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Old Oct 28, 2012, 12:30 PM
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Stef mentioned overdriving LEDs in Post # 7 and Cree offer information on duty cycles % and up to 300% of max rated power.

Dave

Cree PDF

The applications support team at cree is often asked whether it is safe to operate our leDs with pulsed currents above the maximum data-sheet ...


http://www.cree.com/~/media/Files/Cr...sedCurrent.pdf
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Old Oct 28, 2012, 04:09 PM
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Of course! Putting them in series is one way to increase their voltage without using resistors -- in fact I've done it before with the standard wire-ended LEDs, but that was so that I could run them straight off a 3S A123 pack for cabin lights.

The only problem for my present application is I've got a PICAXE-powered strobe-and-switchable-landing-lights unit which has four outputs, all at Rx voltage (5v nominal), and I want to be able to plug any LED (or LED array) into any output, so they all need to be compatible with 5v supply. Two red Crees in series would be fine, because they're rated at 2.1 - 2.5v each, but the green and white ones are 3.4 - 3.9 and 3.0 - 3.5 respectively. Actually, in-line resistors are not so much of a problem with the green and white ones because they're smaller values than for the red one, dropping less voltage, and therefore less watts. Thinking about it, the reds are dimmer than the other two colours, so maybe two together will have similar visibility to a single green, which is what I want.

Thanks for that link, orraman. For a strobe-light which flashes briefly (1/10 second?) every 1.5 seconds or so (I haven't measure the precise timing), I presume that means the "duty cycle" is less than 10%, which in turn means I can theoretically overdrive by up to 300% of rating? Even if it does, I don't think I'm going to go that far, for peace of mind. We'll see how things look when I attach the lights to a model to compare with my conventional LEDs
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Old Nov 28, 2012, 07:39 AM
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Update, and further question ...

Well, a month on since my last post, and a set of three Crees has been installed properly in my model (instead of just taped on the outside) and are BRILLIANT I've put resistors in to keep them all at 500mA, despite the fact that some of the colours are rated for 700 and, I if I remember correctly, 1,000mA; and all three (red, green, white) show up at my normal flying altitude (about 150ft) in what passes for daylight at this time of year.

At the moment all three are strobes, so heating doesn't seem to be a problem. But I've read that heatsinks are required for continuous duty, such as landing lights. How on earth do I heatsink a surface-mounted LED like these? Do I solder its + and - pads to, say, thick copper strips, instead of to a normal printed circuit board?
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Old Nov 28, 2012, 11:36 AM
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Originally Posted by abenn View Post
How on earth do I heatsink a surface-mounted LED like these? Do I solder its + and - pads to, say, thick copper strips, instead of to a normal printed circuit board?
Simply GLUE them on an aluminium bar, sheet or whatever ... with epoxy ...
I've tested a Green Cree to its limits = works fine.
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Old Nov 28, 2012, 01:51 PM
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Arghhhh ... my Crees are different from yours Mine are tiny, with contact strips on the back for surface-mounting -- I think they're the tiny square that I can see on the board inside your lens. I've soldered a couple of 24g wires to the strips on the back so I can then use them like regular LEDs, and I suppose I could still glue them onto an aluminium sheet so long as I take care not to let the terminals touch the aluminium. I suppose I could also surface-mount them onto a tiny PCB so that I can then connect wires at the front like yours, and then glue the PCB onto a heatsink.

Only thing that concerns me is that epoxy might not transfer the heat, and it softens with heat so it might come unstuck and/or let the terminals short out.

What model number are your Crees, please?
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Old Nov 29, 2012, 12:32 PM
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Quote:
Originally Posted by abenn View Post
Well, a month on since my last post, and a set of three Crees has been installed properly in my model (instead of just taped on the outside) and are BRILLIANT I've put resistors in to keep them all at 500mA, despite the fact that some of the colours are rated for 700 and, I if I remember correctly, 1,000mA; and all three (red, green, white) show up at my normal flying altitude (about 150ft) in what passes for daylight at this time of year.

At the moment all three are strobes, so heating doesn't seem to be a problem. But I've read that heatsinks are required for continuous duty, such as landing lights. How on earth do I heatsink a surface-mounted LED like these? Do I solder its + and - pads to, say, thick copper strips, instead of to a normal printed circuit board?
You need a metal core pcb (MCPCB) to mount up your bare LED's. With the MCPCB you are then set to mount to an aluminum heat sink. You don't mention which family of Cree's the LED's belong in, so just search for the MCPCB for your style. Tin the pads real good, place the LED, and heat up the bottom of the MCPCB with an iron, till the solder reflows. Or use solder paste and skillet method.

Back in post #12 I mentioned a place to buy premounted leds (they have bare MCPCB's too), and a linear regulator chip for constant current. Those AMC7135's can mount up to a SOIC to DIP adapter board. These chips can be hooked up in parallel, or stacked, to provide additional current.

Sounds like you are on the path to some LED fun, good luck!
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Old Nov 29, 2012, 02:10 PM
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Thanks for that info, nickelflipper. I hadn't heard of MCPCB until now and, even with a Google search, I can't find any UK source of bare board. But my regular supplier does have premounted LEDs, and also 7-cell MCPCBs for my XP-E Crees. I think I'll get one 7-cell board and see if it can be reduced in size to carry only one Cree -- 20mm (I think that's its diameter) is a bit large for some of my applications. I'll continue to use my bare Crees (I bought two each of red, green, white) without heat sinks for strobes, and I'll buy premounted ones for landing lights.

I didn't follow up your previous mention of the AMC7135 because I couldn't find a UK source. In my application -- individual Crees each running off a regulated 5v -- what's the benefit of the regulator chip over a simple resistor in each LED lead?
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Old Nov 29, 2012, 04:38 PM
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recorded my LEDs an hour ago;

30w RGB LED as police strobes (1 min 56 sec)


daytime video (ran at 1/5 powerso I won't burn my fingers)

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Old Nov 29, 2012, 07:40 PM
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Quote:
Originally Posted by abenn View Post
Thanks for that info, nickelflipper. I hadn't heard of MCPCB until now and, even with a Google search, I can't find any UK source of bare board. But my regular supplier does have premounted LEDs, and also 7-cell MCPCBs for my XP-E Crees. I think I'll get one 7-cell board and see if it can be reduced in size to carry only one Cree -- 20mm (I think that's its diameter) is a bit large for some of my applications. I'll continue to use my bare Crees (I bought two each of red, green, white) without heat sinks for strobes, and I'll buy premounted ones for landing lights.

I didn't follow up your previous mention of the AMC7135 because I couldn't find a UK source. In my application -- individual Crees each running off a regulated 5v -- what's the benefit of the regulator chip over a simple resistor in each LED lead?
I see Newark (i.e. Farnell on your side) is kind of hit and miss on premounted XP-E's, might find what you are looking for there.

Not a huge advantage with the AMC7135's over your resistors, it will still burn off excess heat. Perhaps a better application would be if you ever decide on a separate battery (6V max) to supply the LED's . You get constant current regulation down to the forward voltage (Vf) of the led plus a couple of tenths of a volt. Another advantage would be the slim profile or form factor as opposed to the 1W+ resistors?

The AMC7135 chips are of Chinese manufacture, and are commonly used in driving li-ion powered LED flashlights. The AMC7135 chips could be cannibalized from drivers, which can be had for cheap from the Chinese electronic suppliers like DX, KaiDomain, etc. and possibly on ebay.
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Old Nov 30, 2012, 02:38 AM
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Thanks again nickelflipper. Farnell is one of the sites I regularly use, and also RS Components which is where I found the board and some premounted Crees.

I've got some pretty small (physically) 1W resistors which I install in the leads from LED to controller. I guess a current-regulator chip would be handy if I wanted to use them with variable input voltage, but I'd have to get something different from the AMC7135 because its current seems to be less than the Crees want. For the moment I'll keep it simple with resistors I'll just have to remember to change the resisitors if I ever change the input voltage.
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Old Nov 30, 2012, 03:21 AM
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Hello Everyone,

I just ordered a blue strobe that can be controlled via an unused channel . I didn't fly with it yet but the light output seems very impressive. It hurt my eyes while testing.
The strobe cost me $9.00 and a few bucks for shipping totaling $15.00. The only thing was that they had the wire markings mixed so I ended up mixing the signal and negative wires. After correcting everything worked.

The store is located in Turkey it took 10 days to get it in the US.

http://www.flytron.com/rc-light-syst...robe-blue.html
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Last edited by kintash; Nov 30, 2012 at 03:23 AM. Reason: Provide more accurate information
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Old Nov 30, 2012, 04:29 PM
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ISRAEL
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30w RGB LEDs now sold $11.99/ea incl.shipping (ebay)
my camcorder distorts colors at low light , actual colors are pure Red,Bluue,Green and mixes, somehow lots of purple seen on recordings

today:
30W RGB LED onboard flying wing (1 min 10 sec)
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