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02-24-2010

18 LED dimmable LED lamp

G. Forrest

Introduction

This circuit is a dimmable white LED lamp array with 18 LEDs. The lamp brightness is regulated as long as the input voltage is above 10.5V. A low-dropout analog voltage regulator is used for a simple and relatively efficient design. The lamp produces enough light to use as a a reading lamp or a small work lamp.

18 LED dimmable LED lamp

18 LED dimmable LED lamp

Specifications

  • Power Requirements:
    • Input Voltage: 10.5-16V DC
    • Input Current: 11-150mA at 12VDC

Theory

The 12V DC input voltage is routed through the 1A fuse and the on/off switch. The 1N4001 diode acts as a crowbar device. If reverse polarity is applied, the fuse will blow and the rest of the circuitry will be protected. Power is sent to the LM2941CT voltage regulator IC. The regulator is wired to produce a voltage range from 5.5V (dim) to 8.3V (bright).

The 4.7K resistor across the 1K brightness adjustment potentiometer produces a non-linear brightness adjustment to compensate for the eye's logarithmic brightness perception response. The LEDs are organized in six series groups of three with a 24 ohm current limiting resistor on each group. This arrangement limits the maximum current through each LED group to around 20mA.

Use

Connect the DC input terminals to a 12V source, such as a 12V lead acid battery. Be sure to observe the correct polarity. Turn the power switch on and adjust the brightness adjustment for the desired brightness.

Parts

  • 1X LM2941CT low-dropout voltage regulator
  • 1X 1A DC rated fuse
  • 1X DC switch
  • 1X 1N4001 diode
  • 2X 1K 1/4W resistors
  • 2X 4.7K 1/4W resistors
  • 6X 24 ohm 1/4W resistors
  • 1X 1K linear potentiometer
  • 18X 5mm white LEDs, 20mA max
  • 1X 22uF 16V electrolytic capacitor
  • 1X 100nF 25V monoblock capacitor

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