Connecting LEDs to a power supply(LED drivers).

LED power supplies(LED drivers):


LEDs are very sensitive to voltage. They only work on a certain voltage, and if run higher , they will stop working. A 3.0 voltage led, will have problems at 3.5. It will then change resistance and draw alot more power, and then heat up, and run out.

The only effective way to power an LED is to use a Constant Current LED driver that keeps the LED at a certain voltage by limiting the current. Normally small LED run at 350ma, but the newer ones can run higher at 700ma or even as high as 1400ma.  The effect(watt) of the led is calculated by multiplying its voltage with its current. A Cree XM-L LED(3.3 volt) connected to a 700ma driver will run at (3.3 x 0,7)= 2.3 watt.

Normally the LEDs are intented to work at a certain nominal current. The osram golden dragon red LED has a nominal current of 400ma. It can be run at 800ma, but will then generate more heat, and will also go up in voltage by a small amount. (from 2.0 to 2.2). It will then be abit less effective but with proper cooling will give alot more light at the same cost. A Cree XM-L cool white LED however will run at 700ma nominally, but can easily run at 1400ma with proper cooling, and have a maximum current at 3000ma.

A typical small LED powersupply looks like this:(click picture for full size)

This one is 12volt output and 1440ma, and so it generates about 17.2 watt. (12 x 1.440).

If 3 xm-l LEDs are run at 3.5 volt each in serial. They will draw 3x3.5 volt = 10.5 volt, and at 1440 ma, they will consume 15 watt. So an aluminium surface + 3 xm-l leds + this driver will be enough for a 15watt lighting system. However we also need some materials to connect them in series + thermal paste etc.

Here's a link to a very good explanation on LED-drivers https://www.elfaelektronikk.no/elfa3~no_en/elfa/mime?file=tmp/en_drivers.pdf

Connecting LEDs in series to a surface:

The following picture shows several Cree XPG LEDs connected in series using:

1. A layer of Kapton tape to properly insulate the electrical current to not touch the heatsink(the aluminium plate)

2 Copper tape to conduct the electrical current between the LEDs. This btw also acts as a heat sink, since the LEDs are soldered to the copper which, also is a very good thermal conductor.

3 The XPG-LEDs are glued to the metal plate by either thermal(but not electrical) conductive pads, double sided capton tape, or ceramic heat paste(the kind used between prosessors and heatsinks in PC's). It's important that the circuit is electrically insulated from the heatsink, as not to cause a short circuit.(click picture for full size)

Note: It's also possible to use anodized aluminium as a heatsink that doesnt conduct electricity. But we prefer this method.

Connecting LEDs in parallel:
connecting LEDs in paralell is abit trickier. If you connect two led's in parallel and use a 1400ma LED driver(powersupply), both will draw 700ma if they're exactly the same. If however one of the LEDs generate more heat, it will also change and draw more current and the other will the loose the same amount of current etc. So in order to connect LEDs or chains of LEDs om paralell, we will ned some sort of stabilizer.

A typical stabilizer can be a simple resistor(will reduce effiency), and/or a fuse on each parallel chain that stops the current if it exceeds a certain value(this is problematic since it also will overwhelm the other leds in paralell and will cause a chain reaction blowing all the fuses). 

lm3466 from national semiconductors.

Several Lm3466 can be inter-connected on a circuit board with a connecting led driver, and a few resistors and capacitators, and distribute the current evenly. A circuit board with 4 of these can change a 2000ma led driver into 4x 500ma channels. Technically , the board/resistors etc will draw some of the voltage, so a 48v 2000ma psu(about 100w), will be reduced to about 4x (44volt 500ma). This is a very economical way to drive a high amount of leds in several channels.

Arduino boards/shields:

This will probably be an article for itself, but an arduino is a circuit board that can be connected via usb from a computer and programmed to control a specific purpose board called a "shield". With an arduino:

And an LED driver arduino shield:

it should be possible to connect a normal off the shelf power supply and drive 3 chains of LEDs and also control them via the arduino board to regulate blooming/vegging colour profiles.