LED Resistor value-how low can I go?

[Rocktifier]

Rather trivial.....So I cranked the diffused green led in my grapvine as high as I dared...used a 300 or 400 Ohm resistor.  Is my led gonna "burn out" soon or am I good to go? Thanks

[bigmufffuzzwizz]

You should be fine, especially w/ a diffused LED. I believe you can go as low as 100R, that's what the guy from effects connection sent me (paired with the LED's) with an early order. It will burn out faster than if you used a 4.7k but not within any immediate use. I believe LED's lifespan is something like 50-100 years.

[Rocktifier]

Cool, thanks! 

[stecykmi]

according a datasheet (probably not the same one as the manufacturer, but they're probably very similar), a green diffused LED can sink 40mA without burning. At this much current, it has a voltage drop of about 2.6V.

Thus:

( Vbattery - Vf ) / If = R

(9.2 - 2.6 ) / 40mA = 295 ohms

Edit: should be 165ohms, messed up the math.

That's right at the limit of what it's rated for. I used 9.2 V for the battery as a bit of a safety factor. This does mean that if you ever use a 18V supply, that diode is toast for sure, possibly even with a 12V supply.

As a side note, that's the formula for calculating the current limiting resistor for any LED; the power source voltage minus the voltage drop across the diode (listed in the datasheet) divided by the current to be sunk through the diode. Intensity of the light is a function of (ie is directly related to) the amount of current going through the diode, which is usually shown as a graph in datasheets. note that the voltage drop across the diode depends on the current being sunk, so you have to figure out the current before the voltage drop. for reference, If = 10mA is in most cases "bright enough".

[shawnee]

Ohms law at it's finest!
One thing though- I did the calculation and I think it comes out to 165 ohms. I think you added 9.2 and 2.6 instead of subtracted. The correct fomula is to subtract right?

By the way- your explanations are so good. Thanks for taking the time to make the application of these formulas simple to understand.

[stecykmi]

ya you're right it's 165ohms, I punched the wrong button on my calculator. it is in fact subtraction.

[Haberdasher]

Yes, thank God it's 165R.  The other day I was beginning to wonder if I was off mah rocker.