Regulated Dual 3 Watt White LED Lamp

This project involves constructing an energy efficient dual 3 Watt white LED array that runs on 12VDC at 700mA. The lamp is useful for indoor indirect room lighting. The light that is produced does not flicker and has a regulated brightness from 11V to the maximum operating voltage. The Avago 3W white LEDs are rated at 3 watts (nominal). Never stare directly at this lamp when it is running at full operating power, it is dangerously bright.


Regulated Dual 3 Watt White LED Lamp

Specifications:
  • Nominal operating voltage: 12V DC
  • Nominal operating current: 700 mA
  • Maximum operating voltage: 16V DC
  • Power consumption at 12.0V: 8.4W
  • Minimum voltage for regulated light: 11V DC
  • Leds produce light down to: 6V DC
  • White LED voltage: 3.7V DC each
  • White LED input power: 2.59 W each
  • Voltage across regulator when current becomes regulated: 3.3V DC
Theory:
The lamp is wired as a current loop which includes the power supply, the LED series string and the 700 mA current regulator circuit. The LM317 and 1.8 ohm 5 Watt resistor act as a current regulator that limits the loop current to 700 mA. Two 100nF monoblock capacitors bypasses the LM317 IC.

Construction:
The LEDs and current regulator circuit were mounted on a 3" x 6" piece of blank double-sided circuit board stock. The two LEDs and the LM317 regulator were soldered directly to the circuit board copper using a 200/240W soldering gun. The circuit board should be pre-tinned with solder and soldering should be done quickly to avoid overheating the parts.

The Avago LEDs come in several variations, the part used has an electrically isolated mounting tab, this feature is necessary for preventing a short circuit. If you can't find an LED with an isolated tab, be sure to mount each LED on its own isolated copper board. As of 2012, the specified LEDs are no longer available but there are plenty of other 700mA,3.7V white LEDs available. Just be sure to select one that you can easily solder to and connect to a heat sink.

The first version of this prototype ran too hot, so an aluminum heat sink was bolted to the board near the voltage regulator. Soldering the LM317 tab directly to the circuit board makes the board electrically hot at 1.8V, the board should not be allowed to come into contact with any live conductors. The LM317 may be optionally mounted on an insulated spacer to electrically isolate the board, in this arrangement, a TO-220 heat sink should be directly attached to the LM317.

The two resistors shown in the photo are combined in parallel to make a 1.8 ohm resistance, a single part would be a better choice. The entire circuit board can be mounted inside of a piece of aluminum "U" channel using plastic spacers for electrical isolation. The "U" channel reflects the bright light away from the side.

Use:
Connect this circuit to a 12VDC power supply or other power source such as a solar-charged lead acid battery. Be sure to observe the correct polarity. Look away from the LEDs and apply power. Again, do not stare directly at the LEDs. Prolonged exposure may harm your vision. A switch-mode power supply rated at 12VDC and 1 Amp or more is the most energy-efficient way to power one of these devices from line power. Your author liked this lamp so much that he constructed three of them to run in parallel and light his main electronics work bench.

Parts:
1x LM317 TO-220 case 1A adjustable voltage regulator
1x 1.8 ohm 5W resistor
2x 100nF 35V or higher monolythic capacitor
2x Avago ASMT-MYE2-NMN00 isolated tab 700mA,3.7V warm white LED or equivalent
3"x6" piece of double-sided copper circuit board stock
Miscellaneous wire, solder lugs, termination strips and hardware,
Large aluminum mounting plate, heat sinks if necessary.
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