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stevef22

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How to Reduce/Resist 9 volts to 3 volts?

I would ilke to know what Ohm Resistor I would need to resist 9 Volts (6 AAA batteries each 1.5volts) down to 3 Volts.

I am powering 3 white LEDS that require 3 AAA batteries to run. I do not want to burn up the LEDs

Here is the LED Fact sheet

5mm LED lamp:
Product Information
SKU       LED5W
Manufacturer       Various
Stock Type       New
Shipping Info       0.4g, type L
 
Technical Summary
RoHS       Yes
Package       F 5mm
Packaging       bulk
Dimensions       F5mm
Lead Spacing       2.54mm
Color       White
Brightness       13000 ~ 15000mcd
Wavelength       465 - 470nm
Lens       Waterclear, White
Forward Voltage       3.5V
Current Rating       20mA
Reverse Voltage (V)       5
Continuous forward current(mA)       20
Peak forward Current(mA)       40
Power Dissipation (mW)       100
Operation Temperature Range()       -20~80
Storage Temperature Range()       -20~80
ASKER CERTIFIED SOLUTION
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Dave Baldwin
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phoffric

Use two AAA batteries.
or
tap the 3 V off the bottom two AAA cells      (This will run down those two cells sooner than the rest
or
use voltage regulator circuit. (will describe in more detail if you wish.
Note in the diagram the direction of current flow through the LED is forward bias (basically a short circuit).  The resistor is current limiting to protect the LED.  Size the resistor to bring the circuit current down to the level that is safe for the LED in use.
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ASKER

Here are some additional pictures to help issustrate my project. I would like to add this to my question so you understand my situation better. I would like to add 3 more AAA cells to the project. I want to make this as energy efficient as possible. Where would I place the resistors? Do resistors help save battery life?

View the video to see my circuit in action.

http://www.youtube.com/watch?v=S3lan6DHzyA
Picture-002-360x480.jpg
Picture-006-640x480.jpg
Picture-006-640x480.jpg
Picture-010-640x480.jpg
Do you have a schematic for that board?  Do you have a part number from the manufacturer?
No schematic for board. : ( Called company and they would not give it to me. I do know that origional LEDs that came with the candle are amber, the front of the packaging said lasts 300Hrs on two AA batteries. Does this help?
If you connect pairs of AAA batteries in series then connect those pairs in parallel, you will have 6 volts without needing a resistor. Like this diagram. [] is a battery and __ is wire
_____
[]  []  []
[]  []  []
Without a schematic, it's hard to give you good advice.  Your original question didn't mention anything but LEDs.  And the white LEDs have twice the forward voltage that amber one's do.  If there is any active parts for a power supply on that board, no telling what adding batteries or resistors will do.
The 20mA rating is a maximum; they will work just fine, if a little dimmer, on less current.  First evaluate (using an ammeter, a variety of resistors, your eyes, and your sense of aesthetics) how little current you are willing to run through them.  The less current, the longer the batteries will last.  Measure the voltage drop across the LED at your chosen current level.

Now you use this chosen current and measured voltage drop to design the rest of the circuit instead of the nominal 3.5V and 20mA.

Another trick I haven't seen anybody mention yet:  Two LEDs in series with a combined ~7V voltage drop and ~2V wasted across a resistor use the same current as one LED with a ~3.5V voltage drop and ~5.5V wasted across a resistor.  It's an easy 2-for-1 deal.