LED from Batteries?

S

screamingacorn

Limp Gawd
Joined
May 10, 2002
Messages
171
Does anyone know how to power a series of LEDs from batteries? If one battery is 1.2v does that mean that two batteries equal 2.4v? :confused:

Thx.
 
Yep, 2 1.2v batteries in series, that means the minus side of one goes to the plus side of the other, yields 2.4 volts. I assume you're using rechargables, you might want to actually use 3 or 4 batteries if you intend on running several leds. You'll have to run them all in parallel.

Here's a good calculator to calculate the resistance you need. http://www.metku.net/?sect=view&n=1&path=mods/ledcalc/index_eng
 
Same as mentioned before: Just remember that the resistor is important, without it all your LED's will can burn out. You will probably need 3 batteries at least since with 2.4 volts and a resistor you will not have enough voltage for them to light bright enough.
 
If you connect the LED to a voltage very near it's forward voltage then it will conduct pretty near it's rated current level just fine. However, it's not really a good idea, because then the LED will overcurrent itself if the voltage happens to go above the forward voltage, and it will dim more as the voltage dips under.

The best setup is to use a series resistor and arranging the LEDs and batteries so you need the smallest resistor possible, that way you will be dissipating the least amount of power in the resistor.
 
Yes, a series resistor is ESSENTIAL. Yes, an LED will run just fine if you hook it up to it's rated voltage.....for a while. The problem is that diodes (and all semiconductors) conduct better the hotter they get. So your LED, as it warms up (it will--it's not 100% efficient), will conduct more current, which will cause it to get warmer, which makes it conduct more current, etc, until the whole thing burns out. It's called "thermal runaway". A resistor is important in order to prevent thermal runaway from happening.

Here's how you would do it:
Code: 
|----^^^---|>|-----|
|----^^^---|>|-----|
|----^^^---|>|-----|
|                  |
|+                 |
Battery            |
|-                 |
|                  |
|------------------|

The ^^^ are resistors, the |>| are the LEDs.
The value of the resistors are as follows:
R = (Voltage from battery - LED voltage rating)/current rating of LED
 
Thanks for clearing that up! That said, which is better (longer lasting) nimh or lithium? Rechargeable or not. Don't lithiums put out more volts which means less batteries?
 
You might want to add a voltage regulator so that you can keep the LEDs at the same brightness for the entire life of the batteries.

A 1.2 volt battery is only 1.2 volts when its fully charged.
 
Lithiums have a higher energy density than NiMH batteries, but they are also more expensive. They also have more volts per cell, and often come with multiple cells, so they would run at a higher voltage but lower amp hour rating. Lithium batteries are also hard to find except for specific items like cellphones. You could of course use such a battery, but I've heard it's difficult to charge lithium batteries without the proper charger.
 
as someone said earlier.. you might want to use a voltage regulartor.. so that the light stays at the same brightness
 
I would suggest you not use a voltage regulator. It will unnecessarily drain your batteries faster then need be. Just stick with a simple resistor, unless a constant light level if important to you.
 
A voltage regulator is not totally necessary. You see, LEDs don't work like regular incandescent bulbs. As the voltage from the battery falls as the battery drains, incandescent bulbs lose even more of their light into the infrared spectrum. LEDs may get slightly dimmer as your batteries start to fail, but they will provide plenty of usable light for a long time after the batteries drop from full voltage.

Putting a voltage regulator in will make things worse--it will draw more current and shorten the battery life more than it will help by maintaining the voltage.
 
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