Discussion in 'The Projects Forum' started by Vilkas, May 12, 2011.

1. Vilkas Thread Starter New Member

May 12, 2011
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Ok so yes this is another led thread, but just bear with me, Ive done my homework and am almost ready to build, I just need to fine-tune my design.

I plan to make an led light panel using somewhere between 144-162 Red and Blue leds.
here are the specs:
RED
max forward current: 30ma ( I assume the recommended current would be 20ma?????)
forward voltage: 1.8-2.4v

BLUE:
max forward current: 30ma
forward voltage: 2.8-3.4v

I will be supplying the leds with 20v DC @ 350ma from a typical wall adaptor I have lying around that was actually originally intended to power leds.

Here is my first design:
http://www.flickr.com/photos/62746482@N02/5714922930/

and here is my second design:
http://www.flickr.com/photos/62746482@N02/5714922932/

obviously the overlap of copper strips will be seperated by a 'bridge' of rubber.

I think the 7 rows of red will eat more power and so a smaller resistor would be needed meaning less power wastage and less heat produced by the resistor? is this correct?

Because the specs for the voltages of the led vary I am having trouble calculating resistor value. a row of 6 red leds eats 10.8-14.4v while the blue row of 6 eats 16.8-20.4v
I crunched these numbers using ohms law and got resistor values for the red=470 ohm (@2veach) - 330 ohm (@2.3v)
Blue=82 ohm - 68 ohm
These were calculated with the leds using 20ma each.

Do you think I should be using larger resitsors eg 470 ohm or should I be using the weaker 330 ohm ones???

One more question: Can i, using the same design and power source make a panel that uses say 400 leds? My understanding is that it dosent matter how many banks/strips of leds you have as long as you dont use more than 6 blue leds in one strip and more than 8 leds in the red strip. Is this correct?

so yea, thats about it. What do you think?

2. CDRIVE Senior Member

Jul 1, 2008
2,223
99
While I didn't check your resistor values I did make a basic observation. You have a total of 24 series strings of LEDs, with each string drawing 20mA. This adds up to 480mA. You're wallwart's rating is 20V@350mA. Either loose some strings or get a bigger supply.

3. wmodavis Well-Known Member

Oct 23, 2010
737
150
The circuit is designed to let the smoke out of the power supply.

4. Vilkas Thread Starter New Member

May 12, 2011
4
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^Right ok, I didnt know you had to add up all the amps for each string. Coincidently I do actually have another (the same make) power supply that puts out 20V DC @ 750ma I will use that one instead. This suits me better as I planned to add a few more strings anyway.

The most important question i need answered is how many amps to run through my leds. They are rated at max forward current 30ma but I know most people put 20ma through. do you think I could get away with 25ma or would this heat up the leds too much?

Other than upping the power supply could you make any other suggestions?
Im not expecting you to check all my math, Im just really new to all this and am not even 100% sure what im doing.

thanks for the advice so far.

5. CDRIVE Senior Member

Jul 1, 2008
2,223
99
30mA is a max rating and should be avoided. If you run your LEDs @ 20mA they will likely operate trouble free for the rest of your life. Your 750mA supply should do just fine.

6. Vilkas Thread Starter New Member

May 12, 2011
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Cool, thats what I thought.

My brother came home yesterday with an led array he made at school.
I noticed that instead of soldering the components they were simply glued together with a hot glue gun. Any advantages/dissadvantages of doing this??

7. CDRIVE Senior Member

Jul 1, 2008
2,223
99
EEK!!! No advantage at all. Don't do it! You'll be inviting endless intermittent connections.

May 12, 2011
4
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