# [SOLVED] LED Resistance Calculation



## Seeno (Aug 21, 2011)

:4-dontkno A few years ago, I was using a downloaded (vs. online) LED resistance calculator but I lost it somewhere along the way. It seemed to work pretty well but I can’t find it to reinstall. The only input it required was the LED’s voltage and current. It assumed the voltage drop to be 2.0 (I think) for all cases. I’ve found numerous online calculators but they all want you to input the drop as well. Obviously the voltage drop is important so why do you suppose the original calculator used an estimated value? Why is actual value not listed on the LED packaging? For the purpose of calculating resistance what is the customary way of determining voltage drop? Surely one isn’t expected to put the LED in a circuit and measure it with a meter. Or is one? I’m probably overlooking something obvious. :grin: Seeno


----------



## gcavan (Aug 13, 2009)

*Re: LED Resistance Calculation*



> For the purpose of calculating resistance what is the customary way of determining voltage drop?


If I need an exact number I go the data sheet of the individual diode. 

2.0 volts is a good estimate to use in basic applications. Red, yellow and green LED's typically have a forward voltage (Fv) of 2.0 to 2.2 volts. Infrared LED's are typically lower (1.5 - 1.7) and the Fv of blue and white LED's are usually in the order of 3.6 volts.

PS: One does not typically use the 'resistance' of the LED in calculating the necessary current limiting resistor. Instead, subtact the total forward voltage of all the series LED's from the supply voltage and divide the result by the maximum forward current of the LED's.


----------



## octaneman (May 13, 2009)

*Re: LED Resistance Calculation*

Hi Seeno


In the electronics forum on top there is a thread marked technicians tool box, there is a zip file called LIRCv2 you can download, its an LED calculator which may assist you.


----------



## Seeno (Aug 21, 2011)

*Re: LED Resistance Calculation*

Thanks, gcavan and octaneman! Good info in spite of my confusingly complicated question. Where I said "_for the purpose of calculating resistance_" I should have said "for the purpose of _determining the value of the current limiting resistor_." So, for a critical application use the data sheet. For projects using the cheepo chain store assortment with no i.d. or data the rule of thumb is probably adequate. I'll check out the calculator in the "toolbox" thread. I wonder if there's enough interest in LED's to justify its own sub-forum.


----------

