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12V, 3W High Power LED Assembly

by logan, published

12V, 3W High Power LED Assembly by logan Jul 28, 2012

Description

NOTE: I updated some errors in the original design, including: incorrectly low resistor values (LEDs are 1W, not 1A) and reversed wiring on the 4-pin headers. Also, I moved the control header to a place where it is less likely to cast a shadow.

This is a design for a 12V LED board that hold 3 high power (1W) white LEDs. The board is sized to be fabricated by iTead studios for ~$13 for a lot of 10. Fully populated, the BOM is < $3/board with the power LEDs being the most expensive part ($0.30/ea for me on ebay).

WIth the specified resistor sizes, the board will consume about 4W of power. I intend to use it to illuminate my Printrbot, and as such the 4-pin molex connector (KK) is pinned out to connect to a Floppy Power connector on a standard ATX power supply. Two power connectors are included so that multiple modules may be chained.

There is an optional power transistor and control header that you can connect to spare 3.3/5V I/O lines to make the board controllable.

If there's interest, I can upload CAM files for easy submission to iTead.

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The board is designed so that there's a large square of copper underneath the LEDs with an exposed pad (the large square of unconnected copper in the photos). If you use solder paste and an oven, you can bond the LED to the board for good heat dissipation. Since I used an iron, I put thermal paste under the LEDs instead (an iron can't heat the copper sink enough to melt the solder).

So far so good after several months of continuous usage, though I'm running the LEDs closer to 0.5W rather than their full rated power.
I'd suggest heat sinks for the LEDs.

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Instructions

Bill of Materials

  • 1xBoard
  • 3x1W White LEDs
  • 4x36 ohm, 1/4W resistors (or larger, use 100ohm if you use the thermal paste technique described below)
  • 2x4-pin Molex KK, right angle locking pin headers
  • 4x#6 screws for mounting

If you want the LEDs to be digitally controllable:

  • 1xIRF8714 N-Channel MOSFET (or equivalent SOIC-8 FET)
  • 1x2-pin Molex KK, straight locking pin header

Assembly

Load the brd and sch files into EagleCAD (free version will do) and generate CAM files for your PCB manufacturer. The boards are just under 5cm x 5cm so that they may be fabricated by iTead studios for just under $1.50ea (lots of 10)

Solder the LEDs - The LEDs have a pad on the bottom that should be soldered to the board for heat dissipation. I could not solder the thermal pad on the under side of the LED with a regular iron (you need a heat gun or reflow oven). Instead, I coated the bottom surface with a thin layer of thermal paste (the kind you would use on a PC CPU heat sink). It may shorten the life of the LEDs, though my first few boards have been functioning well for the last few weeks without showing signs of dimming. The easiest way to solder the surface-mount leads of the LEDs is to first wet the solder pads with solder and a generous amount of flux, then place the LED and gently press each lead into the solder with the iron (remelting it in the process).

Solder the connectors and resistors - You need at least one 4-pin connector for power, you only need the second one if you want to chain LED boards. NOTE: The first version of this board had the wires to the connectors reversed. The assembled board in the photo compensated for this by mounting the power connector on the bottom side (thus reversing the pins). If you use the latest board and schematic layout, the connector should be mounted to the top side of the board (opposite what is in the photo). When in dobut, check how it connects to a floppy connector on a PC power supply. The yellow wire on the supply is +12V, ensure that connects to the resistors on the LED board.

OPTIONAL: Connect the solder bridge - There is a small solder bridge next to the optional MOSFET. If you close this bridge with solder, the lights will be "always on."

OPTIONAL: Solder control connector and FET - If you leave the solder bridge open, you can control the LEDs with the 2-pin Molex connector. Solder in the Molex connector and the FET. Any SOIC-8 N-channel MOSFET with a current capacity > 1A and a working voltage > 12V will do. Just make sure the gate lines up with the trace going to the control header.

I'd suggest heat sinks for the LEDs.
The board is designed so that there's a large square of copper underneath the LEDs with an exposed pad (the large square of unconnected copper in the photos). If you use solder paste and an oven, you can bond the LED to the board for good heat dissipation. Since I used an iron, I put thermal paste under the LEDs instead (an iron can't heat the copper sink enough to melt the solder).

So far so good after several months of continuous usage, though I'm running the LEDs closer to 0.5W rather than their full rated power.
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