Voltage Dilemma

IlluminatingBikr

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Feb 26, 2003
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Yesterday I made my first flashlight! /ubbthreads/images/graemlins/grin.gif

It runs off of two AA's and has 2 white 5mm LEDs in parallel, and a 150 ohm resistor.

The problem is, I am getting around 3 volts from the 2 AA's. The LEDs are rated at 3.6 volts, with a maximum voltage of 4V.

<ul type="square"> [*]How much brighter would the LEDs be if they get between 3.6V and 4.0V rather than the current 3 volts? [*]Is there any way I can get more voltage without using lithiums? (Lithiums are like $9.00 for 2xAA! /ubbthreads/images/graemlins/broke.gif) [/list]
 

highlandsun

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Aug 11, 2002
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Los Angeles, CA
Buy the lithium AAs from BatteryStation, he's selling them for only around $2 each.

Get rid of the resistor. With such a low battery voltage, you don't need it.
 

IlluminatingBikr

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Feb 26, 2003
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[ QUOTE ]
highlandsun said:
Buy the lithium AAs from BatteryStation, he's selling them for only around $2 each.

Get rid of the resistor. With such a low battery voltage, you don't need it.

[/ QUOTE ]

Good idea, I'll check Batterystation out.

I tried the circuit with no resistor, and it seemed to get hot quickly, and it wasn't too much brighter. With the resitor it stays cooler.

If i get liths, do I need the resistor?
 

CM

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The resistors limit the current to the LED. If it gets hot without getting much brighter, you're better off sticking with regular AA's with the resistor. It's curious that the LED's get hot when their forward voltage is much less than the voltage of the 2AA's /ubbthreads/images/graemlins/icon3.gif
 

Steelwolf

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Something doesn't add up if the LEDs get hot on only 2 alkaline AAs, since the supply voltage is less than the rated LED forward voltage. If you can, measure the current flowing with and without the resistor. You should usually keep the current below 40mA. 30mA is a good operating current (balance of runtime, brightness and heat).

Most direct drive for 5mm LED flashlights use 3 alkaline cells for a total of 4.5V supply. Some will rely on the internal resistance of the cells to regulate the current, some will put in anything from 10ohms to 33ohms in line to limit the current.

Try it out and see.
 

INRETECH

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Aug 22, 2002
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Check the current with a CURRENT METER, do not allow the current to exceed 20-25ma per LED; adjust the resistor accord
 
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