Driving LEDs directly from the output pins without using a resistor

[Netphemera]

I've been building projects with the Arduino Due for a while now. I've been building many different multi-LED projects using the digital output pins 24 - 51. The LEDs that I've been using are the 5mm bright white LEDs with a Forward Voltage of about 3.4V.

I was concerned at first, but I've been successfully driving the LEDs without using any current-limiting resistor. I've been building a lot of flashing devices, so I'm generally not trying to light more than one or two LEDs at a time. I've found that many LEDs have a FV of about 3.2 - 3.4 volts. Is there any reason why I shouldn't drive these LEDs directly from the digital outputs without using a current-limiting resistor if they have a 3.2 or 3.4 FV?

I'm planning on building some 5x5x5 or 6x6x6 LED cubes and driving them with the Due. Construction will be greatly simplified using the Due since I will not need any multiplexing circuitry for these sized cubes. Would there be any problems to build these LED cubes without using any current-limiting resistors?

[Grumpy_Mike]

Yes there is a reason. Without a resistor you have no control over the current to an LED. Bad for the LED and bad for the driver circuit. Damage is caused althought it might not be immediately apparent. The forward voltage drop is not stable with age or temprature.

Why people keep on trying to remove a one cent component is beyond me. It is one of the more stupid things you can do in electronics.

[retrolefty]

Yes there is a reason. Without a resistor you have no control over the current to an LED. Bad for the LED and bad for the driver circuit. Damage is caused althought it might not be immediately apparent. The forward voltage drop is not stable with age or temprature.

Why people keep on trying to remove a one cent component is beyond me. It is one of the more stupid things you can do in electronics.

Face it GM, this is a battle that will never be over or won, only continually fought by valiant knights for the protection of output pins, leds, and children, everywhere evil exists. 

Lefty

[Grumpy_Mike]

Face it GM, this is a battle that will never be over or won,

Yes but the teacher in me keeps on trying, just like the labors of Sisyphus, surely the patron Saint of educators ( except he predates saints by some way )

As a punishment for his trickery, King Sisyphus was made to roll a huge boulder up a steep hill. Before he could reach the top, however, the massive stone would always roll back down, forcing him to begin again.

That is the new intake of students  :~
From
http://en.wikipedia.org/wiki/Sisyphus

[Netphemera]

GM, can you or anyone else here offer me a bit more assistance. I'm still confused with the theory and application of using these LEDs with the Due.

What would be the best practices when attaching these high-brightness LEDs:
    3.4V forward drop (3.0 ~ 3.6v)
    Max continuous current 20mA
    Resister needed (at 5V): 82 ohm

Should I really be adding tiny valued series resistors? It would certainly be within the fudge-factor to add a small, 10 ohm resistor. I have a background in electrical theory, but I simply don't see the point of adding resistors with such small values.

Or, perhaps, is the answer to simply not use these LEDs with the Due? (As I mentioned earlier they seem to work fine.) Should I simply stop using the LEDs with FV near the output voltage of digital pins? I have plenty of LEDs with FV of 2.2. Should I use those instead?

[Grumpy_Mike]

Using an LED with a forward volts drop close to the maximum output voltage is a tricky design problem. Traditionally the solution has been to use an LED driver that includes a voltage boost converter. However as 5V is avaliable on the board a simpler soloution would be to just drive a transistor so you have enough voltage to use a sensible value of resistor.
The current output of the Due is a lot less than the 8 bit Arduinos, depending on what pin you use. Some white LEDs I have used have very good brightness at only 3mA or so. That will allow you to use a resistor of arround 330R or so directly.
This gives you control over the current. Other methods are simple one chip 20mA constant current drivers. A lot of manufacturers have just this year introduced their own range of these.
So driving white LEDs from 3V3 electronics is not as straight forward as it is with 5V electronics.

[Netphemera]

Thanks for clarifying things for me. It make much more sense now. These are all great solutions. I'm still committed to driving my LED cube with the Due, so I'll pick one that works best for me. I'll look into those LED driver chips. They sound perfect.

[Osgeld]

the second a led lights it produces heat, that drops its fv and is consumes more current. this continues until it reaches a point where its not really going to get hotter, but you already passed into the danger zone.

low power leds may take a long time, but you are still slow cooking them. a resistor, even a smaa one at least keeps thinngs within limits