Yet another question about LEDs and resistors...

[baby y0sh]

Okay - so, I have a specific question about my new setup. I am making a simple halloween pumpkin as my first project, which involves a RPi Zero controlling 2 yellow LEDs and 1 orange LED. I set it all up and it works great using the 220 ohm resistors that came with my Pi. Now that I am diving into buying the final LEDs, resistors, and wires for the project, I realized that not all LEDs are the same and require different resistors (did I mention I am a beginner?)! So, the yellow LEDs (2.25 Vf, 16 mA, 3.3 source voltage --> 68 Ohms/1/8 W) seems pretty straight forward for me, however, works fine with the 220 Ohm one that came with the Pi. However, the orange LED has a forward voltage of 3.6 V and lit up spectacularly with the 220 ohm resistor. This is what I do not understand. It shouldn't have lit up at all, right? What am I missing? Here are the specs for the orange LED:

[PhatFil]

https://www.evilmadscientist.com/2012/r ... -for-leds/

there is a lookup table down the page a bit but it should answer your Q's

[pidd]

The forward voltage at 3.6V is when it is running at 20mA and is only a "typical" rating.

At 5ma the Vf could be smaller - full spec sheets have graphs of typical, min and max values at various brightness and current levels.

"Typical" generally has to err on the side of the user, if you supplied 3.6V you would expect it to light although you might not expect it to draw exactly 20ma at 3.6V

A lot of suppliers don't give full specs these day, if they have a batch of orange LEDs that need 3.7V, there is nothing in the specs that hangs them.

[baby y0sh]

The example table? I am not sure how that answers my question. Not that it doesn't - i just still do not understand.

[PhatFil]

The example table? I am not sure how that answers my question. Not that it doesn't - i just still do not understand.

My fault, tbh i scanned the first few lines of your OP and assumed you were looking for resistor values like many other people have asked for before. (rtfp perhaps) and the table i referred to is just a lookup for that.. thought i could help by sharing a bookmark..

[baby y0sh]

I see - so there is a voltage range dependent on current. Just one question: For the above LED, what resistor should I use... or rather, what Vf should I use to calculate the appropriate resistor strength?

[markkuk]

If the LED is bright enough with the 220 Ohm resistor, then just use that one. The purpose of the resistor is to prevent too large currents, so using one that's larger than the theoretical optimum isn't a problem.

[Burngate]

Ok, found the source of your screenshot
https://www.ledsupply.com/leds/5mm-led- ... wing-angle
And I have a screenshot you may find interesting - it's from a Kingbright green LED datasheet, but the basics should apply to most LEDs.

L-934GD.png (17.57 KiB) Viewed 90 times

As you can see, although it's specified as having Vf of 2.2v, it starts to conduct well below that.

With the limited information you have about the orange LED (Vf = 3.6v at 10mA?), you can't do any real calculations. As you say, if it really needed 3.6v it wouldn't light, so you're now into the realm of genuine engineering - otherwise known as suck-it-and-see.

And it doesn't need the maximum current to be quite bright - your eyes are somewhat logarithmic, so doubling the amount of light doesn't make things look twice as bright. That's without taking into consideration your eyes' spectral response!

I'd try various resistor values, and select one that makes it just bright enough, but not too bright.