Solar street light control circuit

The ideal solar street light controller should have the following functions: ① battery components and battery reverse connection protection; ② load overcurrent, short circuit and surge protection; ③ battery open circuit protection, overcharge and overvoltage protection, overdischarge and undervoltage protection; ④Line lightning protection: ⑤Light control, time control, power reduction control function; ⑥Various working status display function; ⑦Night anti-reverse discharge protection; ⑧Environmental temperature compensation function, etc.

The principle block diagram of the solar street light control circuit is shown in Figure 1. Using a single-chip microcomputer as the control circuit can make the charging process simple and efficient, and select a series control circuit. The single-chip PWM control system has the ability to track the maximum power point of photovoltaic components, which improves the utilization of photovoltaic cells. The PWM control system can also reduce the frequency and time of the charging pulse when the battery tends to be fully charged, so that the change of the average charging current during the charging process is more in line with the state of charge of the battery, and truly realizes the charging from 0% to 100%.

Solar street light control circuit
Figure 1 – The principle block diagram of the solar street light control circuit

The charging of the battery by the battery components is divided into three stages: direct charge, float charge and trickle charge. When designing the circuit, it is necessary to make temperature correction compensation for the charging and discharging voltage set points of the battery, even if the voltage setting value of each charging and discharging stage is automatically adjusted with temperature changes. The temperature compensation must meet the technical conditions of the battery, and the single cell uses -4mV/℃ as the reference value.

The following describes the circuit composition and working principle of a street lamp controller. The specific circuit is shown in Figure 2. It is composed of a charging circuit, a discharging circuit, a working status indicating circuit, and a temperature compensation circuit.

Solar street light control circuit
Figure 2 – Circuit schematic diagram of solar street light controller

In Figure 2, the battery DC is the working power supply of the controller circuit and the power supply of the entire street lamp; C1, C3, C4 are high-frequency filter capacitors, which are used to filter out high-frequency clutter induced or generated by battery components and loads, and reduce the interference to the single-chip microcomputer and control system; varistor RV1 is used to absorb the lightning surge voltage that enters the controller through battery components and lines; VT4, VD6 and other components constitute a voltage stabilizing circuit, which stabilizes the 12V input voltage of the battery to 10V for the controller circuit to work, and prevents the influence of the battery voltage change on the control circuit; VT2, VD4, etc. form a 5V voltage stabilizing circuit to supply power to the microcontroller and related circuits; the voltage stabilizing diodes VD1 and VD3 are components for MOS tube gate protection; resistors R1, R2, R12, R28 and diode VD5 compose the output voltage detection circuit of solar cell module, and the various states of the output voltage of the battery assembly are input to the MCU circuit through pin 3 of the MCU chip IC1, and the street lamps can be light-controlled switch through the photosensitivity of the battery assembly; R19, R24, C6, etc. form a battery voltage detection circuit, which reflects the status of the battery terminal voltage to the single-chip microcomputer, and the single-chip microcomputer controls the corresponding charging stages according to the status of the battery terminal voltage; Pin 1 of the microcontroller IC is the positive power supply pin, pin 14 is the ground wire of the controller, that is, the negative electrode of the battery, and pin 4 is the selection function end of the lead-acid battery and the gel battery, which can be selected by the opening and closing of the S1 switch; VT3 is a transistor that controls the output. When VT3 is turned on, the MOS output control transistor VT8 turns off the power supply to the load, and the output protection (undervoltage) indicator LED1 lights up.

Charging process: When the battery assembly is exposed to sunlight, the voltage signal is input through pin 3 of IC1, and its internal A/D input conversion circuit realizes the sampling, measurement and comparison of the battery assembly voltage, when the output voltage of the battery module exceeds 6V, the solar charging indicator LED5 lights up and the charging process is started. When the battery capacity is low, pin 2 of IC1 outputs a high level, VT5 is cut off, VT1 is cut off, and VT6 and VT7 are on. The battery component current flows from the battery component positive electrode-battery positive electrode-battery negative electrode → VT6 → VT7-battery component negative electrode to quickly charge the battery. As the voltage at both ends of the battery continues to rise, the battery capacity indicators LED2, LED3, and LED4 will light up in turn to show the battery capacity status. During the charging process, when the battery terminal voltage reaches 13.6V and can last for 30s, the circuit automatically switches to the PWM floating state, IC1 pin 2 changes from high level to output PWM signal, the frequency is 30Hz, through VT5, VT1 control VT6, VT7 turn on and off, for the battery float charge.

During the floating charging process of the battery, as the voltage of the battery terminal changes, the pulse width of the charging current is changed accordingly, adjusting the change of the charging current, and repeating this way. After the PWM floating charge state makes the battery terminal voltage reach the overcharge protection voltage value of 14.6V, and can continue to maintain for more than 30s, the entire charging process is basically completed. If you still need to run current charging, the circuit outputs a relatively narrow PWM pulse current for intermittent charging, the intermittent time is more than 30min.

Discharge process: The discharge circuit is composed of R25, R27, VT8, VD3 and lighting lamp load. When the battery voltage is higher than 11V, the two ends of the load can output the mixed electric energy of the battery and the battery assembly. When the battery voltage drops to 11V, IC1’s pin 10 outputs a high level, turning on VT3 and turning off VT8. At the same time, the under-voltage indicator LED1 lights up, and the over-discharge protection works. Because the characteristics of lead-acid batteries determine that they cannot be in a state of loss for a long time, the batteries protected by over-discharge must be charged in time, and the system only allows the batteries to restore power to the load when the battery is charged to 12.5V.

The battery capacity indicator is composed of LED2, LED3 and LED4. When LED4 is on, it indicates that the battery capacity is greater than 75%, and the terminal voltage is above 12.8V; when LED3 is on, it means that the battery capacity is greater than 25% and less than 75%, and the terminal voltage is 11.8~12.8V; when LED2 is on, it means that the capacity is less than 25%, and the terminal voltage is 11~11.8V. When the voltage drops to close to 11V, LED2 flashes. At this time, the system requires the load to be turned off to protect the battery; if it is not turned off, the system will forcibly cut off the power supply to the load after 3 minutes, and the under-voltage indicator LED1 will light up.

Light control to turn on the light: in the evening, when the ambient illuminance drops to 5~101x, the output voltage of the battery panel is less than 6V and reaches the circuit start-up point. IC1 is delayed for 10 minutes and then confirmed, VT8 is turned on and the load power is turned on, and the light is automatically turned on. At dawn in the morning, when the ambient light reaches a certain illuminance, the output voltage of the battery panel is higher than 6V, the controller delays again for 10 minutes and then confirms, VT8 is cut off, and the light is automatically turned off.

VT6 is a MOS protection device that prevents the battery from discharging back to the battery panel at night or when the sun is insufficient. When IC1’s pin 3 detects that the solar panel voltage is lower than 11.3V, it automatically turns off VT6. R20, R21, and VD8 form a battery ambient temperature compensation circuit. VD8 changes the voltage of IC1’s pin 11 with temperature changes. It is converted by IC1’s internal A/D circuit, and then processed by software to change the voltage setting value of each charging and discharging stage. The compensation coefficient is -25mV/℃.