Mains powered backup lights for solar day lamps

Article By : Vijay Deshpande

Mains powered backup light is a cost effective and elegant solution for solar day lamps. User gets constant light under all sunlight conditions.

A simple, reliable and low cost Solar Day Lamp (SDL) is useful in reducing the monthly power bills. SDL without any energy storage element, suffers from frequent changes in the light intensity on cloudy days. Also, backup lights are required after sunset. Using mains power present in the building, it is possible to provide backup lighting. It’s a simple and low cost option. Two types of backup systems are proposed here:

1) Relay based ON/OFF backup lighting system.
2) PWM based intensity controlled backup lighting system.

SDL which uses 4 PV panels is described in the article “Solar day lamp designs provide low-cost lighting solutions, Part 1” [1]. Depending upon the sunlight intensity, voltage Vpv of series connected 4 PV panels, varies from 60 V to 70 V, and the power output varies from 4 to 40 W. This variation in the PV power changes the LED light intensity very widely. To overcome this problem, backup system design is given below.

On/Off backup system

Backup lamps powered from Mains Supply are mounted near the SDL or in the same enclosure. These lamps are operated based on the light intensity level of SDL, as set by the user. The interconnection diagram of backup lamps is shown in Figure 1.

Figure 1 Interconnection diagram of backup lamps.

The backup lamps are connected to the mains supply through a manual ON/OFF switch and a Solid State Relay (SSR). A small AC-DC converter is connected to the mains supply for getting control power (Vcc = 5V, few mA).

Figure 2 shows the circuit diagram. It consists of a solar day lamp working on 4 PV panels connected in series. Each panel generates 17.5 V. Voltage Vpv is sensed using Opto-coupler IC1 (MCT2E). To limit the current through photodiode of IC1, current limiting resistors R1, R2, R3 (POT) and two Zener diodes ZD1 and ZD2 are connected in series. Pot R3 is provided for user to set the desired SDL light intensity at which backup lamps should turn ON.

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

At Vpv = 70 V:

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

The rated forward current of photodiode is 20 mA. Hence the currents are well within the rated value, and in a fairly linear region of the photodiode characteristics.

Resistor R4 is connected to the emitter (pin 4) of the phototransistor. The emitter voltage is sensed using comparator CMP1 of IC2 (LM393). Emitter is connected to inverting input of CMP1. Non-inverting input is held at Vcc/2 using R5 and R6 potential divider. When emitter voltage reduces below Vcc/2, CMP1 output goes high. The SSR connected at the output (pin1) turns ON.

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In order to reduce chattering of SSR, hysteresis has to be introduced. When SSR is ON, voltage at Pin1 gets clamped to about 3 V. Hence, Pin 1 cannot produce effective hysteresis. For that purpose, CMP2 is used. The INV pin 6 of CMP2 is held at about 2 V. When Pin1 is high, CMP2 output at Pin7 is also high (5 V). When Pin1 goes low, CMP2 output also goes low. Resistor R12 introduces required hysteresis at Pin 3. We can add bulk capacitor C1 (10000 uF/ 100 V) for Vpv. This will filter out short term fluctuations in Vpv. However, C1 adds to the cost. If hysteresis eliminates chattering, then C1 can be made optional.

Note: The current transfer ratio (CTR) of opto-couplers varies from device to device. This impacts the value of R4. Hence, it is recommended to use a trim pot for R4, to set required value of emitter voltage.

This simple circuit provides good backup lighting whenever the SDL light intensity falls below the limit set by the user. The number of backup lamps which can be controlled is decided by the current rating of SSR. It can find applications in places like warehouses, office receptions, washrooms etc.

PWM-based backup

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

Figure 3 Circuit diagram of PWM-based backup.

In this figure, only PWM signal generation is shown. The PV interface circuit up to the Opto-coupler MCT2E remains same as shown in Figure 2. In this circuit, IC3 (LM3524) is used for PWM generation. This IC has an internal Op-amp (pins 1,2 and 9). It is configured as unity gain difference amplifier using 10kΩ 1% resistances R25, R26, R27 and R28.

To the inverting pin1 of IC3, phototransistor emitter is connected through R27. Non-inverting pin2 is connected to Vcc through R25. PWM is generated by two output transistors of IC3. PWM output is available at emitters EA and EB of these transistors.  When photo transistor emitter voltage is zero, PWM signal has 100 % duty cycle. As the photo transistor emitter voltage increases, PWM duty cycle goes on reducing. Figure 4 shows the duty cycle variation with emitter voltage.

This scheme uses dimmer lamps having PWM control input. These lamps produce light intensity proportional to PWM duty cycle. As the SDL intensity reduces, the PWM duty cycle increases, in turn backup lamp intensity increases. The sum of two lamp intensities is constant. Thus, user is always assured of constant light output irrespective of sunlight conditions. Therefore this system gives constant light, while maximizing the power savings.

Thus the proposed backup lighting systems eliminate the drawbacks of SDL, without requiring costly batteries and it’s maintenance.

Note: The PWM signal has to be distributed to all the backup lamps. Therefore, it is necessary to use one Opto-coupler for each lamp for isolating the PWM signal.

Figure 4 PWM duty cycle variation (yellow trace) with emitter voltage (blue trace).

Using the above two schemes, we can provide assured amount of light to the user, irrespective of sunlight conditions. This will help utilize the solar power to the maximum extent, without compromising on the performance. Further, being off grid, the SDL helps in reducing the load on the already overburdened grid. To conclude, SDL with Mains power backup system provides low cost lighting solution to homes, offices, companies, hospitals, warehouses etc.


[1] Solar day lamp designs use passive and active current limiting circuits

[2] Solar day lamp designs provide low-cost lighting solutions, Part 1

[3] Solar day lamp designs provide low-cost lighting solutions, Part 2

This article was originally published on EDN.

Vijay Deshpande worked as an electronics hardware engineer for more than 30 years in various industries. After retirement mainly working on low cost, Off-Grid, solar lighting systems.


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