# How to calculate max # of LEDs?



## plux79 (Mar 25, 2010)

Hi,

This is my first post and I had been reading a lot from the site. Great information and great site to learn about LED.

I have a question and hope that the experts could help me out.

How to calculate the max number of leds to be driven by a driver? 

For example, if I have a power supply of 4 x 1.5V AA batteries in series supplying power to 700mA Buckpuck (3021-D-I-700), how many Luxeon rebel LEDs can it drive efficiently in series or parallel (or without having a drastic drop in performance)?

Will increasing voltage, buckpuck is rated at 5V-32V, means I could drive more LEDs? 

Thank you in advance.


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## znomit (Mar 25, 2010)

You add up the forward voltage Vf of the LEDs and then add whatever overhead the driver needs. 

The buckpuck claims a 2V overhead. 
LED Vf is around 3.5V, so you need a 5.5V battery to run 1 LED, 9V to run 2... though the datasheet says 6V minimum

In practice 4AA works ok driving a single LED though you'll lose dimming ability(needs 5V) somewhere along the way as your batteries run down. From my limited experience the overhead is lower than they claim. I'm pretty sure the overhead is closer to 1 than 2V for a single led. 

Heres the datasheet
http://www.luxdrive.com/download.php?id=4&pdf=3021_3023-BuckPuck.pdf


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## spencer (Mar 25, 2010)

From 4xAA you can only drive 1. Even if you could drive more it wouldn't be a good idea because 4xAA alkaline wouldn't like the load very much. I believe even driving 1 LED you will see better performance from a NiMH battery.


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## Neondiod (Mar 25, 2010)

If you wire the leds up in parallell you can drive as many as you like, but the current will be less for each led. 10 pcs of led in parallell give them about 70 mA each and at that drive level you will have a higher lumens per watt then a single led at 700 mA. The cost per lumen will be worse ofcourse..


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## plux79 (Mar 26, 2010)

Thank you for the reply.

Ok, forget what I said about AA battery. Let's assume a 12V or 24V power supply.

@znomit: Thank you for the explanation, so let me get this straight. If I have a 24V power supply, does that mean I can run only 6 Luxeon rebel LEDs (or 2 luxeon rebel stars) that has forward voltage of 3.15V. 

Ok, that previous case was using a low power LED driver. How about using high power driver like xitanium 120VAC, 12w, 700mA? How to calculate how many leds can it drive in parallel?

Thank you again.


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## blasterman (Mar 28, 2010)

plux79 said:


> If I have a 24V power supply, does that mean I can run only 6 Luxeon rebel LEDs (or 2 luxeon rebel stars) that has forward voltage of 3.15V.


 
If the power supply has 24v out then it should be able to handle up to 7 at 3.15 (??). If it's a current regulated supply (typical Xitanium) then it will handle 6 at 3.15 vF in series easily. 

If it's a non current regulated supply you'll be safer using 7 LEDs - with 6 LEDs you'll need a resistor. 



> How about using high power driver like xitanium 120VAC, 12w, 700mA? How to calculate how many leds can it drive in parallel?


 
As many as you want (provided you don't exceed the total voltage per string). If you have one string it wil get the full 700mA. If you have two parallel strings then each will get 350mA. Four strings will deliver 175mA to each. I have several 700mA Xitaniums running a mix of Rebels and Crees in two parallel strings because they ended up being too bright for the commercial job. So, I split them in two and they run exactly where I want them at 350mA.


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