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# Which cell &amp; how many do I need?? Answered

I'm new to this whole electrical engineering thing. Thx to instructables, I got a crash course in LEDs & wiring them up. I need to make a prototype LED array. How do I determine the right size & qty of cells (9v, AA, etc.) needed to run an array for a decent amount of time for an end consumer? What's the equation for figuring this out? Will the Ohm's Law equation (which I'm new to) work for me here? Anyone's help would be greatly appreciated!

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ok, well, first determine what battery you want to use then how many Leds you want, and of what color i can help you from there...

Kewl. I appreciate it. Well the prototype I'm thinking about would have an array of 20 SM-LEDs (4 prong). I believe I will be wiring them in parallel with their own resistors. Now the specs on the SM-LEDs I've been messing with are 3.5v 20mA white. I need a battery configuration that is portable/lightweight and long-enough lasting. With that, the design I'm thinking of could accommodate 1 or 2 9v or 3-4 AA or something like this. Is there an equation that I can run the numbers & see what fits best?

ok, well running the math... here: http://ledcalc.com/

i find that you would need 4+ volts, so 3 AA (4.5v), each led wired to it's own resistor, then all the LED + resistor combos are wired in parallel with each other, using this configuration, you would need a 56 ohm resistor, it would consume 357Milli amps

or

4 AA batteries (6 v) wired the same way, resistor 150 ohm, current draw: 333 Milli amps

or

one 9v battery, two LEDs wired in series with one resistor, then in parallel with the rest, 120 ohm resistor current draw:167 Milli Amps (it will eat the 9V in about 2 hours)

more help?

K. I'm running the numbers through that calculator. Trying to see if I understand the equation. 1 Ah= 1000mAh. Does 1Ah = 1 hr of burn time? If so, in the examples above, does the 167mA & 333mA total current consumption equal 1.67 & 3.33 hours of burn time respectively? Or am I way off still?? It looks to me from Energizers AA specs, that the "AA" gets the best life for use.

1000mah = 1 hour of 1amp = 1000 hours of 1ma (and anywhere in between those two)

so say a battery has 2000mah, you draw draw 500ma you can draw 500ma for 4 hours

Ok kewl. That makes sense. I appreciate the help. If I have any more questions, I'll get back to you.

In the case of leds wired in parallel, the voltage drop across them is the same, so supplying 3.5V is sufficient here.

If portability is not a serious concern, consider purchasing a 5V AC Adaptor ("Wall Cube") such as this one:
http://accessories.us.dell.com/sna/productdetail.aspx?sku=A0963896&cs=04&c=us&l=en&dgc=SS&cid=30098&lid=674098

Then, a simply cut off the connector and solder the connections or use standard screw terminals on a solderless breadboard.

Hope that helps,
Purduecer

Portability is def a main issue. The only plug-in characteristic would be if I decided to use rechargeable cells. I don't think I'll be doing that though. Thx.