Looking for some clarity on 1w led power

I am having a hard time working the math and figuring out exactly what I need so I thought a question to others might help.

I am trying to determine the best way to power a led light.  Here is what I am trying to accomplish.  

I have 12"x12" box that will hold 4 round circuit boards.  Each board has positions for 9 1w leds.  3 for red, 3 for green, and 3 for blue.  Each color is in series.  My goal is to use a 12v power supply for each box.  So we are talking about trying to power 36 1w leds all together but they may or may not be all on at the same time.  The box will be dmx controlled by a computer so that I can adjust the 3 leds in each color on each board.  So its possible that I might only have 1 color on 1 board on for example.  Or I could have one color on all 4 boards on (12 leds) at any given time.  Or they could all be on.

My led specs
Red (610-630nm):DC Forward Voltage:2.2V~2.6V  Forward Current: 350mA
green (510-530nm):DC Forward Voltage:3.2V~3.6V Forward Current: 350mA
blue (460-465nm):DC Forward Voltage:3.2V~3.6V Forward Current: 350mA 

So a couple things that I am having trouble with.  I know that I can power this with 12v using a high amp (15) source but then I need different resistors with high wattage ratings for different colors I would think?  That seems like a lot of power wasted.  So then I can go with a constant current driver but I can't seem to find one that would apower that many leds at once.  Do I need to do two constant current drivers?  Is there a better way?  and what about the fact that each color on each board may be on or off at various times?  Basically using the dmx controller to colorwash the different corners of the 12 x 12 box.  Will that affect the power drawn and change the supply that I need?


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gmoon2 years ago

If you plan to use DXM control, you'll need modules for the LEDs--so they respond to commands. Essentially a "receiver/driver" module. Those modules will either supply the necessary current themselves, or the power supply needs of the module will be specified.

Maybe that's the "round circuit boards" you describe? If so, there will be supply voltage / current draw specifications for the boards.

Whether or not they can handle LEDs in series or parallel, and be adjusted to the voltage drops of different LEDs (which also differs for both the type/color of LED and the number in series or parallel) is going to depend on the modules.

It seems very unlikely that a modern LED driver board would require current-limiting resistors--the module would likely control the current directly...

joecolafrancesco (author)  gmoon2 years ago
Tried to attach an image of the board and controller but the forum won't let me. Says down for maintenance. Anyways, the circuit board is just a 4 inch round aluminum disk with solder pads for 9 1w led's. There isn't any electronics on the board. The controller I am using for dmx is a 18 channel generic one off ebay from china so I have no specs on it. Its a simple rgb controller that has a terminals for power input of 12v and outputs for 6 rgb led links. I don't believe the controller is constant current. I use a benchtop 12v 4amp power supply on the input side of the controller. Then I put my volt meter inline with 3 led's in series on one fhe outputs. I measure 1.5 amps going thru the volt meter. If I turn on another color, then the amps go down on that line that the volt meter is on. So that means it's not constant right? I also assume that the 1.5 amps is the current being drawn thru the 3 led's right?

Here is a link to a picture since I couldn't upload it

OK, I can see the pic. If you can find a spec sheet for the controller, that would be a big help. Without that, there is no way to be certain.

Many of these boards are configured to power 12V LED rgb light strips and modules, which is a standard. If you want to add a current-limiting resistor to each series string of LEDs, to configure them for 12V operation, that might be the safest approach--with each color you'll have a voltage drop between 7 and 10V (three LEDs in series), so it shouldn't be too difficult (although that's burning off a fair amount of current, especially for red).

It's likely the boards are limiting current (based on 12V modules), but possibly with PWM, and your VOM can't read AC current.

Again--difficult to know for sure without the docs...

joecolafrancesco (author)  gmoon2 years ago

So looks like the board is configured to output at 1amp max per channel. For 3 blue or 3 green in series, i measure almost exactly 1 amp on my DMM. I would have thought it should be a little more as the 3 led's in series should pull 1050ma? I did not put a resistor in line but pretty sure I would need one since voltage is gonna be too high without one. For red, I put in a 3w 15ohm resistor and amps came down to around 650ma which seems to low. Everything works but the red seems a little dimmer. The colors don't match what they should be because of it. So it's looking like I need a 4.7 ohm resistor in line for the blue and green series will help the voltage on those lines but what about the red? The math seems to point to a 15ohm resistor and that is also what a led calculator is spitting out when base it off of 3 led's with a Vf of 2.5v each. But I need a 3w so the resistor doesn't fry and the milliamps seem low. Any thoughts around this? I don't really see any other option for this type of setup.

Does your DMM measure AC current? Otherwise it's just guessing (much like us without the specs). With the assumption that the drivers are pulse width mod, PWM rather than DC.

So if the controller boards are constant current (not for 12V strings) and supply 1 amp, you should be safe without any current limiting resistors...assuming each LED is 350mA max (although 50mA, or 5% isn't much of a safety margin).

The first thing you should get is proper drivers for the LED's - for every one.

The driver makes sure the expensive LED has a proper lifetime, especially if it is a Cree.

Best thing is the drivers are available for a wide range of input voltages, so it should be fairly easy to get it sorted out.