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Needing help with RGB LED chip PSU


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Needing some help here and showing my ignorance in LED technology.


What I want to do is build an RGB LED footlight on a budget. To make it work I needed a single LED chip bolted onto a heatsink.

Found a nice DMX controller, tested the whole concept with small 5050 chips and all was good.

Then it became time to supercharge it and I ordered a handful of 20w RGB chips and that is where things got interesting.

Here's the specs:

DC Forward Voltage (VF): Red 13-15V, Green 18-20V, Blue 18-20V

DC Forward current (IF): 350mA


If I feed the controller with 12V DC, Red and Green are fine but Blue is negligible. If I feed it 20V DC, Blue and Green are fine but Red blows.

The chips are common anode and the RGB controls are cathode.

Obviously I need to reduce the voltage to the Red LED's - but how?


Or are there smarter solutions out there?

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You need a completely different approach to the supply. Raw LED's need a constant current supply. You are using constant voltage. A cheap way of turning a CV psu into an approximately CC psu is to put a series resistor in the circuit but for a higher powered LED this is rather inefficient. Look up constant current LED drivers then come back for further help



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As DrV says you need constant current and for that power of LED you have to use a switching regulator, otherwise everything gets hot and smokes.


Look at these from Eldoled, they make DMX-controlled constant current driver PCBs for embedded use. (You just put in a DC power supply)



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You could use a resistor in series with the red , its not elegant and it gets hot, but it will work.




gives 15R at 1.8W, 3W wirewound would be fine.


DMX current driver is the proper answer, will be described as 350mA driver , typically other currents slectable and needs to have an allowable V in of more than 20V, in use a 24V supply will be fine and the driver will settle the voltage to the LEDs.

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Thank you all for the quick responses.


Don, looking at those examples the problem I have is that the output from the DMX controller, and the input for the chip, is V+ /R-/G-/B-.

Do I put the LM317 at the V+ end or at each individual RGB output? If in the RGB output, should I use a LM337?


To get everyone on the same page, this is what I am trying to make work at the moment:



DMX control:


(note the ad says 12v but the specs show 9v - 24v)

LED RGB chip:



The plan is to have 2 chips per footlight in parallel.

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Have done something similar when couple hundred channels of current drivers was over budget.


In that case used wirewound resistors, with a stable power supply it dosen`t make a whole lot of difference. 24V supply probably 5W wirewound.


317 will get very hot and the programming resistor gets hot, advantage of using 317 is its a true current source and current delivered to LED will remain same despite supply voltage fluctuations.

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Got me a handful of LM317 and resistors. Initially put the LM317 on the V+ side of the chip. Worked but had weird effects when bringing up Green and Blue, they were limited by the Red current. Bringing up Green and Blue without the Red was fine.

Then placed the LM317 in-line with the Red only and that worked much better.


Question: Should I put LM317 current limiters in line with each feed?

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The Blue and Green are noticeable less bright with the 350mA limiters in line.

At the moment I have 2 RGB chips in parallel with the LM317 limiters in-line.

Should I change the value of the resistor to allow for the two chips? How should I calculate that?

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Looking at the specs for the LED



Model : 20W

Color: RGB

DC Forward Voltage (VF): Red 13-15V, Greed 18-20V, Blue 18-20V

DC Forward current (IF): 350mA

Out put Lumens: Red 260-300LM, Green 400-600LM, Blue 150-200LM

Wave Length : Red 620-625nm , Green 515-520nm, Blue 455-460nm

Beam Angel: 140 degrees

Life span: >50,000 hours


You need three separate drivers for the three colours because their lumens are different, so you can adjust the current to suit the colour emitter.


If you want to put two RGB LED's in the same location, you need six LM317 drivers as the power supply is a 24V one, so you cannot put two emitters in series, which is the normal setup in a LED PAR.


This is because the LED chip you are using is made up of multiple LED's in series on the die. Have a look at 3W RGB LED Emitter and scroll down to see VF of individual emitters. Red is 2V Green and Blue 3V so your LED chip appears to have six emitters in series.


Measure the voltage across each colour to see if you are within the required voltage range of the above specs.


Interesting to see a new LED colour "greed" in the Forward Voltage specs.

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With the LEDs in parallel the driver's limited 350mA current output will be split (possibly asymmetrically based on the tolerance of the forward voltage) between both LEDs.

Typically LEDs are used in series, as they all get the same current through them and thus should be the same brightness.


As above, you'd need a much higher voltage to go in series, so you'll need another 3 current sources.

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In fact that difference in lumens for the colours is normal and you wouldn't normally adjust the drive current because of it. The lumen measurement is weighted by the sensitivity of the human eye so green gets higher numbers but in reality they will not look much different to each other.

If the blue/green look dim you probably don't have enough voltage going in, so the Lm317 is not regulating - you need a few volts above the vf of the leds.

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