Regulated Dual 3 Watt White LED Lamp

(C) 2010-2015 G. Forrest Cook

This circuit can be powered by a CirKits SCC3 or SPC3 solar circuit kit.

Dual 3 Watt LED lamp prototype 1

First prototype with Avago LEDs on a copper plated printed circuit board

Dual 3 Watt LED lamp prototype 2

Second prototype with Ligitek 100 lumen LEDs on an aluminum heat sink

Dual 3 Watt LED lamp schematic

Regulated Dual 3 Watt White LED Lamp

Introduction

This project involves the construction of an energy-efficient dual 3 Watt white LED array that runs on 12VDC at 700mA. The lamp is useful for indoor indirect workspace lighting. The light that is produced does not flicker and has a regulated brightness from 11V to the maximum operating voltage. The circuit uses a DC current regulator so it does not produce any radio frequency interference (RFI). Never stare directly at this lamp when it is running at full operating power, it is dangerously bright.

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 capacitors bypass the LM317 IC to prevent oscillation.

Construction

In the first prototype of this circuit, 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.

In the second prototype, two Ligitek LEDs and an LM317 regulator were bolted to an aluminum heat sink. Heat sink grease was used for thermal conductivity between the LED MCPCBs and the heat sink. The LM317 was mounted using an insulating washer and plastic shoulder washer, this isolates the IC so that the heat sink is electrically insulated from the circuitry. A small two conductor terminal strip was used to connect the power wires to the circuit.

The Avago LEDs shown in the first prototype photo have been discontinued by the manufacturer. Fortunately a wide variety of newer LEDs with similar current ratings (700mA) are available. Jameco.com part 2006908 LEDs were used in the second prototype. The LED manufacturers have standardized the mounting of high power LEDs by installing their parts on hexagonal MCPCB (Indus Star) circuit boards. These small LED boards can be mounted to a larger heat sink using two 4-40 machine screws. Be sure to include insulating washers around the screws on the top of the star PCB so that the mounting hardware does not short out to the LED wiring contacts.

Lightek 313H1 star

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 substrate 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 first photo are combined in parallel to make a 1.8 ohm resistance, a single 1.81 ohm resistor was used in the second prototype. 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.

The wiring for this project was done with 22 gauge tinned copper wire inside of teflon insulated tubing. Any general purpose wire would be suitable for this project but teflon insulation can survive direct soldering iron contact without melting.

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 before applying power. Never 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.

Parts

1x LM317 TO-220 case 1A adjustable voltage regulator
1x 1.8 ohm 5W resistor
2x 100nF 35V monolythic capacitors
2x Jameco 2006908 700mA, 3.7V warm white LEDs or equivalent
3"x6" piece of double-sided copper circuit board stock or an aluminum heat sink
Miscellaneous wire, terminal strips and hardware

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