help needed for direct drive 10 crees

[aljsk8]

currently i have a tripod mounted floodlight that i use for lighting up an area outside at night

it consists of a aluminum tripod about 1 - 1.5m high with 2 halogen globes attached

i am converting this unit to led (to increase runtime from the SLA 12v 12ah battery)

i have ordered 10 leds (cree P4 - didnt want premium cree for this application)

im 95% sure i want to direct drive this system and i dont want to use resistors. battery voltage hot off charger is 12.8v

heat sink size and thickness is not a problem - it can literally be any size (but the smaller the better)

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so here are my questions

1) if a string of leds is set up in series (say 3) and powered from 12.8v
would this be the same brightness as a string of 4 leds

to rephrase
12.8v / 3leds = 4.2v each - so these will be at 1amp or more i think. 12.8v / 4leds = 3.2v each so these will be less than an amp. so when these 2 are compared would the over all brightness be the same? is there a preferred method?

here are my options
a)2 parallel strings of 5leds in series
b)3 parallel strings of 3leds in series (and lose one)
c)3 parallel strings of 3leds / 4leds / 3leds in series

pick one for me and explain why its the best

2)what size (thickness and area) heat sink will i need (anything up to about 200mm x 200mm is fine for this application)

id like a minimum amount for the recommendation you make in question 1

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i really need help on this and cant do it without your help so if you read this post and know some helpful information please reply

thanks in advance

Alex

 

[scott.cr]

Check out the LED calculator I found online. This will give the answers you seek. (Hint: You'll need resistors.)

 

[Mash]

I hate resistors too, especially in a battery powered setup, thinking of all that juice just being dumped away!
SO I dig where you are coming from, and therefor would reccomend you look into drivers.
They are IMO essential for this kind of setup, as they regulate the current, and remove a lot of guesswork and fiddle factor. I say this having tried to run Crees with tens of PSUs etc etc, and you end up running in circles, eg this psu gives too much with 3 Crees and not enough for two etc
Just get buckpucks (or powerpuck depending on your desired setup) or a couple of 18V max drivers from kai, and you will be a happy chappy, knowing your LEDs are being driven correctly and safely, and efficiently.

 

[Gunner12]

You could use 2-3 of these drivers and drive 6-9 Crees. 2-3 separate 3 Cree strings, that way you could also turn off a string and lower the output.

According to my calculations, you should get about 420 lumen with each string with Cree P4 LEDs. So around 840 lumen for 2 strings or 1260 for 3 strings.

If you direct drive 10 Crees, that might be hard, I would recommend 2 strings of 4 Crees wired in parallel. The other two, I'm not sure. So 12.8/4 = 3.2v per Cree. Each Cree will be drawing around 250 mA and produce around 60 lumen each, so 480 lumen total for 8 LEDs. I think a driver would be better. A 3 Cree string would mean about 4.2v per LED. At that voltage, a Cree will draw an unhealthy amount of power, 2+ amps.

Heatsink? maybe a big hunk of metal or a/many CPU heat-sinks and a fan.

 

[Wok]

I also use car batteries for power source in a camping situation.The two uses are..

1] Light for cooking.

Here I have an open aluminium channel 40mm x 20mm x 2mm, 2m long.
Uses 4 emitters in series - no control.
Reasoning: Enough light for cooking without attracting too many insects.I originally drove the emitters at 700mA but too bright = lots of bugs.

2] General light.

Here an aluminium channel 25mm x 20mm x 3mm, 300mm long [needed to retofit useless fluro light]
Uses 12 emitters; 4 parallel strings of 3 in series driven by 700mA Buckdriver.
Reasoning: Lower heat buildup as tube is closed.

Note: 1] Cree emitters
2] No optics/reflectors - flood output
3] System is useable when supply is high ie: campsite with mains power,generator or car engine running.

 

[aljsk8]

cheers for the help guys

gunner - would 4.2v really be pushing them at over 2amps??

i want to push the leds as close to 1amp as possible
but not to worried about pushing them harder - say 1.2a with massive heatsink

im really after the 1500 - 1800 lumen mark

(180 x 10)

i might have to look into drivers but i could really do without the cost

i dont mind being a little in-efficient as the current setup uses 100w and lasts 1hr and give approx 2000lumens and anything has to be better than that!

any more advice guys??

thanks

Alex

 

[Mash]

At 4.2V you are definately seriously overdriving things!
Cost wise, the drivers wont add up to much, the Kai driver is $3, and if it saves you one blown LED it has paid for itself, not to mention the saving in time and effort of setup/experimentation!
There is a good deal on Buckpucks over at Marketplace, for $10 each, but you need to buy 5, maybe you can use a few and sell the rest here.
PS With certain buckpucks you can dim the setup too!

 

[Gunner12]

A Cree at around 3.7-3.8v draws 1300 to 1600 mA. So at 4.2v, it would be drawing way over 2 amps, which is really bad for the LED. If you can buy two more LEDs, you could get 4 strings of 3 Crees each and get around 1770 lumen Lumen of output at the emitter. The previous numbers were using numbers at 700 mA, the driver drives at 755 mA. Buying 4 drivers an.d 2 Crees will bump your price up $22.32 including shipping if you buy these drivers and these Crees.

 

[legtu]

cheapest route is to just use resistors to limit current...

 

[aljsk8]

ill be asking some resistor questions tomorrow

thanks for all the help guys - im still undecided on this one

Alex

 

[aljsk8]

been away for a few days

back to the lights

so if i use a setup with 9 crees and i want them at least 1000ma but 1200ma would be ok too (mabee more)

here is my circuit



where do i put the resistor / resistors

option A, B, C or D

and what size to get 1000ma or 1200ma

thanks

Alex

 

[VidPro]

resisters dont loose any more than curcuits do WHEN the voltages are really close, do the math. you got say 12.8v to 10.5V and you need to drop a Volt or 2 , 2/12 - 1/6 = about 17% find me a driver that does that well? (course i am bad at math, just good at RUNTIME )

put each of the resiters in each of the strings like 3 on position C, OR position D, either way, no advantage from both D&C , and no advantage from an ADDITIONAL A & B resiter.

single resiters on A & B would be "ok" but not "best" if you can put one on each paralleled series string like position C&D its better. also having A& B resiters Added to the curcuit would be "ok" but not "best" , but great for say dimming the whole rack at once.

Dont overdrive a resistered setup, over TIME and HEAT they will lower in Vf (voltage on the led) so if you start at spec for your max current, it will overdrive itself without any help from you

on the other hand, dont assume that the Led calculator included ALL of the possible resistance, you may need slightly less resistance than the calculation, to get a high drive, due to wires and connections, and battery droop on load, and everything.

also try and avoid the chaining, where the last Series string in the row has the least voltage, prefer to have each led series string be connected with the exact same lenght of wire so the voltage to each string is more similar.

 

[aljsk8]

so its 3 seperate resistors

ok so how do i use this calculator to figure out the size

or can someone do the maths and figure it out for me

many thanks

Alex

 

[VidPro]

you calculate based on your input voltage, which would be MAX ever about 14v and your LED total voltage at the current your going to run, somewhere around 3.5-4volts, you get a good guess, then have enough different resisters, or parellel/series resisters till it works about right.

there are led calculators that you put in your input voltage, the voltage of EACH led and and the number of leds in the string, it gets pretty close, and is a good start.

Each series set, is in and amongst itself a seperate connection to the battery, so you dont have to calculate the parellel connection.
(hmm how to say that better)
Figure out the resistance for the 3X leds on the voltage you have, then just do that 3 times, when placing resistance on each string.

the ONLY real way to "perfect" it after having done the math, and getting a good idea of the total, is to use an AMPMETER or DMM with ampmeter, and meter your final results in the curcuit as it will be. you will then adjust it a bit.

adjusting UP can be done by reducing the resistance, and Parelleling up resisters reduces the resistance total by relative ammounts.
so 2X 4Ohm resisters in parellel is then 2 Ohms.

|----{~~}-----|
|----{~~}-----|

adjusting DOWN can be done by increasing the resistance , or seriesing more than one resister, again relative to the number you series.
so 2X 4 ohm resisters in SERIES is then 8 ohms.

|----{~~}-------{~~}-----|

i dont do math i get close, measure it IN the curcuit, "debug" it live with a meter.
Remember Meter leads, and meter Shunts, have a lot of resistance, using fat copper for meter leads reduces that a lot.

i always use overrated resisters, cause a burnt resister becomes less resistance usually, and metal oxide (type) resisters are nice, cheap, and work great.

 

[Cypis]

 

[aljsk8]

wow guys this is all super info

vidpro

this system is powered by a sla that is never hooked up to an engine
after charging way beyond full the max is 12.8v (not 14)

so id be happy with 12.8 at correct

after the advice given it will probably be a good ide to head for 1000ma just to be safe then if things do change ive got some safety margin

cypis

thats a sweet chart / diagram - very helpful (can i get that program anywhere - i have 1 like it but not as good)

can the wattage be higher as long as the resistance of the resistor is the correct value?

any tips on where to get resistors for this?

thanks again guys - very helpful

Alex

 

[VidPro]

so id be happy with 12.8 at correct

Yes, if its not on any charger thing at the time.

 

[TorchBoy]

thats a sweet chart / diagram - very helpful (can i get that program anywhere - i have 1 like it but not as good)

As Scott mentioned earlier, it's at http://ledcalc.com/.