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how to limit to current to led's? Answered

hi all

I have a diy led project. 
My batteries are dying really fast, I did a litte bit a research and find out that my led are drawing alot of current from my 3 AA's

I wire my 36 led's in a parallel fashion
voltage supply 4.5v
vf = 3.4 per each led
If = 20 ma
 currently Not running any resistor through the leds. 


What is the best way to make the battery last longer or to limit the current that is being supply led's?


thanks in advance!!!!!



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user
mathewsBest Answer (author)2010-06-28

I found that pulsing the LEDs used about half of the power.

If you connect the clock output to a decade counter, you could then connect the transistor bases to the outputs of the decade counter, divided equally over 9 of the 10 outputs.

You could experiment with the resistor in series (or use a potentiometer) to find a brightness which suits, or:


Vs = 4.5V
Vf = 3.4V
if = 20mA


Vr = Vs - Vf
Vr = 4.5 - 3.4
Vr = 1.1V

V = iR
R = V/i
R = 1.1/0.02
R = 55Ohm
                                            So use a 56Ohm resistor in series with each LED.


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user
lemonie (author)2010-06-28


Like rick' said use resistors.
But I'd suggest you get a higher voltage supply, run some in series so you've got the same 20mA going through more than one.
The only reason the LEDs are still working is that the AAs can't supply quite enough current to "fry" them.

L

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rickharris (author)2010-06-27

http://led.linear1.org/led.wiz will do the calculation for you

BUT 36 x 20ma  is around 0.7 of an amp no wonder your small batteries are dying fast.

You need to look to setting up a series parallel arrangement (see the web site) and perhaps going to larger batteries.


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