3 Watt LED array with 1 watt resistors; Power Supply questions

Thread Starter

LEDwire

Joined Mar 1, 2015
12
Hello,

I just finished wiring an LED array with several series of 5 3-watt LEDs (~2.3 volt fV, 13.8 V power supply, 350 mA max current) with a 4 ohm, 1-watt ceramic resistor wired to each series to limit current.

I just realized these resistors may be limiting the brightness of my LEDs. If the resistors are rated at 1 watt, and the LEDs are 3 watts, will they limit the brightness of my array?

Also:
I have a pyramid PS52KX 52 amp power supply that has an input of 115 volts 60 hz and 1000 watts. Its output is 12-15 volts and 40 amp constant/52 amp surge.
I have 540 3 watt LEDs in my array at the moment. 540 x 3 = 1620 watts. Do I need a power supply with more watts to get the most output from my LEDs?
The power supply, at the moment, "flickers" and will put out close to 30 amps then drop down to 20 amps or so, and this happens on and off while the array is lit up. Any idea what could be causing this behavior?

Thank you.
 

Dodgydave

Joined Jun 22, 2012
9,917
WOW,your leds need 1620W and your psu is 1000W, definitely need a bigger psu, by the way what are you making , a Cannabis Farm!
 

mcgyvr

Joined Oct 15, 2009
5,394
So... I'll try to keep this fairly simple.. Because you absolutely need to understand the basics before you even attempt to build such a MASSIVE light..

First.. 3W LEDs is just a generic term to attempt to classify LEDs into wattage groups.. (and a huge amount of rounding up)
2.3Vf x 350mA = .805 Watts.. not 3W..
The 3W comes from the fact that some may have a Vf closer to 3V and can be run up to 1000mA max.. and 3 x 1000mA (1 amp) = 3W

Second.. LEDs need to be FED current.. hence what the resistor is doing..
However a resistor is really a bad choice for "high" power LEDs as #1 the Vf can/does fluctuate (which will in turn change the amount of current that the LED is being fed).
A "constant current" supply (commonly referred to as an LED Driver when used with LEDs) is a much better choice as it is more efficient AND will self adjust to ensure exactly 350mA is delivered to the LEDs.

The 1W is simply how much heat that resistor can typically safely dissipate.

Assuming 5 LEDs in series with a vf of 2.3 thats 11.5V..
With your 13.8V supply the resistor is dissipating
(13.8-11.5) x .350 = .805 watts
One should ALWAYS double (or triple) the calculated wattage so a resistor doesn't run too hot..
I would have used a 2W resistor.. (well I would have used an LED driver instead)..
The circuit will function identically but the resistor will run much cooler to the touch..


Now you have a 4 ohm resistor.. Lets do some math for that..
(13.8-11.5)/4 = .575 Amps (or 575mA)..
So you are feeding those LEDs 575mA... NOT 350mA

So lets start with that and see how that sinks in...

Google "Ohms wheel" and see where the math comes from...
 
Last edited:

mcgyvr

Joined Oct 15, 2009
5,394
Little more math..
Assuming the same strings/4 ohm resistor..
If your large supply is outputting 12V..
Then (12-11.5)/4 = .125 Amps (125mA).. now you are drastically underdriving them..
If your large supply is outputting 15V...
Then (15-11.5)/4 = .875 Amps (875mA).. now you are overdriving them..

With big LEDs.. I'm 100% in the use a LED driver camp..
I would highly recommend eliminating the resistor and using a Meanwell LDD driver (350mA version)..
That will ensure your LEDs are always getting 350mA..
But it does require your supply voltage be approx 2V higher than the Vf..
 

mcgyvr

Joined Oct 15, 2009
5,394
Flickering..
With LED drivers "flickering" typically happens when the VF of the string is less than or greater than the rated DC output voltage of the driver.. So it flickers trying to maintain the constant current..
Anytime I've seen that its typically too many LEDs in a string and removing one solves that problem..
Not saying thats the problem here.. but more than likely the supply cannot "supply" and is in a "safety..oops mode"..
 

mcgyvr

Joined Oct 15, 2009
5,394
And Vf is only an "average".. It could be 1.8V or it could be 2.5V (typically the datasheet will show a min/max/average Vf)
Slight changes/differences in the Vf can really change the math above...
Hence why a constant current LED driver is better.. It adapts to that change as best it can..
A resistor is "fixed" and cannot adapt..
 

mcgyvr

Joined Oct 15, 2009
5,394
At least you got close on the resistor (by some stroke of luck I'd guess).. way too many people think LEDs are just like lightbulbs and you just need to attach a power supply to them..
That always ends in "smoke"...
 

Thread Starter

LEDwire

Joined Mar 1, 2015
12
Thanks for your messages. That helps clarify things.

I did check the forward voltage of each LED with a voltmeter and grouped them so they averaged about 2.3 volts across each 5 LED string.

Is it possible to connect an LED driver and simply unsolder the resistors and keep the array wired as is in 5 LEDs per string? You mentioned an LED driver requires 2 volts higher than forward voltage--is this total per string or per LED?

Thank you. I really appreciate your help.
 

Thread Starter

LEDwire

Joined Mar 1, 2015
12
So if I were to purchase one of these, could I use it with my current arrangement and set my power supply to 14.5 volts (3 v for the driver, 11.5 [2.3 fV * 5] for the LEDs) and the current would be constant at 350 or 300 mA?

If the LEDs I have are rated for max current of 350mA, but I use them only for 20 minutes at a time (this whole LED array is for a skin therapy application, treatment times last 20 minutes), would this significantly affect their working life? Or would it be better to get a 300 mA driver and run them a bit below max?

Thanks again.
 

John P

Joined Oct 14, 2008
1,876
And I really hope you're mounting those LEDs on a good heat sink. Even an "efficient" light source like an LED turns most of the energy it receives into heat.
 

mcgyvr

Joined Oct 15, 2009
5,394
WOW..
Did you do any temperature testing prior to spending ALL that time?

I really expect a "greatly" reduced lifespan like that unless you only turn them on for a few minutes or so..

I can't believe you actually did that.. That goes in my WTF folder for future use/laughs..
 

Thread Starter

LEDwire

Joined Mar 1, 2015
12
It's a light therapy array for rosacea (a skin condition that responds favorably to red light exposure).

I use a box fan to cool it while it's in operation. The LEDs are typically lit for 20 minutes per day like that.
 

Thread Starter

LEDwire

Joined Mar 1, 2015
12
So, mcgyvr, if you were going to produce something with that density of 3W LEDs for this application, what would you do differently?

Thanks.
 
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