How to make and work the solar cell phone charger

This article introduces a solar mobile phone charger, which uses solar panels to charge mobile phone batteries after DC voltage conversion through circuits, and can automatically stop charging after the battery is fully charged, solving the problem of the mobile phone battery suddenly running out of power when you are out and the charger is not around or you can't find a place to charge, which affects the normal use of the mobile phone.

How it works

When using solar cells, the output voltage is unstable and the output current is small due to the large change of sunlight and the high internal resistance. Therefore, a DC conversion circuit is needed to convert the voltage for charging the mobile 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 tube 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 in the cut-off state. At this time, the high-frequency transformer T1 stores energy through the primary coil Np; when the switch tube VT1 is turned off, the secondary coil Ns is 5 negative and 6 positive, and the energy stored in the high-frequency transformer T1 is output to the load after rectification by VD1 and filtering by capacitor C3.

The working principle of the circuit is briefly described as follows:

Transistor VT1 is a switching power supply tube, which forms a self-excited oscillation circuit with T1, R1, R3, C2, etc. After the input power is added, the current flows to the base of VT1 through the starting resistor R1, making VT1 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 3 positive and 4 negative induced voltages, so that VT1 obtains a positive feedback voltage with a positive base and a negative emitter. This voltage injects base current into VT1 through C2 and R3 to further increase the collector current of VT1. The positive feedback generates an avalanche process, which makes VT1 saturated and turned on. 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 the base current change of VT1 cannot satisfy its continued saturation, VT1 exits the saturation region and enters the amplification region.

After VT1 enters the amplification state, its collector current drops from the maximum value before the amplification state, generating 3 negative and 4 positive induced voltages in the feedback coil Nb, which reduces the base current of VT1 and its collector current decreases accordingly. The positive feedback avalanche process occurs again and VT1 is quickly cut off.

After VT1 is turned off, the energy stored in transformer T1 is provided to the load, and the 5-negative and 6-positive voltage generated by the secondary coil Ns is rectified and filtered by diode VD1, and 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, making it turn on again and flip again to reach the saturation state, and the circuit oscillates repeatedly like this.

R5, R6, VD2, VT2 and other components form a voltage limiting circuit to protect the battery from overcharging. Here, a 3.6V mobile phone battery is taken as an example, and its charging limit voltage is 4.2V. During the battery charging process, the battery voltage gradually rises. When the charging voltage is greater than 4.2V, the voltage-stabilizing diode VD2 starts to conduct after the voltage is divided by R5 and R6, making VT2 conduct. The shunt effect of VT2 reduces the base current of VT1, thereby reducing the collector current Ic of VT1, achieving the effect of limiting the output voltage. At this time, the circuit stops charging the battery with a large 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, and 2SC2500, 2SC1008, etc. can be used. VD1 is a voltage regulator diode with a voltage regulation value of 3V.

High-frequency transformer T1 should be made by yourself, using E16 ferrite core, Np is wound with φ0.21 enameled wire for 26 turns, Nb is wound with φ0.21 enameled wire for 8 turns, and Ns is wound with φ0.41 enameled wire for 15 turns. When winding, pay attention to the starting end of each coil, so as to avoid the circuit not vibrating or abnormal output voltage. When assembling, a layer of plastic film with a thickness of about 0.03mm is placed between the two cores as the core air gap.

The solar panel uses 4 silicon solar panels with an area of ​​6cm×6cm. Its no-load output voltage is 4V, and the output voltage is 3V when the working current is 40mA. Since the working efficiency of the DC converter increases with the increase of input voltage, the 4 solar panels are connected in series and the input voltage of the circuit is 12V. The reader can decide the number and connection method according to the specifications of the solar panels you can buy.

The parameters of other components are shown in Figure 1.

The printed circuit board is shown in Figure 2 and has a size of 45×26mm2.

After installation, connect the solar panel and place it in the sun. The circuit output voltage 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, otherwise it can be increased. The circuit working current is related to the intensity of sunlight. Normally, it is about 40mA, and the charging current is about 85mA.