Jul 19, 2015, 08:44 PM
Registered User
United States, CA, San Diego
Joined Mar 2012
2,575 Posts
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LED/resistor - final voltage issue

I'm just trying to power a single 2.2v orange (blinking) LED using the calculator here:
http://ledcalculator.net/

Power supply: 5.23v (just what my battery is charged to at the moment, just a reference for now)
LED forward voltage: 2-2.2v
Current rating: 20ma

The online calculator mentioned above says to use about a 220ohm resistor for this application (if I want about a 15ma current draw).

But here's my problem: when I connect it all together, with even a 300ohm resistor inline, I still get a +3v reading at my 2.2v LED. Which means too much voltage is being supplied still, even though I used an even higher resistor than suggested. It doesn't sound like my 2.2v leds would last very long at +3v.

Something is wrong here. Am I looking at this right? Does the answer have something to do with the wattage staying the same even though the voltage reads higher (like as if the voltage were higher and current appropriately lower to equal the same wattage in the end). Otherwise I'm lost at this point.

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Last edited by finguz; Jul 19, 2015 at 10:16 PM.
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 Jul 19, 2015, 10:43 PM Registered User United States, CA, San Diego Joined Mar 2012 2,575 Posts Maybe my meter is broken. It has been acting up lately but I just thought it had a broken solder joint on the connectors.
 Jul 21, 2015, 09:06 PM Dark Yet?? United States, VA, Narrows Joined Dec 2009 1,666 Posts So you have 5.2V on one end of the resistor and 3V on the other end, for a total of 2.2V across 300 ohms? That would only be 7ma, no danger to the LED. The voltage across the LED is a specification at a specific current. I suspect you are not putting enough current thru the LED to drive the forward voltage down that low. The LED is a active device. It's forward voltage changes with current. The resistor tells no lies.

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