstable and the output current is also low, requiring the use of a DC conversion circuit converts the voltage to charge the cell phone battery. The DC conversion circuit is shown in Figure 1. It is a single-tube DC conversion circuit in the form of a single-ended flyback converter circuit. When the switch VT1 is turned on, the induced voltage of the primary coil NP of the high frequency transformer T1 is 1 positive and 2 negative, the secondary coil Ns is 5 positive and 6 negative, and the rectifier diode VD1 is cut off -off state. At this time, the high frequency transformer T1 passes through the primary coil Np and stores energy when the switching tube VT1 is turned off, the coilThe secondary Ns is 5 negative and 6 positive, and the energy stored in the top. -the frequency transformer T1 is rectified by VD1 and filtered by the capacitor C3 before being sent to the load.
The working principle of the circuit is briefly described as follows:
Transistor VT1 is a switching power tube. It forms a self-excited oscillation circuit with T1, R1, R3. , C2, etc. After adding the input power, current flows to the base of VT1 through the start resistor R1, causing VT1 to conduct.
After VT1 is turned on, the input DC voltage is applied 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, which causes VT1 to obtain the base voltage. The positive feedback voltage is extremely positive and the emitter is extremely negative. VSThis voltage injects a base current into VT1 via C2 and R3 to further increase the collector current of VT1. avalanche process, causing saturation and conduction of VT1. 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 decreases when changing the base current of VT1 cannot satisfy its continuous saturation. VT1? exits The saturation zone enters the amplification zone.
After VT1 enters the amplification state, its collector current decreases from the maximum value before the amplification state, and an induced voltage of 3 negative and 4 positive is generated in the feedback coil Nb, which reduces the base current. of the VT1 and its collector. The current decreases accordingly, positive feedback occurst again by an avalanche process and VT1 is quickly cut off.
After VT1 is cut off, the energy stored in transformer T1 is supplied to the load. The 5 negative and 6 positive voltages generated by the secondary coil Ns are rectified and filtered by the diode VD1, then one. A DC voltage is obtained on C3 to charge the cell phone battery.
When VT1 is cut off, the input voltage of the DC power supply 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 and making him a driver again. , returns again to reach saturation and the circuit oscillates repeatedly.
R5, R6, VD2, VT2, etc. form a voltage limiting circuit to protect the battery against overcharging. Here, taking a 3.6V mobile phone battery as an example, the charging limit voltage is 4.2V. Duringthe battery charging process, the battery voltage gradually increases. When the charging voltage is higher than 4.2V, the voltage regulator diode VD2 turns on after being divided by R5 and R6. It begins to conduct, causing VT2 to conduct. The bypass effect of VT2 reduces the base current of VT1, 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 low current to maintain the battery voltage at 4.2 V.
Component selection, installation and debugging
VT1 requires Icm>0.5A, hEF is 50-100, 2SC2500, 2SC1008, etc. can be used, VD1 is a Zener diode with a stable voltage value of 3 V.
The high frequency transformer T1 must be homemade, using an E16 ferrite core. Np is wound with 26 turns of enameled wire φ0.21, Nb is wound with 8 turns of enameled wirelé φ0.21 and. Ns is wound with φ0.41 enameled wire over 15 turns. When winding, pay attention to the starting ends of each coil so as not to cause the circuit to vibrate or the output voltage to be abnormal. During assembly, a layer of plastic film with a thickness of about 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 6 cm × 6 cm. 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 increasing voltage input, four solar panels are used in series. At this point, the input voltage of the circuit is 12V. Readers can decide how many and connection methods to use according to the specifications of the solar panels you canbuy.
See Figure 1 for parameters of other components.
The printed circuit is shown in Figure 2, the size is 45×26 mm2.
After installation is complete, connect the solar panel and place it in the sun. At no load, the output voltage of the circuit is about 4.2V. When the no-load output voltage is greater than 4.2V, it can be appropriately reduced the resistance of R5, otherwise increase the resistance of R5. The operating current of the circuit is related to the intensity of sunlight. Normally it is around 40 mA. The charging current is approximately 85 mA.
First of all, solar energy can charge lithium batteries
If you realize that there are already such cell phones on sale
It is recommended not to use this type of charging method. The main charging method
It can be used as a backup when there is no power
The charging interface, solar energy is DC power supply. power supply
It is recommended to add a Schottky diode and an electrolytic capacitor
Then connect it to your charger and use it