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Unread 10-06-2012, 02:13 AM   #6
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Join Date: Jul 2009
Posts: 2,292
Originally Posted by AshofRebirth View Post
I'm not sure if you're serious here, but I apologize anyway. I didn't mean for it to appear so phallic. XD;

Oh and thanks everyone! I think that answered my question.

Followup question though: Each LED has a 3V capacity right(is that even there right word..?). If I did two of the 1.5 v batteries or even the 3V, wouldn't I not need a resistor at all?
The activation voltage of the LED varies, reds tend to be around 1.8V, blues can be as high as 3.7V. Other colors tend to be in between these two numbers. Also it changes with temperature, and as the LED ages. For this reason you should almost always use a resistor, unless you are limiting the current in some other fashion (ie. a constant-current driver).

If you do not know what value resistor to use (or you want the circuit to work with any voltage) you can construct a constant current source using the commonly available LM317 integrated circuit (should be able to find one at radio shack, etc.) e=1 sources
(Replace the 100 ohm resistor with a wire. Replace the 1.25 ohm resistor with a resistor between 1K and 100 ohm (less resistance -> brighter LED). The voltage drop across the LM317 will be roughly 3-4V minimum, so use a 9V battery if you choose to use this circuit. The benefit here is the current will stay constant regardless of the voltage of the battery and the LED (or chain of LEDs in series), as long as a battery with a high enough voltage is used. So for instance you could power 6 red LEDs (around 12V) in series with 2x 9V batteries (around 18V) in series.

If you go the series resistor route, start with a value like 1K and see if that is sufficiently bright. If not follow the instructions online and calculate the proper value for the resistor. Most small low-power LEDs can handle up to 20mA without burning up.

There is honestly nothing wrong with wiring LEDs in parallel if it makes sense for the working voltage of the battery you are using (ie. a 3V battery and 1.8V LEDs). Just make sure each LED has its own current-limiting resistor if you choose to do this. The reason people generally prefer to wire in series is because only one resistor is required, and it's easier to string them together.

Last edited by Access : 10-06-2012 at 02:18 AM.
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