With the development of the economy and the progress of society, people have put forward higher and higher requirements for energy. Finding new energy has become an urgent issue facing humanity. Solar power is considered to be the most important energy source in the 21st century because solar power has the cleanliness, safety, and extensiveness and sufficiency of thermal power, hydropower, and nuclear power. The storage of solar energy is the key to the development of solar energy products. At present, various batteries are mainly used, but the long charging time, short life and non-environmental protection of the battery have always been the bottleneck for the development of solar energy products, and supercapacitors as a fast charging, long life, green Environmentally friendly energy storage components, which bring new vitality to the development of solar energy products. This paper describes in detail the design and implementation of a supercapacitor solar lawn light. The lawn lamp combines the advantages of solar energy and supercapacitor. It can actively emit light without installing other power sources. It can also automatically control the switch according to the intensity of ambient light, and it is easy to install, no wiring, stable and reliable. , maintenance-free, environmentally friendly, non-polluting, long service life, can be widely used in square green space, community lawns and other places.
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1 Design choice
1.1 choice of light source
As LED technology has achieved a key breakthrough, the performance price ratio has also been greatly improved. Today's LEDs have a lifespan of more than 100 000h and low operating voltage, making them ideal for use on solar lawn lights. In addition, the LED is powered by low-voltage DC, and its light source control cost is low, it can adjust the brightness and darkness, and can be frequently switched, and will not adversely affect the performance of the LED. Therefore, from the aspects of reliability, cost performance, color temperature and luminous efficiency, a super bright LED with a rated voltage of 3.3 V and an operating current of 6 mA can be selected as the light source. Because the lawn lamp not only has a decorative effect, but also has a certain lighting function, it can be used with 8 LEDs.
1.2 Solar cell selection
The solar cell is a semiconductor device that converts solar energy into electrical energy according to the photovoltaic effect principle of the semiconductor PN junction, and is the core device of the supercapacitor solar lawn lamp. The performance of solar cells directly determines the energy conversion efficiency and the stability of the output voltage, and also directly determines the performance of the supercapacitor solar lawn lamp. Therefore, the design should use a single-crystal silicon solar cell with better cost performance.
Since the total annual solar radiation and the average peak sunshine hours in different regions of the earth are different, the design of the solar lawn lamp is related to the geographical location of the lamp, and the relationship between the rated output power of the solar cell module and the input power of the lamp. It is about 2 to 4:1, and the specific ratio is determined according to the daily working hours of the lamp and the lighting requirements for continuous rainy days. The power of the solar cell of this system is 3.3VÃ—0.006Ã—8=0.1584 W. Assuming 12 hours of work per day, the efficiency of solar cell work is 40%, and the effective working time per day is 5 hours, then 3 W/6 V can be selected. Solar battery.
1.3 choice of super capacitor
Since the input energy of the solar cell is extremely unstable, and the lawn lamp only emits light when the ambient light is weak, it is necessary to configure the power storage system to work effectively. At present, lead-acid batteries, Ni-Cd batteries or Ni-H batteries are commonly used, but the use of batteries as storage systems has many drawbacks: first, rechargeable batteries have a limited number of charging times (less than 1000 times) and have a short service life; Because of its chemical structure, it can not be charged with large current. Third, the battery needs to have anti-overcharge, over-discharge and temperature compensation control circuits, and the control circuit is more complicated. Fourth, the rechargeable battery is mainly used. The chemical reaction is used for charging and discharging, and the waste in the battery will pollute the environment and is not an environmentally friendly product.
Therefore, supercapacitors are used as energy storage components in this product. The capacitor has a large capacitance of the Farad stage, superior charge retention capability, and a very small leakage current, and the voltage drop rate is less than 5% in 8 hours; no special charging circuit and control discharge circuit are required, charging is rapid, and it can be used only It is charged above its leakage current (typically about 1 mA), so even on a cloudy day, the solar cell can charge the supercapacitor; compared with the battery, its overcharge and overdischarge do not constitute its lifetime. Negative impact, high reliability, long service life (charge and discharge cycle life of more than 100,000 times); in addition, it also has excellent temperature performance, can be used normally in the ambient temperature of -40 Â° C ~ 75 Â° C; no pollution, is a Green power supply; solderable, and there is no such problem as battery contact is not strong.
The solar lawn lamp product uses a 5.0V/225 F capacitor module composed of six 2.5 V/150 F supercapacitors produced by Jinzhou Kaimei Energy Co., Ltd., and its single capacitor product has an ESR (DC) of only 20 mÎ© and a diameter of 25 mm. The height is 48 mm. The charging time of the 5 V/225 F supercapacitor is (at a charging current of 450 mA):
Where: C-capacitor rated capacity; â–³ U-capacitor operating voltage change; I-capacitor charging current; t-capacitor charging time.
The supercapacitor discharge time is:
Where: ESR is the DC internal resistance of the capacitor. The 5 V/225 F supercapacitor can be discharged from 5 V to 0.6 V.
2 system control circuit design
2.1 Charging circuit
The charging circuit in this system is composed of anti-overvoltage and anti-reverse charging circuit. Figure 1 shows its charging circuit. The ZD1 in the figure is a 5.6V Zener diode. When the battery voltage is higher than 5.6 V, Q2 is turned on. The entire current of the silicon battery is consumed by the resistor Q2. When the voltage of the silicon battery drops below 5.6 V, the Q2 is turned off, and the silicon battery charges the super capacitor and simultaneously protects the super capacitor.
The anti-charge control circuit can ensure that when the solar cell input voltage is lower than the super capacitor voltage, the super capacitor does not reversely charge the solar cell to avoid unnecessary energy loss. The reverse charge control can be done by diode D1 in Figure 1. This diode uses a Schottky diode because the turn-on voltage drop of the Schottky diode is lower than that of a normal diode.
2.2 drive LED voltage regulator circuit
Since the selected LED lamp has a rated voltage of 3.3 V and the supercapacitor has a voltage of 5.0 V, and the supercapacitor voltage is continuously reduced during discharge, a voltage stabilizing circuit is required to drive the LED.
This article selects VFM switching DC/DC boost regulator manufactured by CMOS technology and has very low quiescent current. The chip includes VFM control circuit, LX switch drive transistor, reference voltage unit, oscillator, error comparison amplifier and voltage sampling resistor. , LX switch protection circuit, etc. Only one inductor, one output capacitor, and one Schottky diode are required outside the chip to provide a stable low noise output voltage. The regulator features 8Î¼A of very low input current, low ripple, low noise, 80% efficiency, and a very low startup voltage of 0.6 V with an output voltage accuracy of Â±2.5%. This part of the circuit is shown in Figure 2.
The inductance and diode of the 331C peripheral will affect the conversion efficiency. In addition, the capacitance and inductance will also affect the output ripple. therefore. The design should choose the appropriate inductor, capacitor and Schottky diode for higher conversion efficiency and lower ripple and noise. According to the data sheet of the 331C, this design selects a 47Î¼H and less than 0.5 Î© inductor and a 47Î¼F low ESR tantalum capacitor. The diode used for rectification has a great influence on the efficiency of DC-DC. Although the ordinary diode can make the DC-DC circuit work normally, it will reduce the efficiency of the DC-DC converter by 5% to 10%, so the D3 adopts the positive guide. Schottky diode lN5817 with low pass voltage and short reaction time.
2.3 light control circuit
Supercapacitor solar lawn lights require a light-controlled switching circuit to control the switching of the LEDs. To illuminate the LED when the ambient light is dimmed and to turn off the LED when the light is bright. Some light-controlled switch circuits use a photoresistor to switch LEDs, and solar cells can also be used directly as photosensitive switches because solar cells have better characteristics than photoresistors. This part of the circuit is shown in Figure 3.
In Fig. 3, U1 is a six-inverter 74HC14 with a Schmitt trigger, and the multivibrator is composed of a second inverter and R5 and C1. In this way, when the daytime light is bright, the solar cell voltage rises, the 74HC14 pin is high, the 74HC14 pin 2 is low, the 8 pin also outputs a low level, the transistor Q1 is off, and the LED is turned off. When the light is dimmed, when the solar cell voltage drops to a certain level, the HC14 pin is low, the HC14 pin is high, the multivibrator starts to oscillate, and the Y4 output is high and low. When Q2 is turned on and off, it is only necessary to adjust the values â€‹â€‹of R6 and C1 to adjust the flash frequency to the human eye. The biggest advantage of this is that the working time of the lawn lamp can be extended. After actual testing, the circuit can work continuously for more than 10 hours every day, which can fully meet the user's needs.
The design of the supercapacitor solar lawn adopts super capacitor and integrated DC-DC chip, so the whole circuit is simple in design and high in reliability, and belongs to green environmental protection products. Currently, this design has been successfully applied to actual products.
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