Hello guys i am really confused about the whole resistor issue. I have visited so many sites that tells you what resistor you need but i don't know the volt drop that asks to complete or the current.
I want to power 3 RED LEDs with a 9 Volt battery
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Red LEDs usually have a 1.7V drop. If the LEDs are wired in series, they will drop about 5.1V. 9V - 5.1V leaves about 3.9V across the current limit resistor.
I'm going to further assume that you want to have the LEDs run at 20 mA max. So: 3.9V / 0.02 Amps = 195 Ohms. The closest standard (E12) resistors are 180 or 220 Ohms. I'd choose 220 Ohms.
Now let's see what happens as the battery dies. A standard Alkaline battery is considered to be dead when its' terminal voltage drops to about 1V under load. A 9V battery contains 6 cells. 6 * 1V = 6V. (6V - 5.1V) / 220 Ohms ~= 4 mA. The LEDs will be lit but dim.
The red LEDs I've used have had a forward voltage of about 1.8 volts (but there may be some newer technologies with higher voltages). Typical 5 mm LEDs usually have a recommended maximum current of 20 - 30 mA, but do produce ample light at lower currents.
So, three LEDs will drop 3 x 1.8 volts = 5.4 volts, which leaves 3.6 volts across the series resistor. I usually aim for 10 mA current, so the resistor is R = E/I = 3.6/.010 = 360 ohms. The resistor value is not critical - a higher value will reduce the current, and make the LED dimmer.
Lots of "ballpark" figures and guessing here. Red LED's are typically ABOUT 1.8V [as others have mentioned] but the actual voltage varies for each device. The current you are pumping through it will also effect the voltage slightly. A quick Google search for "red LED datasheet" got me devices with voltages ranging from 1.65V to 2.25V so 1.8 is about average.
USUALLY, the limiting resistor will average out these variations. If you put them in series, however, you COULD get one LED running at 1.9V and another at 1.6V... which impacts the voltage across the resistor... and your overall brightness is lower. It might not be a big change, but it CAN happen. If this isn't an issue, put the LEDs in series: you use less current, and batteries last longer.
For greater control [at the expense of using more current] you can have the LEDs in parallel, with one limiting resistor on each LED. 9v - 1.8V = 7.2V, which at 10mA is 720 ohms [or 360 ohms for 20mA]. 390, 470, and 560 ohm resistors are common values which should work. If you find one LED is brighter/dimmer, you can change its resistor: especially useful if you ever decide to mix LEDs with different colors.