Design of LED lighting for solar lawn lamp based on single chip microcomputer

1 Introduction

As an emerging green energy source, solar energy has been rapidly promoted and applied with its incomparable advantages. It is generally believed that energy-saving lamps can save 4/5 of energy, which is a great innovation, but LEDs can save 1/4 more energy than energy-saving lamps, which is a great innovation in solid light sources. In addition, LEDs also have the advantages of high light quality, basically no radiation, reliable and durable, and extremely low maintenance costs. They are typical green lighting sources. The successful development of ultra-bright LEDs has greatly reduced the cost of using solar lamps, making it reach or approach the cost quotation of the initial installation of industrial frequency AC lighting systems, and has the advantages of protecting the environment, easy installation, safe operation, and economical energy saving. This article mainly introduces some knowledge about it, hoping to give you some inspiration.

2 Definition and structural composition of LED solar lawn lights

Solar lawn lights mainly use the energy of solar cells to work. When sunlight shines on the solar cells during the day, the light energy is converted into electrical energy and stored in the battery. The battery then provides power for the LED (light-emitting diode) of the lawn lights at night. Its advantages are mainly safety, energy saving, convenience, and environmental protection. It is suitable for beautification and lighting of green lawns in residential communities and beautification and decoration of park lawns. The structure of LED solar lawn lights: It consists of solar cell modules (photovoltaic panels), ultra-bright LED lights (light sources), maintenance-free rechargeable batteries, automatic control circuits, lamps, etc.

3 System composition, control principle and circuit principle of solar lawn lamp

3.1 System composition of solar lawn lights

LED solar Lawn lights are an independent power generation system. It can independently convert solar energy into electrical energy, and can convert electrical energy into heat energy for lighting and decoration without the need for wire transmission. An independent photovoltaic system generally consists of the following three parts: solar cell components; power electronic equipment such as charge and discharge controllers, inverters, test instruments and computer monitoring, and batteries or other energy storage and auxiliary power generation equipment. The photovoltaic system has the following characteristics: no rotating parts, no noise; no air pollution, no wastewater discharge; no combustion process, no need for fuel; simple maintenance and low maintenance costs; good operating reliability and stability; solar cells as key components have a long service life, and the life of crystalline silicon solar cells can reach more than 25 years; it is easy to expand the scale of power generation according to needs.

Figure 4-0 Several common LED solar lawn lights

Figure 4-1 is a typical schematic diagram of a photovoltaic system supplying DC loads. It includes several main components in the photovoltaic system: Photovoltaic module array: It is composed of solar cell modules (also called photovoltaic cell modules) connected in series and parallel according to system requirements. It converts solar energy into electrical energy output under sunlight. It is the core component of the solar photovoltaic system.

LED solar lawn light is a small solar power supply system (Figure 4-2 is a simple solar power supply system). Its structure is very simple and mainly consists of solar panels, charge and discharge controllers, batteries, lighting circuits and lamp poles (as shown in Figure 4-3).

3.2 Control Principle of Solar Lawn Light

The controller of solar lawn lamp is mainly used to control the charging and discharging of batteries. Figure 4-4 is a basic charging and discharging controller. In this figure, a basic photovoltaic application system is composed of photovoltaic panels, batteries, solar controllers and loads. The switches K1 and K3 here are charging switches, and K3 is a discharging switch, both of which are part of the solar control center. The opening and closing of the switches in the figure are determined by the control circuit according to the charging and discharging status of the system. When the battery is fully charged, the charging switch is disconnected, and the charging switch is closed when charging is required; when the battery is discharged, K2 is closed, otherwise it is disconnected. These control circuits can use voltage comparison boost charging and discharging circuits composed of triodes, resistors, capacitors, and inductors, or light control circuits, or voltage hysteresis comparators composed of integrated operational amplifiers, or single-chip microcomputers. In view of the low cost, the former is generally used.

Generally speaking, a qualified solar charge and discharge controller has the following charge and discharge protection modes:

a Direct charging protection point voltage: Direct charging is also called urgent charging, which belongs to fast charging. Generally, the battery is charged with a large current and a relatively high voltage when the battery voltage is low. However, there is a control point, also called a protection point, which is the value in the table above. When the battery terminal voltage is higher than these protection values ​​during charging, direct charging should be stopped. The direct charging protection point voltage is generally also the "overcharge protection point" voltage. The battery terminal voltage cannot be higher than this protection point during charging, otherwise it will cause overcharging, which is harmful to the battery.

b. Voltage of equalization control point: After the direct charge is completed, the battery will generally be left idle for a period of time by the charge and discharge controller to allow its voltage to drop naturally. When it drops to the "recovery voltage" value, it will enter the equalization state. Why is equalization designed? That is, after the direct charge is completed, some batteries may "lag behind" (the terminal voltage is relatively low). In order to pull these individual molecules back and make all the battery terminal voltages uniform and consistent, it is necessary to charge them for a short time with a high voltage and a moderate current. It can be seen that the so-called equalization is "balanced charging". The equalization time should not be too long, generally a few minutes to more than ten minutes. Setting the time too long is harmful. For a small system equipped with one or two batteries, equalization is not very meaningful. Therefore, street light controllers generally do not have equalization, but only two stages.

c Floating charge control point voltage: Generally, after equalization charging is completed, the battery is left to stand for a period of time to allow the terminal voltage to drop naturally. When it drops to the "maintenance voltage" point, it enters the floating charge state. At present, PWM (i.e. pulse width modulation) mode is adopted, which is similar to "trickle charging" (i.e. low current charging). The battery voltage is charged a little bit when it is low, and a little bit when it is low, one by one, to prevent the battery temperature from continuing to rise. This is very good for the battery, because the internal temperature of the battery has a great influence on charging and discharging. In fact, the PWM mode is mainly designed to stabilize the terminal voltage of the battery, and the battery charging current is reduced by adjusting the pulse width. This is a very scientific charging management system. Specifically, in the late stage of charging, when the remaining capacity (SOC) of the battery is >80%, the charging current must be reduced to prevent excessive gas release (oxygen, hydrogen and acid gas) due to overcharging.

d. Over-discharge protection termination voltage: This is easier to understand. The battery discharge cannot be lower than this value, which is stipulated in the national standard. Although battery manufacturers also have their own protection parameters (enterprise standard or industry standard), they will eventually move closer to the national standard. It should be noted that for safety reasons, the 12V battery over-discharge protection point voltage is generally artificially added with 0.3V as temperature compensation or zero drift correction of the control circuit. In this way, the over-discharge protection point voltage of the 12V battery is 11.10V, and the over-discharge protection point voltage of the 24V system is 22.20V. At present, many manufacturers of charge and discharge controllers use the 22.2V (24V system) standard.

3.3 Design of Solar Lawn Lamp Charge and Discharge Controller

As the interface circuit between photovoltaic cells and lead-acid batteries, charging controllers are generally expected to operate at the maximum power point to achieve higher efficiency. However, while achieving maximum power point tracking (MPPT), battery charging control also needs to be considered. Currently, the commonly used main circuit topologies include buck converters, boost converters, and Cuk converters. Generally, the output voltage of photovoltaic cells fluctuates greatly, while Buck converters or Boost converters can only perform buck or boost conversion. As a result, photovoltaic cells cannot fully operate at the maximum power point over a large range, resulting in reduced system efficiency. At the same time, the input current ripple of the Buck converter is large. If a storage capacitor is not added to the input end, the system will work in an intermittent state, which will cause the output current of the photovoltaic cell to be intermittent and cannot be in the best working state; while the output current ripple of the Boost converter is large. Using this current to charge the battery is not conducive to the service life of the battery; the Cuk converter has both boost and buck functions. Applying the Cuk converter to the photovoltaic system charging controller can achieve maximum power point tracking in a large range, which is conducive to improving system efficiency. Therefore, the Cuk converter is often selected as the main circuit of the charging controller, and its system topology is shown in Figure 3-2.

When the load current of the Cuk converter is continuous, the steady-state process of the circuit is:

1. During the conduction period of the switch tube Vr

During this period, the switch tube Vr is turned on, and the voltage on the capacitor C2 causes the diode D2 to be reverse biased and cut off. At this time, the input current iL2 causes Ll to store energy; the discharge current iL2 of C2 causes L2 to store energy and supply power to the load, as shown in Figure 3-3 (a).

2. During the period when the switch tube Vr is turned off, the switch tube Vr is turned off, the diode D2 is forward biased and turned on, the power supply and the energy release current iLl of Ll charge C2, and at the same time, the energy release current iL2 of L2 maintains the load, as shown in Figure 3.3 (b). Therefore, C2 is charged during the period when Vr is turned off, and C2 discharges to the load during the period when Vr is turned on, and C2 plays the role of energy transfer.

3.4 Circuit Principle of Solar Lawn Light

The circuit principle of solar lawn lamp is relatively simple. Here we will introduce a simple circuit principle of solar lawn lamp. Its controller is realized by using boost circuit.

Component selection: BT1 uses a 3.8V/80mA solar panel, monocrystalline silicon is preferred, polycrystalline silicon is second; BT2 uses two 1.2V/600mA Ni-Cd batteries. If you need to increase the luminosity or extend the time, you can increase the power of the solar panel and battery accordingly. The β of VQ2, VQ3, and VQ5 is about 200, and VQ4 needs a transistor with a large β value. VD1 should try to choose a tube with low voltage, such as a germanium tube or a Schottky diode. LEDs can choose white, blue, and green ultra-high brightness scattered light or concentrated light. When low-voltage drop LEDs such as red, yellow, and orange are selected, the circuit needs to be reset. R3 and R5 are recommended to use 1% precision resistors; R4 uses a photoresistor with a bright resistance of 10kΩ~20kΩ and a dark resistance of more than 1MΩ. Other resistors can use ordinary carbon film (1/4)W and (1/8)W resistors. L1 uses a (1/4)W color inductor with a small DC impedance.

Working principle of the circuit: When there is sunlight during the day, BT1 converts light energy into electrical energy, and VD1 charges BT2. Due to the light, the photoresistor is low-resistance, and VQ4 is turned off at a low level. When there is no light at night, the photoresistor is high-resistance, VQ4 is turned on, and VQ2 is also turned on at a low level. The DC boost circuit composed of VQ3, VQ5, C2, R6, and L1 works, and the LED is powered and emits light.

The core of the DC boost circuit is a complementary tube oscillator circuit, and its working process is: when VQ2 is turned on, the power supply charges C2 through L1, R6, and VQ4. Since the voltage across C2 cannot mutate, the VQ3 b pole is at a high level, and VQ3 is not turned on. As C2 charges, its voltage drop becomes higher and higher, and the VQ3 b pole potential becomes lower and lower. When it is as low as the VQ3 turn-on voltage, VQ3 is turned on, and VQ5 is turned on successively. C2 discharges through the VQ5 ce junction, the power supply, and the VQ3 eb junction (since VQ2 is turned on, we assume that its ec junction is short-circuited, and the VQ3 e pole is directly connected to the positive pole of the power supply).

After the discharge, VQ3 is turned off, VQ5 is turned off, and the power supply charges C2 again. Then VQ3 is turned on, VQ5 is turned on, and C2 discharges. This is repeated, and the circuit forms an oscillation. During the oscillation process, when VQ5 is turned on, the power supply is connected to the ground through L1 and VQ5 ce junction, and the current is stored through L1. When VQ5 is turned off, L1 generates an induced electromotive force, which is superimposed with the power supply to drive the LED, and the LED emits light. The battery voltage can be increased to directly drive the LED to improve efficiency, but as the battery voltage increases, the corresponding solar cell price also increases significantly. As long as the circuit components are set appropriately, its efficiency is still acceptable. When the charge is not enough during the day (such as encountering rainy days, etc.), BT2 may be over-discharged, which will damage the battery. For this reason, R5 is added to form an over-discharge protection: when the battery voltage drops to 2V, due to the voltage division of R5, the base potential of VQ4 is not enough to turn on VQ4, thereby protecting the battery. Increasing R5 will affect the conduction depth of VQ4.

4 Light source advantages and selection

At present, most lawn lamps use LED as the light source. LED has a long life span of more than 100,000 hours and a low operating voltage, which is very suitable for use in solar lawn lamps. In particular, LED technology has undergone a key breakthrough, and its characteristics have been greatly improved in the past five years, and its performance-price ratio has also been greatly improved. In addition, LED is powered by low-voltage DC, and its light source control cost is low, making it possible to adjust the brightness and frequent switching without adversely affecting the performance of the LED. It can also easily control the color, change the distribution of light, and produce dynamic illusions, so it is particularly suitable for solar lawn lamps.

However, LED has many inherent characteristics. If you don't pay attention to it when using it, it will cause adverse consequences. The luminous efficiency of LED currently sold on the market can only reach 15lmW, which is only 1/3 of the three-color primary color high-efficiency energy-saving lamp. The luminous efficiency of the three-color primary color high-efficiency energy-saving lamp can reach 50-60lmW. From the price point of view, the current production cost per 1m: three-color primary color high-efficiency energy-saving lamp (including electronic ballast) is 0.022 yuan, and the price of φ5mm white light LED in 2002 was 1.9-3.0 yuan. The current production cost per 1m is very different. From the service life point of view, the service life of the three-color primary color high-efficiency energy-saving lamp (including electronic ballast) can reach 6000h, and the LED can reach more than 100,000h. On the surface, the service life of LED is dozens of times that of the three-color primary color high-efficiency energy-saving lamp (including electronic ballast), but this is not the case. At present, most solar lawn lights use ultra-bright white LEDs. The time (lifespan) for the ultra-bright white LED to maintain 50% of the initial intensity at 20mA is less than 10,000 hours. The Institute of Electric Light Sources of Fudan University has proved the above argument. That is to say, in many cases, LED is not the best solar lawn light source, unless it is a low-end solar lawn light with a service life of only 1-2 years, or a solar lawn light below 1W. For solar lawn lights above 1W, it is best to use three-color primary color high-efficiency energy-saving lamps. At present, some solar lawn lights use 30-40 ultra-bright white LEDs with an input power of more than 2W. In this case, if a three-color primary color high-efficiency energy-saving lamp is used, the price is only 1/10 of that of LED, and the luminous flux is 4 times the original. Fortunately, a 2-10W low-voltage DC three-color primary color high-efficiency energy-saving lamp has been successfully developed, and its lifespan can reach 6,000 hours.

According to the above analysis, we believe that low-power solar lawn lights below 1W, which have the functions of adjusting brightness and frequent switching, should generally use LED as the light source. However, when using ultra-bright white LEDs, special attention should be paid to the problem of lumen maintenance rate, otherwise it is easy to cause quality accidents. For solar lawn lights with higher power, it is more reasonable to use three-color primary color high-efficiency energy-saving lamps. It should be emphasized here that the above conclusions are only the current analysis. When the LED technology level is improved and the price drops, the above conclusions need to be adjusted.

5 Problems encountered

5.1 Photosensitive Sensor

Solar lawn lights need a light-controlled switch. Designers often use photoresistors to automatically switch the lights on and off. In fact, the solar cell itself is an excellent light-sensitive sensor. Using it as a light-sensitive switch has better characteristics than a photoresistor. For a solar lawn light that only uses a 1.2V Ni-Cd battery, the solar cell assembly consists of four solar cells in series, and the voltage is low. The voltage is even lower under weak light, so that the voltage is lower than 0.7V before it gets dark, causing the light-controlled switch to malfunction. In this case, the problem can be solved by adding a transistor for direct coupling and amplification.

5.2 Control the load size according to the battery voltage

Solar lawn lights often have high requirements for the duration of continuous rain, which increases system costs. When the battery voltage drops during continuous rain, we can reduce the number of LEDs connected or reduce the daily lighting time of the solar lawn lights, which can reduce system costs.

5.3 Solar Cell Packaging

At present, there are two main forms of solar cell packaging: lamination and glue dripping. The lamination process can ensure that the solar cell has a working life of more than 25 years. Although glue dripping looks beautiful at the time, the working life of the solar cell is only 1-2 years. Therefore, for low-power solar lawn lights below 1W, glue dripping can be used if there is no high life requirement. For solar lawn lights with a specified service life, it is recommended to use a laminated packaging form.

5.4 Flashing and changing light

Gradually brightening and dimming is a good way to save energy. On the one hand, it can increase the illumination effect of the solar lawn. On the other hand, it can control the average output current of the battery by changing the flashing duty cycle, thereby extending the system working time. Or, under the same conditions, it can reduce the power of the solar cell and the cost will be greatly reduced.

5.5 Switching speed of three-color primary high-efficiency energy-saving lamp

This problem is very important, it even determines the service life of solar lawn lights. The three-color primary color high-efficiency energy-saving lamp has a starting current of up to 10-20 times. When the system is subjected to such a large current, the voltage may drop significantly, and the solar lawn light may not start or start repeatedly until it is damaged.

5.6 Improving the efficiency of boost circuits and their impact on LED lamps

Low-power solar lawn lights generally have a boost circuit. If an oscillating circuit is used, the inductor is boosted. The inductor uses a standard color code inductor. The standard color code inductor uses an open magnetic circuit, and the magnetic flux loss is large, so the circuit efficiency is low. If a closed magnetic circuit is used to make an inductor boost, such as a magnetic ring, the boost circuit efficiency will be greatly improved. The characteristics of LEDs are close to that of a voltage-stabilizing diode . When the operating voltage changes by 0.1V, the operating current may change by about 20mA. For safety, a series current limiting resistor is usually used. The huge energy loss is obviously not suitable for solar lawn lights, and the LED brightness changes with the operating voltage, and the current must be automatically limited, otherwise the LED will be damaged. Generally, the peak current of an LED is 50-100mA. If a high-energy battery is reversed or the battery is unloaded, the peak voltage of the boost circuit is too high and is likely to exceed this limit, damaging the LED.