LED Brightness Chart

[Lord Maul]

Heh, that was me. I stuck the thread too, it's very useful

[Matt Thorn]

I have recently gotten the impression that it is excessive voltage, and not excessive current, that can fry things. I've been commissioned to make two lightsabers for use in a performance to be held at Tokyo's Budokan (as in "Cheap Trick Live at the"... Please tell me someone here remembers that one), and they need to be ultra bright, or rather "hyper" bright, so I went with Hyperdyne's Hyperblades. (The customer has deep pockets.) Battery life is a major concern (they have to maintain maximum brightness for at least 40 minutes), so I got a couple of UltraFire BRC 18650 3000mAh 3.7V li-ions. (Yes, that "3000mAh.") Hyperdyne told me the high current would be no problem.

But I look at the datasheets for LEDs, and find things like "Absolute Maximum DC Foward Current: 1500mA." So, can current kill an LED? I seem to remember reading somewhere on the forum that too much of "something" (I can't recall, but I'm guessing current) actually makes an LED dimmer.

I ask because I have two of these 3000mAh li-ions left over, and have another (potential) project with not enough room in the hilt for more than one battery, and I'm wondering if I could use a 3000mAh li-ion to power, say, a 5-watt blue LedEngin (which has an Absolute Maximum DC Foward Current of 1500mA). Or would bad things happen, like my girlfriend leaving me for a woman (it's happened before), or an asteroid hitting the Earth (ditto), or a $13 LED dying?

Too bad there's no such thing as a 1500mA BuckPuck.

I guess the basic question is, when the chart says "overdriven," how much "overdriving" are we talking about?

[Kal El Rah]

Hey Matt,

A simple explanation to you question is that the mah/ma rating of a battery is that it is the capacity of the battery and has no relation to what it is providing to the circuit you are using, it is just how much current it can provide over time, for example if the LED you are using is rated at 1500 ma, it can theoretically last for 2 hours 3000/1500=2, it isn't that the battery/s will pump out 3 amps to the circuit, but what the circuit needs to operate.

Voltage is more detrimental to the LED as the rating for those is absolute, for example, if the LED you are using is rated at say 3vf/4vf max you really don't want to overdrive it more than its highest rating or it will blow.

Here is an example of an accent LED running off of a US 2.1 connected to the main LUX III R/O outputs from the board, the output from the board is 1500 ma to drive the LUX R/O but the accent is rated at 30ma and as you can see it is working just fine.

http://s646.photobucket.com/albums/u...t=DSCI0203.flv

I hope that helps you out

[Matt Thorn]

That helps a lot, Kal El. Thanks. I know I've read about the distinction between mA and mAh at least twice before on this forum (probably explained by Novastar ), but on certain things I'm a slow learner. The bottom line is, using a 3000mAh 3.7V battery with a blue 5-Watt LedEngin that wants a forward voltage of 3.6V should be no problem...?

[Kal El Rah]

Correct

check this link out:

http://led.linear1.org/1led.wiz?VS=3.7;VF=3.6;ID=1500

[Matt Thorn]

Ah, my dear friend the LED calculator. I use it often, since after 2 years of saber-making I still have trouble wrapping my head around the whole "resistance" thing.

[Kal El Rah]

No worries Matt, just glad that you understood and can finish your project.

[Matt Thorn]

It might be time to update the chart with the newer 5-watt and 10-watt options that are now feasible in a saber. I recently got a LedEngin green 10-watt star. Here's a photo of it running off four 3.7V 900mAh Li-ion batteries with a 1000mA Buckpuck. That's a Luxeon III being run by a CrytsalFocus 3.x next to it.


I didn't bother to charge the latter before taking the photo, and of course the CF-powered blade is "shimmering," so this is not the most scientifically rigorous visual comparison. Take it as a ballpark comparison. In short, the Luxeon III is bright, and the 10-Watt LedEngin is brighter. (Doh.)

Just to give you an idea how much brighter, look carefully at the Luxeon III blade. The side facing the LedEngin 10-watt is actually being lit up by the other blade, so it appears brighter on that side.

Of course, there's a price to pay for all that bright. I have the LED on one Tim's standard copper MHS heatsinks, sitting in an MHS blade holder and 2" extension. The whole thing (top to bottom) becomes uncomfortably hot after just 5 minutes continuous running. I don't know how much it takes to melt a collimator and lens holder, but I don't really want to find out. Also, the LED seems picky about voltage. Although it seems happy to keep running continuously for an unknown amount of time (I didn't have the nerve to let it run longer than about 6 minutes), I find that once I turn it off, next time I turn it on, it will light up for half a second or so and they turn go black. It keeps doing this until I recharge the battery pack. There are five different bins of the green 10-watt, and if I read the datasheet correctly, there seems to be no overlap in (theoretically) acceptable forward voltage between bins. To top it off, there is nothing on either the LED itself or the packaging to indicate which bin it is!

You can see the datasheets for all the LedEngin LEDs here. Scroll down to "LZ-010 Family" to see the 10-watt LEDs.

[Kal El Rah]

Matt, how hot do the batts get? It sounds like the batts are doing ok for the time they are being used, but once you turn it off they have depleted their charge or the current capacity is not enough to power the puck, could be why recharging them solves the problem. Looks to me like you need another battery solution as the 900's don't have the capacity you need.

[Matt Thorn]

Matt, how hot do the batts get? It sounds like the batts are doing ok for the time they are being used, but once you turn it off they have depleted their charge or the current capacity is not enough to power the puck, could be why recharging them solves the problem. Looks to me like you need another battery solution as the 900's don't have the capacity you need.

The batteries don't get warm at all. When I do the same thing without the Buckpuck, again the LED lights up for a second, but instead of going completely dark, it abruptly becomes extremely dim.

I suppose I could try four 18650 Li-ions, but I would only be able to use them in a 1.5" sinktube. (2 of them won't fit side by side in MHS parts.) Alternatively, I could use 6 of the 14500s. Presumably that would be safe if I used a Buckpuck.