Do i need two AA battery or one to power 5 led's with forward voltage of 3.0 -3.2v?

Question

hi... im trying to work out how to make a simple led circuit for a project with kids where we are gonna make light sculptures.. and i am getting confused by all this electronicness...

i have found some what i hope to bright leds and they say the forward voltage is 3.0 - 3.2v and i want to know if i can run them straight off AA batteries... it would be best to run each circuit from one battery but maybe we would need two???

any help/information/ suggestion greatly appreciated...

thanks..

...Livvy*

Answer 1

You need 2 regular AA batteries connect in series to produce 3.2 volts when batteries are new.  Connect all the lazer LEDs in parellel,observe correct polarity of LED. Do not need any resistor to limit the current of each lazer LED because it is rated at 3 volts. Do not use rechargable battery,its has only 1.25 volts.

Answer 2

You need to get over that forward voltage, which will increase as you pull more current to light up, so they might end in 3.5 to 4 V.

I would use 3 D batteries in series to add 4.5 volts and have plenty of mAh stored to light up for days.

Any higher voltage (4 cells or 6 Volts or 9V batteries) will require you to calculate the resistor required to limit the current through the LEDs so you don't fry them.  But energy will be lost in the resistor as heat, so excess voltage is not needed, is waste, unless you don't have possibility of lowering voltage.  The Resistor should be calculated by (Vbatt-Vleds)/(Led Current)  so 9V battery would require a resistor=(9V-3.2V)/(0.020A) ohms or R=290 Ohms.  Power dissipated in resistor is Current^2*R so (0.02*0.02*290)=0.116Watts or 1/4 watt would be enough to dissipate the heat generated at resistor.

Answer 3

You need more than 2. You cannot run LEDs without a series resistor or you run a large risk of burning them out. And you should drop 2 volts at least in the resistor, which means you need 5-6 volts worth of battery, which is 4 AA batteries. You may get by with 3, which is 4.5 volts if you can live with variations in brightness.

Bottom line, you can NEVER run LEDs straight off a battery or voltage supply without a series resistor.

And each LED needs it's own resistor.

Calculate the resistor as follows:

Vb is battery voltage, Vf is LED voltage, If is LED current.

R = (Vb-Vf)/If

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Answer 4

2 batteries in series will just barely light them up when they are all in parallel with each other, and the polarities are matched -- all anodes together, all cathodes together.

It is better to use 4 AA cells (6 volts) and a resistor in series with the LEDs.  The resistor should be any value between 18 and 36 Ohms (for 5 LEDs in parallel), and it should be rated for 1/2 watt (and it will get warm).  RadioShack has these resistors in their parts cabinets.  The smaller the value of resistor, the brighter the LEDs will be.

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Answer 5

You would need two as a minimum in series, as the are only 1.5 volts each.  Although LEDs are very low power consumption, they still do require some!  You are better off using a 9 volt battery.  Of course the total number of LEDs also needs to be added into the equation if more that 5 or so!