Low power LED corridor lights

This corridor light made of high-brightness light-emitting diodes is turned off during the day and automatically lights up after dark. The annual power consumption does not exceed 2 degrees, and the service life can reach 10 years.

Circuit working principle

The circuit of the corridor light is shown in the figure below. The circuit consists of a capacitor step-down circuit, a rectifier circuit, an LED light-emitting circuit, and a photoelectric control circuit.

After the 220V AC is stepped down and limited by capacitors C1 and R1, the AC voltage at points A and B is about 15V. It is rectified by VD1~VD4. A DC voltage of about 14V is obtained on C2 as the working voltage of high-brightness light-emitting diodes VD5~VD8. The working current of the light-emitting diodes is about 14mA. Since capacitor C1 does not consume active power, the power consumed by the discharge resistor can be ignored, so the power consumption of the entire circuit is about 15×0.014≈0-2(W).
In order to further save energy and extend the service life of high brightness LED, a photoelectric control circuit composed of photoresistor R2, resistor R3 and transistor VT1 is added to the circuit. At night, the resistance of photoresistor R2 can reach more than 100K. At this time, the DC bias voltage provided to the base of VT1 by the voltage across C2 after voltage division by R2 and R3 is very small, VT1 is cut off, and there is no effect on the operation of the LED. During the day, due to the photoelectric effect, the resistance of R2 can be reduced to less than 1OK. At this time, VT1 is turned on and close to saturation. Since the maximum current through C1 can only reach 15mA, due to the shunt of VTl, the voltage on C2 can drop to less than 4V.
Because the working voltage of each LED must reach more than 3V to start working, and the four series must reach more than 12V, so the LED cannot emit light normally at this time.
Then, why does the power consumption of the circuit decrease after VT1 shunt "short circuit"? This is because the capacitor voltage reduction circuit can be regarded as a constant current source when the output voltage is not too high, and its power consumption is proportional to the load resistance. After VT1 shunt causes the voltage on C2 to drop, it is equivalent to the load resistance decreasing, so the power consumption decreases. Take the AC voltage at both ends of A and B dropping to 5V as an example, assuming that the AC current passing through C2 reaches 15mA, then the power consumption of the circuit is about 5×0.015≈0.08(W). Therefore, taking the average lighting of 12 hours every night as an example, the annual power consumption is: (0.2×12+0.08×12)×365≈1.2(kWh), that is, 1.2 kWh.

Component selection and installation

In the figure below, C1 uses a 0.22μ 250VAC metallized polyester film capacitor. R2 uses a CDS photoresistor, model GL4516, whose bright resistance (10Lux) is 5~10KΩ and dark resistance is 0.6MΩ. Other types of CDS photoresistors can also be used. When the parameters are different, just adjust the resistance value of R3 appropriately. VD5~VD8 use high-brightness white light-emitting diodes with a rated working current of 20mA. Other component parameters are shown in the figure below.
The corridor lights can be reassembled using old energy-saving lamps. Remove the original lamp tube, and the hole left is just the exit hole of the light-emitting diode. Since the exit hole is relatively large, four small holes can be made in the corresponding position of the light-emitting diode on cardboard as a substrate. When assembling, there is no need to remake the circuit board. The original circuit board can be used for assembly, leaving 4 rectifier diodes, removing the original 400V electrolytic capacitor and replacing it with a 100μ25V electrolytic capacitor, removing the redundant components on the circuit board, and using the original solder joints to lap-weld the other components on the corridor lamp. Open a hole in the upper part of the circular shell as the light-transmitting hole of the photoresistor to facilitate the external light to shine on the photoresistor.
After installation, turn on the power. Under normal indoor light, if the light-transmitting hole of the photoresistor is blocked, the light will light up, and the light will go out after it is removed, indicating that the working state of the lamp is normal. Finally, install the lamp in the corridor used. If the light is lit when the light is slightly dim during the day, the resistance value of R3 can be appropriately increased; on the contrary, if the light cannot be lit when the light is dim at night, the resistance value of R3 can be appropriately reduced. Under normal circumstances, no debugging is required. The corridor lamp after lighting up.
Note: Since the entire circuit is directly connected to a 220V AC power supply, pay attention to safety during production and debugging.