Solar lawn lights have the characteristics of safety, energy saving, environmental protection, and easy installation. It mainly uses the energy of solar cells to supply power to the lawn lamp. When sunlight shines on the solar cell during the day, the solar cell converts the light energy into electrical energy and stores the electrical energy in the storage battery through the control circuit. After dark, the electric energy in the storage battery supplies power to the LED light source of the lawn lamp through the control circuit.
At dawn on the second day, the battery stops supplying power to the light source, the lawn lamp goes out, and the solar battery continues to charge the battery, repeating and cyclical work. The control circuit of the solar lawn lamp allows the lawn lamp to work in the above-mentioned manner through the strength of the external light. The composition and brief working principle of several commonly used control circuits are introduced below.
Figure 1 shows an early solar lawn lamp control circuit. It uses a photoresistor to detect the intensity of light. When there is sunlight, the electric energy generated by the solar cell charges the battery DC through D1. The photoresistor R2 also exhibits a low resistance value, making the BG2 base extremely low and cut off. When there is no light at night, the solar battery stops charging the battery, and the setting of D1 prevents the battery from discharging back to the solar battery. At the same time, the photoresistor changes from low resistance to high resistance, BG2 is turned on, and the BG1 base is also turned on at a low level. The DC boost circuit composed of BG3, BG4, C2, R5, L, etc. is energized to work, and the LED emits light. The DC boost circuit is actually a complementary oscillating circuit, and its working process: when BG1 is turned on, the power supply charges C2 through L, R5, and BG2. Since the voltage across C2 cannot change suddenly, the base of BG3 is extremely high and BG3 is not turned on; with the charging of C2, the voltage drop is getting higher and higher, and the base potential of BG3 is getting lower and lower; when the BG3 turn-on voltage is low, BG3 turns on, BG4 turns on, and C2 discharges through BG4. After the discharge is completed, BG3 and BG4 are turned off again, and the power supply recharges C2. This cycle repeats and the circuit forms oscillation. During the oscillation process, when BG4 is turned on, the power supply passes through L to ground, and the current passes through L to store energy. When BG4 is cut off, induced electromotive force is generated at both ends of L, which drives the LED to emit light after being superimposed with the power supply voltage.
In order to prevent the battery from over-discharging, R4 and BG2 are added to the circuit to form an over-discharge protection. When the battery voltage is as low as 2V, due to the partial pressure of R4, BG2 cannot be turned on, the circuit stops working, and the battery is protected.
When the voltage of the solar cell and the storage battery is increased to 3.6V, the circuit can be simplified, the complementary oscillation boost circuit of BG3 and BG4 is removed, and the LED is directly driven to emit light. The principle is similar to the circuit shown in Figure 2.
Figure 2 is a simple solar lawn lamp circuit, which can also be used in solar lawn lights and solar light-controlled toys. Compared with the circuit shown in Figure 1, this circuit no longer uses a photoresistor to detect the intensity of light to control the operation of the circuit, but uses a solar cell to detect the intensity of light, because the solar cell itself is a very good photosensitive sensor device. When there is sunlight, the electric energy from the solar cell charges the battery DC through the diode D, and the voltage of the solar cell is also applied to the base of BG1 through R1, so that BG1 is turned on, BG2 and BG3 are turned off, and the LED does not emit light. When night comes, the voltage at both ends of the solar cell is almost zero. At this time, BG1 is cut off, BG2 and BG3 are on, and the voltage in the battery is applied to both ends of the LED through K and R4, and the LED emits light. In this circuit, the solar cell doubles as a light control element, and the resistance value of R1 is adjusted, and the working control point of the lamp can be adjusted according to the intensity of the light. The shortcoming of this circuit is that there is no circuit or components to prevent the battery from over-discharging. When the lamp is in the dark for a long time, the electric energy in the battery will basically be exhausted. The switch K is set to prevent the lawn lamp from exhausting the battery power during storage and transportation.
Figure 3 is a currently used lawn lamp control circuit. BG3, BG4, L, C1 and R5 form a complementary oscillatory boost circuit. Its working principle is basically the same as the circuit shown in Figure 1, except that the circuit uses a 1.2V battery for power supply and storage. BG1 and BG2 form a light control switch circuit. When the voltage on the solar cell is lower than 0.9V, BG1 is turned off, BG2 is turned on, the boost circuit composed of BG3, BG4, etc. works, and the LED emits light. When the day is light, the solar cell voltage is higher than 0.9V, BG1 is turned on, BG2 is turned off, and BG3 is turned off at the same time, the circuit stops oscillating, and the LED does not emit light. Adjust the resistance of R2 to adjust the starting point of the switch light. When the battery voltage drops to 0.7~0.8V, the circuit will stop oscillating. Some designers think that the advantage of this circuit is that the lawn lamp can still work when the battery voltage drops to 0.7V. As for the 1.2V battery, it seems to be a bit over-discharged, and long-term over-discharge will definitely affect the service life of the battery. Therefore, some manufacturers have made a little improvement on the basis of the circuit in Figure 3, as shown in Figure 4, that is, a diode D2 is connected in series between the emitter of the BG3 and the anode of the power supply. Due to the connection of D2, the voltage of BG3 entering the amplification area is superimposed about 0.2V, so that the entire circuit stops working when the battery voltage drops to 0.9~1.0V. The service life of the improved circuit battery can be prolonged by about twice.
Figure 5 is a control circuit composed of a solar lawn lamp application specific integrated circuit (ANA6601F) and peripheral components, which includes a charging circuit, a driving circuit, a photosensitive control circuit, a pulse width modulation circuit, etc. The circuit has the advantages of high conversion efficiency (80~85%), wide operating voltage range (0.9~1.4V), output current adjustable at 5~40mA, etc., and has a good battery over-discharge protection function and low-environment brightness opening function. Functions of each pin: pins 1 to 3 are the battery over-discharge protection control terminals, pin 4 is the power ground, pin 5 is the start terminal, pin 6 is the power supply positive, pin 7 is the pulse width adjustment terminal, and pin 8 is the output terminal.
Figure 6 is a solar lawn lamp circuit that uses supercapacitors to store energy. When the ambient light is strong, the solar battery charges the super capacitors C1 and C2 through VD1. When the voltage across the capacitor reaches 0.8V, IC1 (BL8530) starts to work, boosting and outputting 3.3V voltage, providing working power for the control circuit composed of IC2A and peripheral components, and the control circuit starts to work. At this time, the voltage at the inverting input terminal of IC2B is relatively high and outputs low level, which in turn causes IC2A to output low level, VT is cut off, and LED does not emit light. When the ambient light is weak and insufficient to charge C1 and C2, VD1 prevents C1 and C2 from discharging to the solar cell. At the same time, the non-inverting input terminal voltage of IC2B is higher and the output is high. The IC2A light control circuit enters the working state, and the LED lights up. The number of LED lights can be selected from 1 to 5.
Solar lawn lights are actually an independent solar power system. Therefore, the control circuit of the lawn lamp is the same as other controllers, in addition to controlling the normal operation of the lamp, it should also have protection functions such as anti-overcharge, anti-overdischarge, and anti-recharge.
The function of preventing overcharging is achieved through several methods. One is to match the power generation capacity of the solar battery with the battery capacity and night power consumption through reasonable calculations, so that the solar battery’s daily power generation can best meet the storage capacity of the battery, and even the battery capacity is designed to be larger. Although the cost of the battery is a bit higher, the control circuit does not need to be specially designed to prevent overcharging. The second is to add an anti-overcharging circuit to the control circuit, that is, to connect a transistor bleeder circuit in series or in parallel in the input circuit, and control the switch of the transistor by identifying the voltage level to discharge the excess solar battery energy through the transistor, to ensure that the battery is not overcharged.
The function of the over-discharge prevention circuit is to protect the battery from being damaged or shortening its service life due to over-discharge. Especially the solar lawn lamp circuit belongs to the low rate discharge state, and the discharge cut-off voltage should not be too low. Therefore, as long as the cut-off voltage of the circuit is adjusted so that the control circuit stops working when the battery reaches the over-discharge protection point, the over-discharge protection can be played. For circuits using 1.2V power supply, the cut-off voltage of the power supply is generally adjusted to 0.9~1.0V.
In the circuits shown in Figure 3 and Figure 5, why use a 1.2V battery for energy storage and power supply instead of two or more batteries in series for power supply? This is because the battery voltage is low, and the voltage of the solar cell that charges the battery can be reduced accordingly. Regardless of the area of each solar cell, its working voltage is only about 0.5V. The solar cell used in the solar lawn lamp is a battery module composed of multiple solar cells connected in series. Under the condition of meeting the power requirements, the lower the voltage, the fewer solar cells connected in series, which is very beneficial to simplify the process and reduce the cost. . Secondly, when multiple batteries are connected in series, the consistency requirements for each battery are higher, and the battery packs formed by series connection of batteries with different performances will have reduced charge and discharge performance and charge and discharge life. This is not as advantageous as using a battery in terms of system reliability and cost reduction.