A simple, reliable and low-cost solar daylight (SDL) can help reduce monthly electricity bills. SDL does not have any energy storage components and will suffer frequent changes in light intensity when it is cloudy. In addition, spare lights are required after sunset. Using the main power source in the building, backup lighting can be provided. This is a simple and low-cost option. Two types of backup systems are proposed here:

1) On/off backup lighting system based on relay. 2) PWM-based intensity control backup lighting system.

SDL uses 4 PV panels as described in the article "Solar Fluorescent Lamp Design Provides Low-Cost Lighting Solutions, Part 1" [1]. According to the sunlight intensity, the voltage Vpv of 4 PV panels connected in series varies from 60 V to 70 V, and the output power varies from 4 to 40 W. This change in PV power greatly changes the LED light intensity. In order to overcome this problem, the design of the backup system is given below.

The spare lamp powered by the main power supply is installed near the SDL or in the same enclosure. These lights operate according to the light intensity level of the SDL set by the user. The wiring diagram of the reversing light is shown in Figure 1.

Figure 1 Wiring diagram of reversing light.

The reversing light is connected to the main power supply through a manual ON/OFF switch and a solid state relay (SSR). A small AC-DC converter is connected to the main power supply to obtain control power (Vcc = 5V, several milliamperes). 

Figure 2 shows the circuit diagram. It consists of solar fluorescent lamps working on 4 PV panels connected in series. Each panel generates 17.5 V. The voltage Vpv is sensed using optocoupler IC1 (MCT2E). In order to limit the current through the IC1 photodiode, current-limiting resistors R1, R2, R3 (POT) and two Zener diodes ZD1 and ZD2 are connected in series. Potentiometer R3 is for users to set the required SDL light intensity, under which the reversing lights should be turned on.

Figure 2 Circuit diagram of ON/OFF backup circuit using SSR.

Minimum photodiode current = (Vpv – ZD1-ZD2-Vd) / (R1 + R2 + R3) = 1.46 mA Maximum photodiode current = (Vpv – ZD1-ZD2-Vd) / (R1 + R2) = 4.77 mA

The rated forward current of the photodiode is 20 mA. Therefore, the current is completely within the rated value and within the rather linear region of the photodiode characteristics.

Resistor R4 is connected to the emitter of the phototransistor (pin 4). Use IC2 (LM393) comparator CMP1 to detect the emitter voltage. The emitter is connected to the inverting input terminal of CMP1. Use the R5 and R6 voltage dividers to keep the non-inverting input at Vcc/2. When the emitter voltage is lower than Vcc/2, the CMP1 output goes high. The SSR connected to the output (pin 1) is turned on.

Wow the engineering world with your unique design: a guide for design concept submission

In order to reduce the jitter of the SSR, hysteresis must be introduced. When the SSR is turned on, the voltage of pin 1 is clamped at about 3 V. Therefore, pin 1 cannot produce effective hysteresis. For this, use CMP2. The INV pin 6 of CMP2 is kept at approximately 2 V. When pin 1 is high, the CMP2 output of pin 7 is also high (5 V). When Pin1 goes low, the CMP2 output also goes low. Resistor R12 introduces the required hysteresis at pin 3. We can add a bulk capacitor C1 (10000 uF/100 V) for Vpv. This will filter out short-term fluctuations in Vpv. However, C1 increases the cost. If the hysteresis eliminates the chattering, C1 can be made optional.

Note: The current transfer ratio (CTR) of the optocoupler varies from device to device. This will affect the value of R4. Therefore, it is recommended to use a trimming potentiometer for R4 to set the desired emitter voltage value.

As long as the SDL light intensity is below the limit set by the user, this simple circuit will provide good backup lighting. The number of controllable reversing lights is determined by the rated current of the SSR. It can be found in warehouses, office receptions, toilets and other places.

The circuit diagram of the backup system using the PWM signal is shown in Figure 3.

Figure 3 PWM-based backup circuit diagram.

In this figure, only PWM signal generation is shown. The PV interface circuit before the photocoupler MCT2E is still shown in Figure 2. In this circuit, IC3 (LM3524) is used for PWM generation. The IC has an internal operational amplifier (pins 1, 2 and 9). It is configured as a unity gain differential amplifier using 10kΩ 1% resistors R25, R26, R27, and R28.

The phototransistor emitter is connected to the inverting pin 1 of IC3 through R27. The non-inverting pin2 is connected to Vcc through R25. PWM is generated by two output transistors of IC3. The emitters EA and EB of these transistors can provide PWM output. When the phototransistor emitter voltage is zero, the PWM signal has a 100% duty cycle. As the emitter voltage of the phototransistor increases, the PWM duty cycle continues to decrease. Figure 4 shows the duty cycle as a function of emitter voltage.

This solution uses dimming lights with PWM control input. These lamps produce light intensity proportional to the PWM duty cycle. As the SDL intensity decreases, the PWM duty cycle increases, which in turn increases the intensity of the reversing lights. The sum of the intensities of the two lamps is constant. Therefore, regardless of the sunlight conditions, the user can always ensure a constant light output. Therefore, the system provides constant light while maximizing power savings.

Therefore, the proposed backup lighting system eliminates the shortcomings of SDL and does not require expensive batteries and maintenance.

Note: PWM signal must be allocated to all reversing lights. Therefore, it is necessary to use an optocoupler for each lamp to isolate the PWM signal.

Figure 4 PWM duty cycle change (yellow curve) and emitter voltage (blue curve).

Using the above two solutions, we can provide users with a guaranteed amount of light regardless of the sunlight conditions. This will help maximize the use of solar energy without compromising performance. In addition, because it is off the grid, SDL helps reduce the load on the already overburdened grid. All in all, SDL with a main power backup system can provide low-cost lighting solutions for homes, offices, companies, hospitals, warehouses, etc.

[1] Fluorescent lamp design uses passive and active current limiting circuits https://www.edn.com/solar-day-lamp-designs-use-passive-and-active-current-limiting-circuits/

[2] Fluorescent lamp design provides low-cost lighting solutions, Part 1 https://www.edn.com/solar-day-lamp-designs-provide-low-cost-lighting-solutions-part-1/

[3] Fluorescent lamp design provides low-cost lighting solutions, Part 2 https://www.edn.com/solar-day-lamp-designs-provide-low-cost-lighting-solutions-part-2/

Vijay Deshpande has worked as an electronic hardware engineer in various industries for more than 30 years. After retirement, he focused on low-cost, off-grid solar lighting systems.

You must log in or register to post comments.