I’ve bought a few LED spots and floods and small led car dome lights and even a monster 300W lightbar and none of them draw the current I’d expect at 12VDC. (the car dome lights) and otherwise most of the lights say something like 9-32VDC input (so they work on 12V or 24V DC systems). When I buy an 18W light from say Kawell, I’d expect it to draw 1.5 amps but it measured 1 amp. I think every light I ever measured drew about 1/2 to 2/3 of the current I expected… (and note that I am running with supplies that can source the current at 12VDC,
I attributed this to Chinese sellers over stating their specs but I think there could be more to the LED light power specifications,. this link has some weird stuff talking about 1W LEDs and 3W LEDs current draw… what am I missing? (note that I am presently looking at buying some lights for my friend’s boat and considering bars that use multiple 3W, 5W, or 10W LEDs (you can find my other thread if interested!).
“LED wattage classifications have been made very confusing by manufacturers such as Cree, Osram and Luxeon, for example a 1W LED is driven at 350mA of current; however a 3W is driven at 700mA. Even though the 3W has only double the current of a 1W, it is somehow classified as consuming triple the wattage.”
Yes Power is measured in watts. and P=I*V. typically total power (P in watts) in a system is expressed as the supply current (I in amps) multiplied by the supply voltage (V in volts). I think you are saying the different LEDs have different forward voltages which is the voltage required to break down the diode junction and produce photons. so if the manufacturers want to say they are using 3W leds and running them at 1.5W then maybe the 3W is a max operating power?? maybe all the power specs are max operating power specs? but then the actual operating power is always set by the light design in terms of the internal current limiting or LED drivers… could it be that if I run these lights at 24VDC (or this light running at 32VDC http://www.ebay.com/itm/171709543087?\_trksid=p2055119.m1438.l2649&ssPageName=STRK%3AMEBIDX%3AIT ), then I will observe the total current and total operating voltage total to their advertised power??? even the link I posted above from the grow light manufacturer was not clear…
Don’t confuse efficiency with output, if you want efficiency run a 5W at 350mw, more efficient than a 3W at 1A, however if you want some light to actualy see something then you need to put in the power and efficiency drops through the floor.
Have a play here http://pct.cree.com/dt/index.html
its slow to load, a bit clunky to use and is for Cree only, but it should clear things up a bit for you, especially the lumins per watt section.
The spec’s the manufactuer state are the recomended maximum.
Thanks david. I understand efficiency as the percentage of power dissipated as creating light vs creating heat. And yes leds can be driven at different operating currents of even pulsed and this will affect light output and efficiency (the sweetspot) and junction temperature (reliability and life)…
But I’m talking about packaged lights which have drivers setting all that and only user control is input voltage. These light specs are for maximum rated power?? Then how would I get that power? Apply the highest rated input voltage & then I get the theoretical lumens out?? Could this help me select between lights constructed with small leds vs large leds??
Most multicell drivers can only supply the rated amps with the max cells, I have a 5A one which is dreadful in this regard, while it is rated as 3.7 to 12.6 it only manages 1.5A with one cell, 2.5A with 2 and only with 3 does it reach 5A, well 5.5A measured at the led, but who’s counting
it’s rated 3.7 to 12.6VDC input for 5A output? and V out? I don’t really know anything about drivers. But my question really relates to how packaged LED light sellers are advertising their lights. Seems like they sum the LED wattage for total power but then never know or say how they are driving the LEDs or how much power is actually used by the light. Again, my experience is 1/2 to 2/3 of the advertised power but sometimes lower and I suspect that has everything to do with what input voltage is applied to the lights… so a 18W light (that has six 3W LEDs) might run at 12W if it’s run at 12VDC in but perhaps it would run at 18W if 24V was applied?? I’m surprised this isn’t well published industry information or at least there might be a general guide like… actual power = Vin/V in-max * rated power…
The sellers have not much interest in testing lights, they just sell them, along with a lot of other types of stock, and the light manufacturers usually do not provide any info.
They just find out how many lumins the led emitter is rated at recomended max amps and use that, this is the max emitter lumins, not out the front lumens, in the case of the XM-L2 the top bin is the U3, about 1200lm at 25c junction temp, but the emitter will probably be a T6 and the j temp will be more like 80c so about 930lm, then you have the loss of lumins (10~20%) through the reflector and lens, getting confused yet, you might not be but the sellers are, and 1200lm will sell more lights than 750lm.
Then you have the resistance to account for, and apart from testing each individual light there is no way to know this with any accuracy, now to the driver, most single cell lights will get quite warm at 3A, and the average person on the street dislikes this, so they only drive it at 2A, with the resulting drop in lumins, but not in sales.
Multi cell and emitter confuse things even more, best bet is to look at the reputable light manufacturers specs, most are reasonably accurate, then keep them in mind when looking at the cheap budget stuff.
