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Hello,
This may have been addressed in one of the threads but can anyone tell me how many LED's can be run on just 4.5 volts or 3 AA batteries? I am hoping to use up to 20 LED's most of which are in parallel. At least 2 will be in series times two for a total of 4. All LED's use 20ma and 16 reguire 3.3 volts, 2 reguire 3.4 volts and 2 reguire 2.4 volts. I might throw in a blinking LED using a 3909 chip. I hope I have given enough information and any input would really be appreciated.

Thanks,
Alan
 

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if you ran them all parallel you'd be drawing 400mA (20x20). At that current a 3 AAs have about 3000 mAh capacity so you'd wear them out after about 7 hours. YMMV.

I don't see how you're going to run any in series with only 4.5 volts supply. Two 2.4v LEDs would need 4.8 volts in series. There's some wiggle room but they'd probably stop lighting way before the rest.
 

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Alan,
Try using this LED calculator and see what you come up with.

http://led.linear1.org/led.wiz

Rob
Iwata Padawan
I like that calculator.

The actual limit on the number of LEDs you can light is the amperage rating of the batteries (or power supply), not the voltage (although that's important). Batteries can deliver high current for a short time, or low current for a long time.

If you were going to size a regulated power supply you'd need 400mA at 4.5 volts, but batteries have the luxury of being able to supply much higher currents.

7 hours is a guess for 20 LEDs at 400mA on 3AAs. There are a lot of variables (temp, service times (how long illuminated each time) battery condition).
 
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