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leds and help

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conntaxman

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Im tring to find a wireing diagram to connect 4 leds together,these are 3 watt each,forward voltage3.3 to 4 and draw is 700ma. Reason is that I want to put 8 of these in my recessed lights in my kitchen , but I want to feed these from a 12 dc battary.
I never figured out a cir. but I could follow a print.
Also how many of these leds could be put in series? Im figuring only about 2 because of the high ma.I have looked all over the net for a cir. to use 4 at once.no dice.
tks
John
 
Starting from 12V, no more than two. If Vf=4, two LEDs drop 8V, leaving 4V across the current limiting resistor. At 700mA, the power dissipated in the resistor is 0.7*4 = 2.8W, so you will need a 5W resistor. The resistance is R=E/I = 4/0.7 = 5.7 Ohms. 5.6 Ohms is a standard value.

To run all four in series, you would need 16V plus the voltage across the current limiting resistor, say another 4V, for a total of 20V. You would use the same 5.6 Ohm 5W resistor...
 
Led's in series

Hi MikeML. thanks for the reply and help.Here is what I was looking at,a LM338 regular.
**broken link removed**
This small cir. they say will put out about 5 amp. The LED'S that I want to use are 3watt,forward V is 3.5 and currant is 700ma. So seeing that 1 amp=1000ma. could I run about 6 of these leds in Series and that would be 4200ma. Then all I have to do is figure out what R1 and R2 would be. They also say that I don't need to use the cap's.
Or just do 2 Cir. using only 4 leds each and put two in each light.
I don't think it's very hard,but I don't play around with electronics all the time,I just follow schematics.
Thanks for all your help so far.
Guess I'll have to go and buy some cheap leds , when I start to figure out R1 andR2. so when I Blow some leds, they wont be the 3 watt leds. LOL.
 
If running LEDs IN SERIES, the current through all of them is the same, namely 700mA. The total current that the supply must deliver is still only 700mA, albeit at a higher voltage. All things being equal, it is much more efficient to put multiple LEDs in series.

If you are confident that the drop across each LED is 3.5V, then you would need 6*3.5 = 21V PLUS the voltage dropped across the current-regulating (ballast) resistor. Per the previous calculation, if you allow for 4V across the ballast resistor, then the total supply voltage required is 21 + 4 = 25V. So design you LM338 for 25V @ ~1A. Don't forget heatsinking sufficient to cool the LM338 and the ballast resistor.

Go look up "BUCK-PUCKs" to see if there is not a driver commercially available.
 
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leds

hi Mike. Im sorry, but I ment in parallel.This would mean that I would need the 3.5vdc but need more amp's.
**broken link removed**
John
 
If you run them in parallel, you will waste a lot of power in each of the REQUIRED series current limiting resistors.
 
leds

If you run them in parallel, you will waste a lot of power in each of the REQUIRED series current limiting resistors.
Hello again,Yes your right.Forgot about that.So back to the First question,Dose anyone have a small cir. to run about 6 or 8 of these leds [3watt/FV=3.5 FC=700ma ]? I know that you cant run all these in series .
tks
John
 
Hello again,Yes your right.Forgot about that.So back to the First question,Dose anyone have a small cir. to run about 6 or 8 of these leds [3watt/FV=3.5 FC=700ma ]? I know that you cant run all these in series .
tks
John

You obviously didn't follow my BUCK_PUCK link above!
 
leds 3watt

You obviously didn't follow my BUCK_PUCK link above!

Hello Mike, I found a person that had a cir. on the net for connecting two 3watt leds and the supply was 12dc.
I posted the link for other people to see and use.Dont know if it was on this site or another.
Looks like many are looking for these small cir. to build off of, but not many people put then on the site,They just keep telling the other person to do this,or use this, no that wont work,but never show them the cir.
I like to help out people ,and most of the time I can tell if they need a blue print to see how its done,so that they can just build it.simple.
About the buck reg,,I want to build this cir,not buy it.
tks
John
 
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