buck pucks and multiple LEDs (same color)

[vadeblade]

Hi,

I did searches all day and could not find the answer.

Or more precisely, I just can not seem to wrap my brain on how to wire up what I want. From what I read all over this forum wiring up multiple high powered LEDs on a single Buck Puck will give less than stellar results.
i site these threads as an example:
http://forums.thecustomsabershop.com/showthread.php?t=2694
http://forums.thecustomsabershop.com/showthread.php?t=5596

However, the Buck Puck data sheet says to do exactly that, wire multiple LEDs to one buck puck....

to quote the datasheet

(buck puck) are designed to supply their current to one or more LED junctions... due to the nature of the buck regulator, the input voltage must always be higher than the total forward voltage drop of the led junction(s)... Figure 3-6 shows 700mA and 1000mA units driving multiple LEDs...

So, if I have a Rebel Star with 3 cool white Endors on it and I managed to cram 12V (900mAh) into my hilt why shouldn't I just run it off one 700mA buck puck? I ask first because I don't want to make magic smoke come out of LEDs.

[LeMoel]

you will be under driving it tho, more leds more underdriving,like an RGB

three endors? that might be way to bright man,if its for filtering then it will over power the filters,and make then fade

i would use a seoul p4-240 lumens or atlas led-272 lumens

[vadeblade]

you will be under driving it tho, more leds more underdriving,like an RGB

three endors? that might be way to bright man,if its for filtering then it will over power the filters,and make then fade

i would use a seoul p4-240 lumens or atlas led-272 lumens

looking...

Lamina Atlas - 282 lm @ 1050 mA 7.5Vf (typical condition)- what can I use to drive that?

Seoul P4 - 240 lm @ 1A ?? forward voltage (max rating - typical is 350mA). Can not find the datasheet.

If running 435 lm is too much then doesn't under-driving the Rebels a good thing? 435 lm is too much of a good thing? I thought we are trying to get LEDs so bright it would make our eyes bleed :p.

[vadeblade]

OK i found the datasheet for the Seoul P4 U bin.

So if I want 240 lm from the P4 I need to drive it @ 1A with a forward voltage of > 3.25v. Now that happens to be the Max rating of the P4. The typical condition it should be run at is 350 mA with forward voltage of 3.25v. Per the datasheet running at max is only guaranteed if we keep the junction temperature below 90 deg celcius. Would heat dissipation be an issue?

Now i have been digging all over the internet and it looks like if I took Rebel with 3 Endors on it and I connected it to one 700 mA buck puck the three LEDs will get around 650 mA each. Since the rebel minimum is 350 mA and typical is 700 mA, overall wouldn't I be getting better performance out of the Rebel Endor under driven than a P4 over driven?

Still new to very high powered LED so please let me know what i am missing.

[LeMoel]

yeah but then u wont have a constant current,so ur sabe will be brighter then it is,constanly changing

[neophyl]

Thats because the normal way to drive multiple leds with one puck is to wire them in series and use a higher voltage.

For example if 1 led needed 3.7v @1000ma then you could wire 3 in series to a 1000ma puck but you would need to supply around 13v (3.7 + 3.7 + 3.7 + puck voltage drop, usually around about 1v). This would give ALL 3 leds 1000ma. Thats why some of the available pucks are designed to handle 24-30v so you can power strings of leds. You have to make sure the driver can handle the voltage if you decide to use that wiring method.

If you wired them in parallel then you could use around 4.7v BUT the available power of the puck would get distributed amongst the leds so if all 3 leds were perfectly matched electrically then each would only get around 330ma. Not good. That along with the fact that 1 led will probably have a slightly lower internal resistance so would get a greater share would mean the light levels have a chance of being uneven too.

[Novastar]

I always love it when Neophyl posts. Bravo x 3.7v!!!!! :) :)