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Old    Joe (ilikebeaverandboats)      Join Date: Jul 2007       05-21-2011, 5:52 PM Reply   
Im going to run some LED's in the bottom of a long board I built, Im conformable soldering, and doing the math involved. I just had a couple questions.
Im planning on running 10 strings of 3 LED's, each set of 3 LED's in series will be run in parallel.
Now, If the LED I want to run has a Forward Voltage of 3.2V, that would mean each string needs 9.6V plus a resister (I will calculate this later once I determine the voltage I need to run). Since voltage is equal when applied in parallel, each string will get 9.6V

Heres my question, Can I just use a 9V battery? will the .6 difference be a huge deal in terms of brightness?

Once I determine the Voltage source I can get the Resistance settled and start gathering supplies.
Old    John Anderson (fly135)      Join Date: Jun 2004       05-22-2011, 8:41 AM Reply   
The 0.6V won't make much difference. You can compensate by adjusting the resistor. How are you calculating the resistor? SInce the voltage of the 3 leds is 9.6V, why are you thinking you need a resistor at all?
Old    "G" (grant_west)      Join Date: Jun 2005       05-22-2011, 8:41 PM Reply   
Sorry Im having a hard time understanding what your trying to do. I think your trying to link LED's and work out the resistance between the 2 ect/
I need to ask why don't you run the SMT LED's and you wont have to do any math just like 12 to each strip and your good to go.
Old    Joe (ilikebeaverandboats)      Join Date: Jul 2007       05-22-2011, 11:46 PM Reply   
From what I read you need to run a resistor to limit the current so they dont burn out right away. In my EE lab I actually lit an LED on fire by pushing a large amount of voltage through it at like 2 amps. I was hoping to use a 9V battery, and calculated the necessary 9.6V by using the "Forward Voltage" of each LED which is 3.2V. since they are in series, adding up the 3 LED's will give you the 9.6V. Then wiring multiple Series of 3 LEDs in parallel would allow each parallel series to get equal voltage of 9V.

Im pretty sure I figured it out though, here is what I decided on. Im gonna do my best to explain it, but i might just give in and draw a picture for you guys!
I also need to go see what radioshack has in stock for LED's, but I read that Blue LED's typically run at 20milliAmps and require a Forward Voltage of 3.2V to produce maximum output. If these numbers change, its pretty simple to re calc what I need, I just want to double check my method with you guys.

Assuming, 9V source, the 3.2V and 20mA needed for each LED, 2 LED's per series, and a total of 20 LEDs in 10 parallel series.
Resistance= (Vs-FV)/C= (9V-(3.2V*2))/.02A= 130 Ohms = resistor for each series of LED's

This will work since the voltage will be the same to each series since they are in parallel, and then the current will stay the same across each series as the voltage drops. Right?
Old    John Anderson (fly135)      Join Date: Jun 2004       05-23-2011, 8:56 AM Reply   
Your calculations are correct.

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