Design of capacitor solar lawn lamp

Introduction

With the development of economy and the progress of society, people have put forward higher and higher requirements for energy, and the search for new energy has become an urgent issue facing mankind. Since solar power generation has the cleanliness, safety, extensiveness and sufficiency of resources that cannot be matched by thermal power, hydropower and nuclear power, solar energy is considered to be the most important energy in the 21st century. 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 environmental protection of batteries have always been the bottleneck of the development of solar energy products. As a fast charging, long life and green environmentally friendly energy storage element, supercapacitors have brought new vitality to the development of solar energy products. This paper introduces the design and implementation method of a supercapacitor solar lawn lamp. The lawn lamp combines the advantages of solar energy and supercapacitors very well. It can actively emit light without installing other power sources, and can automatically control the switch of the lamp according to the intensity of the ambient light. It is easy to install, does not require wiring, works stably and reliably, is maintenance-free, environmentally friendly and pollution-free, and has a long service life. It can be widely used in square green spaces, community lawns and other places.

1 Design Choices

1.1 Choice of light source

Since LED technology has achieved a key breakthrough, the performance-price ratio has also been greatly improved. The current LED life can reach more than 100,000 hours, and the operating voltage is low, which is very suitable for use in solar lawn lights. In addition, LEDs are powered by low-voltage DC, and the light source control cost is low. They can adjust the brightness and can be switched on and off frequently without adversely affecting the performance of the LED. Therefore, considering the reliability, cost-effectiveness, color temperature and luminous efficiency, an ultra-bright LED with a rated voltage of 3.3 V and an operating current of 6 mA can be selected as the light source during design. Since lawn lights not only have a decorative effect, but also have a certain lighting function, 8 LEDs can be selected.

1.2 Selection of solar cells

Solar cells are semiconductor devices that convert sunlight into electrical energy based on the photovoltaic effect principle of semiconductor PN junctions. They are the core components of supercapacitor solar lawn lights. The performance of solar cells directly determines the energy conversion efficiency and output voltage stability, and also directly determines the performance of supercapacitor solar lawn lights. Therefore, monocrystalline silicon solar cells with better cost performance should be used in the design.

Since the annual total solar radiation and average peak sunshine hours in different regions of the earth are different, the design of solar lawn lights is related to the geographical location of the lights. The relationship between the rated output power of the solar cell module and the input power of the lamp is about 2 to 4:1. The specific ratio depends on 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 3.3V×0.006×8=0.1584W. Assuming that it works for 12 hours a day, the efficiency of the solar cell is 40%, and the effective working time is 5 hours a day, a 3W/6V solar cell can be selected.

1.3 Selection of supercapacitors

Since the input energy of solar cells is extremely unstable, and the lawn lamp only emits light when the surrounding light is weak, a storage system must be configured to work effectively. At present, lead-acid batteries, Ni-Cd batteries or Ni-H batteries are commonly used, but there are many defects in using batteries as storage systems: first, the number of charging times of rechargeable batteries is limited (less than 1,000 times) and the service life is short; second, due to the influence of its chemical structure, it cannot be charged with a large current; third, the battery needs to have control circuits such as overcharge prevention, over-discharge prevention and temperature compensation, and the control circuit is relatively complex; fourth, rechargeable batteries mainly use chemical reactions to charge and discharge, and the waste in the battery will pollute the environment, and it is not an environmentally friendly product.

Therefore, supercapacitors are selected as energy storage components in this product. This capacitor has a large capacitance of Farad level, a strong charge retention capacity, and a very small leakage current, with a voltage drop rate of less than 5% in 8 hours; no special charging circuit and controlled discharge circuit are required, and it charges quickly, and can be charged at a state just higher than its leakage current (typical value is about 1 mA), so even on cloudy days, solar cells can charge supercapacitors; compared with batteries, overcharging and over-discharging do not have a negative impact on their lifespan, and they have high reliability and long service life (charge and discharge cycle life is more than 100,000 times); in addition, they have excellent temperature performance and can be used normally in an ambient temperature of -40℃ to 75℃; they are pollution-free and are a green power source; they can be welded, and there are no problems such as loose contact like batteries.

This solar lawn lamp product uses a 5.0V/225F capacitor module composed of 6 2.5V/150F supercapacitors produced by Jinzhou Kaimei Energy Co., Ltd. The ESR (DC) of a single capacitor product is only 20mΩ, with a diameter of 25mm and a height of 48mm. The charging time of a 5V/225F supercapacitor is (under a charging current of 450mA):

In the formula: C-capacitor rated capacity; △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. A 5 V/225 F supercapacitor can be discharged from 5 V to 0.6 V.

2. Design of system control circuit

2.1 Charging Circuit

The charging circuit in this system is composed of overvoltage protection and reverse charging protection circuits. Figure 1 shows the charging circuit. ZD1 in the figure is a 5.6V voltage regulator diode. When the battery voltage is higher than 5.6 V, Q2 is turned on, and all the current of the silicon battery is consumed through the resistor Q2. When the silicon battery voltage drops below 5.6 V, Q2 is turned off, and the silicon battery charges the supercapacitor and protects the supercapacitor at the same time.

The reverse charging control circuit can ensure that when the solar cell input voltage is lower than the supercapacitor voltage, the supercapacitor will not reverse charge the solar cell to avoid unnecessary energy loss. The reverse charging control can be completed by the diode D1 in Figure 1. This diode is a Schottky diode because the conduction voltage drop of the Schottky diode is lower than that of an ordinary diode. 2.2 LED driving voltage stabilization circuit

Since the rated voltage of the selected LED lamp is 3.3 V, and the voltage of the supercapacitor is 5.0 V, and the voltage of the supercapacitor will continue to decrease during the discharge process, a voltage stabilization circuit is required to drive the LED.

This article uses a VFM switch type DC/DC boost regulator manufactured with CMOS technology and extremely low static current. The chip includes VFM control circuit, LX switch drive transistor, reference voltage unit, oscillator, error comparison amplifier, voltage sampling resistor, LX switch protection circuit, etc. Only one inductor, one output capacitor and one Schottky diode are needed outside the chip to provide a stable low-noise output voltage. The regulator has an extremely low input current of 8μA, low ripple and low noise characteristics, an efficiency of 80%, and an extremely low startup voltage of 0.6 V. The output voltage accuracy can reach ±2.5%. This part of the circuit is shown in Figure 2.

Among them, the inductor and diode outside the 331C will affect the conversion efficiency. In addition, the capacitor and inductor will also affect the output ripple. Therefore, when designing, you should choose the appropriate inductor, capacitor and Schottky diode to obtain higher conversion efficiency and lower ripple and noise. According to the data sheet of 331C, this design selects an inductor of 47μH and less than 0.5Ω and a tantalum capacitor with low ESR of 47μF. The diode used for rectification has a great influence on the efficiency of DC-DC. Although ordinary diodes can also make the DC-DC circuit work normally, it will reduce the efficiency of the DC-DC converter by 5% to 10%. Therefore, D3 uses a Schottky diode lN5817 with a lower forward conduction voltage and a shorter reaction time.

2.3 Light-controlled circuit

Supercapacitor solar lawn lights require a light-controlled switch circuit to control the switch of the LED. This allows the LED to be turned on when the ambient light becomes dim, and turned off when the light becomes brighter. Some light-controlled switch circuits use photoresistors to switch the LED, and solar cells can also be used directly as light-sensitive switches, because solar cells have better characteristics than photoresistors. This part of the circuit is shown in Figure 3.

In Figure 3, U1 is a six-phase inverter 74HC14 with a Schmitt trigger, in which the multivibrator is composed of the second inverter, R5 and C1. In this way, when the light becomes brighter during the day, the voltage of the solar cell increases, the 1st pin of the 74HC14 is high, the 2nd pin of the 74HC14 is low, the 8th pin also outputs a low level, the transistor Q1 is cut off, and the LED is turned off. When the light becomes darker, when the voltage of the solar cell drops to a certain level, the 1st pin of the HC14 is low, the 2nd pin of the HC14 is high, the multivibrator starts to oscillate, and outputs high and low levels through Y4, Q2 is sometimes turned on and sometimes cut off, at this time, just adjust the values ​​of R6 and C1 to adjust the flash frequency to a level that the human eye cannot distinguish, and the biggest advantage of doing so is that it can extend the working time of the lawn lamp. According to actual tests, the circuit can work continuously for more than 10 hours a day, which can fully meet the user's needs.

3 Conclusion

The design of this supercapacitor solar lawn uses supercapacitors and integrated DC-DC chips, so the entire circuit design is simple, the reliability is high, and it is a green and environmentally friendly product. At present, this design has been successfully applied to actual products.
..