This lamp circuit using ultra-bright white LEDs provides sufficient light for reading purposes while consuming approximately 3 watts of power. In the case of AC mains failure, the battery backup circuit instantly lights up the LEDs. When the power resumes, the battery supply is automatically disconnected and the lamp circuit again works off AC mains.
The power supply circuit consists of 0-7.5V, 500mA step-down transformer X1, rectifier diodes D1 through D4 and filter capacitor C1. Regulator IC 7805 (IC1) provides regulated 5V to LEDs, so there is no variation in the intensity of the lamp light even if the mains power supply fluctuates. A total of ten white LEDs (LED1 through LED10) are connected in parallel across the 5V power supply. Resistors R1 through R10 (each 56 ohms) are connected in series with the white LEDs to limit the current. To increase the intensity of the lamp light, you can add more LEDs in the same manner; a maximum of 15 LEDs can be used for the lamp.
When power switch S1 is closed, relay RL1 energises to disconnect the 6V, 4Ah battery (connected across N/C contact of relay RL1) from input to regulator IC1 if battery switch S2 is closed. When power switch S1 is open, relay RL1 de-energises and connects the battery to the input of IC1 via N/C contacts of the relay.
Diodes D5 and D6 are reverse-current protection diodes that don’t allow the battery current to flow towards the power supply section. Diode D7 is for reverse polarity protection of the battery. Before connecting the battery, make sure that it is fully charged.
The circuit can be assembled on a general-purpose PCB. Arrange all white LEDs (LED1 through LED10) on the PCB. Now remove the bulb holder from the lamp and fix the PCB (where bulb holder was mounted) such that LED light falls on your book properly. No separate reflectors are required for LEDs as the LEDs have inbuilt lens reflectors. Use a heat-sink for IC1 as indicated in the figure.
Caution. Though you can read for hours without eye strain in this lamp light, don’t directly look at white LEDs for long.
Author : Pranab Kumar Roy
Sourced : EFY
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