CC power source and LED matchup question

Thread Starter

dorydan

Joined Jul 1, 2020
11
Hello, i am going to use a Meanwell LCM-60UDA constant current power supply to power 6 or 7 Cree LEDs (specs in pdf). My goal is to get maximum luminosity and long life. I am thinking the best combo is selecting 600mA @ 84V to power 6 of these LEDs in series. This would allow ~13.5V/LED (near middle of Vf rating 11-15V) and keep the current about 14% below maximum. Can someone confirm i have this correct? Specs and calculations attached.
Thanks!
 

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Jon Chandler

Joined Jun 12, 2008
302
The current supplied (for a constant current supply) is determined solely by the power supply.

The voltage across the LEDs is determined solely by the Vf of the LEDs.

The power supply attempts to maintain the desired current by varying the output voltage. As an example, if there's a near short on the power supply, it's output voltage will be near zero to maintain 600mA (in this case). If there's a large resistance across the power supply, it's going to make every effort to pump 600mA by raising its output voltage to the maximum.

From the info provided, the LEDs' Vf range is not known. The power supply maximum output voltage must be a little greater than the worst-case sum of the LEDs' Vf.

You may have to go up to the next voltage step on the power supply, which will reduce the current to 500mA. There will be a slight drop in LED brightness, but their lifetime will be improved. Remember heatsinks.
 

DickCappels

Joined Aug 21, 2008
9,105
Just a note: The IV characteristics at 25° is only there to give you an idea of what to expect. Running 600 ma x 84 V = 50 watts for the package! You are going to need a very good heatsink to keep the LED from being damaged by getting too hot and after some experimentation you will probably find that you need better than 0.5°C/watt temperature rise at the junction.
 

BobTPH

Joined Jun 5, 2013
5,510
The forward voltage listed in an LED datasheet is not a range of operating voltages. It is the minimum and maximum voltage than any particular LED will have when run at the stated current, in this case 700mA.

So, you might have 6 LEDs that drop 15V at 700mA, which would be 90V. You don’t know for sure what yhe voltage will be at 600mA. It migjt be less than 84 or it might not, you only know that it is less than 90.

In other eorfs, your design is right on yhe edge and might or might not work as intended.

I agree with the other post that said use 500mA, which you can guarantee given that driver and LED.
 

MisterBill2

Joined Jan 23, 2018
12,745
The TS has already mentioned a string of 13.5 volt devices. So the LEDs lack some sort of internal current limiting. So "6 or 7"devices in series will be less than ten watts each. With 600mA as the setting the sum of the forward voltages is probably close to that 13.5 volts x6 =81 volts. So an 84 volt max supply should be able to power six devices with a bit of margin. But not seven. With the 500 mA operation you wil be able to light seven.
Max power / current will not be possible but that is better for the LED long term survival. And probably the difference will be quite small and hard to notice.
BUT BEWARE heat sinking is vital even for very short times of operation at that power level.
 

BobTPH

Joined Jun 5, 2013
5,510
The TS has already mentioned a string of 13.5 volt devices. So the LEDs lack some sort of internal current limiting. So "6 or 7"devices in series will be less than ten watts each. With 600mA as the setting the sum of the forward voltages is probably close to that 13.5 volts x6 =81 volts. So an 84 volt max supply should be able to power six devices with a bit of margin. But not seven. With the 500 mA operation you wil be able to light seven.
Max power / current will not be possible but that is better for the LED long term survival. And probably the difference will be quite small and hard to notice.
BUT BEWARE heat sinking is vital even for very short times of operation at that power level.
Only if all his LEDs are typical. The range is 11-15V. Also, the 13.5 typical Vf is at 700mA. It will be different at 600mA.
 

Jon Chandler

Joined Jun 12, 2008
302
The TS has already mentioned a string of 13.5 volt devices. So the LEDs lack some sort of internal current limiting. So "6 or 7"devices in series will be less than ten watts each. With 600mA as the setting the sum of the forward voltages is probably close to that 13.5 volts x6 =81 volts. So an 84 volt max supply should be able to power six devices with a bit of margin. But not seven. With the 500 mA operation you wil be able to light seven.
Max power / current will not be possible but that is better for the LED long term survival. And probably the difference will be quite small and hard to notice.
BUT BEWARE heat sinking is vital even for very short times of operation at that power level.
No. The TS says Vf ranges from 11 – 15 volts. IF Vf = 13.5, as he's based his calculations on, than the 84 volt compliance voltage provides a slim margin to operate 6 LEDs in series. But the LED Vf is going to be whatever it is based on manufacturing tolerances and temperature. If it's 15 volts (the maximum specified), a string of 6 LEDs will require 90 volts and won't work on 84 volts. This may lead to all sorts of interesting problems, like working sometimes are not others depending on a slight change in ambient temperature.
 

MisterBill2

Joined Jan 23, 2018
12,745
The voltage span of the high power LEDs that i have played with covers much less than one volt, more like 200 millivolts, from first illumination to full rated current. That is one 3.5+ volts LED. The 13.5 devices might even include a resistor, possibly. So it seems that the selected driver has it covered.
 

Audioguru again

Joined Oct 21, 2019
4,980
Those are dangerous Ultraviolet LEDs that are invisible but can destroy your eyes and give skin a sunburn or melanoma.
Don't you want visible light from some LEDs?
 

Thread Starter

dorydan

Joined Jul 1, 2020
11
OK guys, all great comments! You've confirmed - at least in my mind :) - that my calculations were half decent. My takeaway is 84V would PROBABLY run 6 in series IF they had typical Vf and IF there weren't a melt down. HOWEVER, the safe thing to do is to select the 500mA 90V setting which will definitely cover voltage requirements, run a little cooler, and make the LEDs last longer. Hey, if it doesn't have enough output, just flip dip #1 and see what happens! Audioguru, thanks for the warning about UV. This will be used in a curing operation so 395nm wavelength is required. Fortunately, the adhesive folks have built their chemistry around 395+/-nm because, although precautions are required, 395nm is much less dangerous than the shorter wavelength UV lamps of yesteryear.
Thanks again guys!
 

MisterBill2

Joined Jan 23, 2018
12,745
The quoted spec is 11 – 15 volts. Imagining it to be otherwise is foolish.
That is an E-bay listing, not the manufacturer's spec sheet. Half of what we see on ebay is not quite right. and some is total lies. You will not see such a broad forward voltage range in a manufacturer's data sheet. LEDs by themselves do not work that way. As diodes the voltage range between initial conduction and max current is much smaller.
 

Jon Chandler

Joined Jun 12, 2008
302
I believe "the datasheet I LINKED" is standard, commonly understood terminology.

SmartSelect_20220728-014048_Edge.jpg

If you had looked at that datasheet, you would see it's from the LED's manufacturer for the exact part in question.

SmartSelect_20220728-013854_Dropbox.jpg

In this datasheet, you would find the Vf range at 350mA, which is listed as 11 – 15 volts.

SmartSelect_20220728-013929_Dropbox.jpg

And if you had happened to continue to scroll down, you would have found the graph for typical Vf vs current. Please do note, this is the typical value at a given current, and does not account for manufacturing and temperate differences shown in the previous table.

SmartSelect_20220728-014014_Dropbox.jpg

There's also no mention of any included mystery resistors.

At 700 mA, the typical Vf is about 13.6, and a string of six will probably work on 84 volts. But a string of six LEDs within the manufacturer's tolerances might take 90 volts. It's the luck of the draw and if somebody else is paying the bills they may not be happy when the switch is flipped and nothing happens. Datasheet says Vf is 15 volts, that's what you design for if you want to be sure it works.
 
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