Powering a 100W LED at 38V? - Page 1
Forum section: electronics / beginners

author: Dave_R
author_rank: Newbie
timestamp: December 18, 2014, 02:08:12 am
content: Hi Folks,In brief, I have a 100W LED hooked up to a 600W boost converter that is powered by a pair of 12V SLA batteries.The 100W LED is rated with a forward voltage of 30-34V, but at 34V the LED only draws 1.1A. Only when I push the voltage up to 38.5V does the LED pull 100W at around 2.5A. Increasing the voltage past that makes the current shoot off as expected with no perceptible increase in the brightness of the LED.The boost converter is the typical ebay special 600W boost converter with adjustable output current and the LED is also a typical 100W ebay jobbie.I don't have any other means of driving the LED, I don't have anything that can do 100W+. I did test the boost converter with three 12V 50W halogen lamps in series, the current limiting worked with these. Also, the LED is heatsinked and I'm using thick cable throughout.The LED seemed happy running at 38.5V for about an hour up until the battery ran flat, could I just leave it at 38.5V? Am I missing something?Thanks!

author: amyk
author_rank: Super Contributor
timestamp: December 18, 2014, 02:16:46 am
content: LEDs are current-driven devices, they'll have a rated forward current; Vf will vary depending on temperature and manufacturing tolerances. As long as the temperature and current is within limits it will be OK.

author: wraper
author_rank: Unknown
timestamp: December 18, 2014, 02:20:02 am
content: Congratulations! You were ripped off, and your led actually is half of advertised power at best, probably even less. Boost converter probably have fake watts too.

author: tautech
author_rank: Super Contributor
timestamp: December 18, 2014, 02:21:38 am
content: I think our perception of LED wattage re V x A = W does not apply.eg. 3 W downlight LED unit draws 47 mA x 12 V = 0.564 WYet these units are brighter than 55 W halogens.You figure it out.EditMaybe I was tucked too?

author: ludzinc
author_rank: Unknown
timestamp: December 18, 2014, 02:27:22 am
content: Quote from: tautech on December 18, 2014, 02:21:38 amLEDs are more efficient, thus brighter at less power.V x A = W is the POWER consumed by the device. Efficiency of the device is how much of this power becomes usable output...

author: tautech
author_rank: Super Contributor
timestamp: December 18, 2014, 02:58:17 am
content: Quote from: ludzinc on December 18, 2014, 02:27:22 amOf course.back to you.So my units are 0.564 W......try and sell those specs to the public.You missed the point.How does rated LED wattage rating apply to theoretical consumption.IMO it doesn't.Take a 10W LED spotlight, lights up the whole yard like 500 W incandesant, so to compare both examples 50 times more efficient?Got one of those 10 W'ers, I will grab a AC clamp meter and report back.Put them both on the bench for accurate series measurement with bench DMM.Obviously my memory fails me by a factor of nearly 10 times.My Bad.12V 5W LED downlights consume 367mA = 4.404 W230 VAC 20W LED spot consumes 142mA = 32.66 W This measurement was surprising.... considerable losses.So yes after a recheck V x A = W consumed as it always has.

author: wraper
author_rank: Unknown
timestamp: December 18, 2014, 12:38:16 pm
content: Quote from: tautech on December 18, 2014, 02:58:17 amRated power is rated power. Light output is rated in lumens not watts. You can compare effectiveness of led by comparing rated light output related to power consumption. Halogen and incandescent bulbs have lumen rating too, therefore you can directly compare their light output without mystical Chinese watts. There might be LED watt vs incandescent watt comparison on the package, like 9w = 60w, but it is not rated power.QuoteGood led lamp is about 6-7 times more effective than incradescent bulb, not 50 times.QuoteThose are no loses but incorrect measurement/calculation. Because of the low power factor you cannot just multiply voltage * current. Moreover crap AC led bulbs have just a film capacitor in them as a ballast + diode bridge. And they flicker at 100/120Hz frequency.