This article introduces a solar cell phone charger that uses solar panels
to charge the cell phone battery after converting the DC voltage through the
circuit, and can automatically stop charging after the battery is charged. This
solves the problem that the cell phone battery suddenly runs out of power when
going out and the charger Not being around or finding a place to charge affects
the normal use of the phone.
working principle
When solar cells are used, due to large changes in sunlight and relatively
high internal resistance, the output voltage is unstable and the output current
is also small. This requires a DC conversion circuit to convert the voltage and
then charge the mobile phone battery. DC conversion circuit See Figure 1, which
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 in the cut-off state. At this time, the high-frequency transformer
T1 passes through the primary coil Np stores energy; 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 rectified by VD1 and filtered by
the capacitor C3 before being output to the load.
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 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 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 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.
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.
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