http://www.outdoorlanternsonsale.com/led-aaa/
Powering LEDs with AA/AAA/9V Batteries?
I've read a couple ways to do this. If I have 2 LEDs, both require 2.0V each @ 20mA, and I've read that a typical AAA battery puts out 1.5V. So if I put 3 AAA batteries and connect them to the LED, why do I need a resistor if it's just over the 4.0V required? I've read on Ohms Law (R=E/I) so my resistor would be (4.5V - 4.0V) / .02 = 25 Ohm resistor? Is this correct? There is a website with a calculator but it shows a different Ohm resistance? This shows two 27Ohm resistors?
http://led.linear1.org/led.wiz
Also, I read that a 9V has a low mAh rating so I should maybe use AA/AAA batteries to last longer???
billruss, Does a normal AA or AAA put out 1.5V? So 1 Volt = 50 Ohms for a resistor? I thought LEDs drew very little power. So I need 3+ batteries just to turn these 2 LEDs on? How long will they last?
First of all, LEDs always require a series resistor to set the current and avoid damage. The only exception is the occasional one that has a resister built in.
You should adjust the voltages so that the drop across the resistor is at least 2 volts.
The reason is that LEDs have a very steep VI curve and very small changes in voltage cause large changes in current. In addition, the voltage specification of an LED is typically not too accurate, so a 2v diode may actually be 1.8 or 2.2 volts
yet another problem is that this voltage changes a lot with temperature, in a direction that increases current flow, which of course makes it hotter yet, leading to damage.
So, bottom line, you can keep these voltage problems under control by the use of a serial resistor.
Yes, a 9v battery does not have a lot of current capability.
You have 2 leds, both rated at 2 volts, so with them in series, you should have 4 volts across the resistor, or 200 ohms, and a 8 volt supply. For a 9 volt supply, that's 5 volt drop across the resistor, and a 250 ohm resistor.
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