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The data sheet for this LED gives the forward voltage range, at forward current of 20mA, of 2.8 to 3.8V. How can I choose a sensible current-limiting resistor value for this device, given such a wide range of possible forward voltages?

If I have a 5V supply and the forward voltage is 2.8V, I would need a 110-ohm resistor to give 20mA forward current (the rated value). But then if the forward voltage on a particular one was actually 3.8V, I'd only get about 11mA - and considerably less brightness. And of course operating from a 3.3V supply it's going to pretty random whether the thing switches on at all.

Am I misreading something?

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    \$\begingroup\$ No, not misreading anything. If you can't afford to drive the LED with a 20mA constant current source, then you will have to use the 110Ω resistor and accept the variations in brightness. Either that or, choose a different LED. \$\endgroup\$ – Solomon Slow Nov 22 '17 at 15:08
  • \$\begingroup\$ @jameslarge: And give up any hope of driving it from a 3.3V source, am I right? \$\endgroup\$ – Tom Nov 22 '17 at 15:09
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    \$\begingroup\$ For a 3.3 V supply you should use a LED with a lower forward voltage, most red LEDs fit this description. The LED you linked to is a blue one, these simply need a higher voltage. \$\endgroup\$ – Bimpelrekkie Nov 22 '17 at 15:11
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    \$\begingroup\$ It's a pretty poorly specified LED anyway, the Luminous Intensity at 20mA can be anywhere from 28 to 180 mcd! If you're buying enough, the manufacturer will bin them according to Vf for you for a price. \$\endgroup\$ – Finbarr Nov 22 '17 at 15:19
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If you just get a random LED, then you're right. Setting a "good" value will involve picking a resistor that will not cause too much current to flow in the worst condition (highest voltage rail, lowest resistance, highest/lowest temperature, highest/lowest LED Vf), and then live with the variations.

Note however that they are binned. Each bin has a span of 0.2 volts, making it easier to pick a value.

Depending on your LED source, they might already be binned, and the binning should be indicated on the distributor's listing.

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  • \$\begingroup\$ Ah, so if RS Components lists it as a 3.3V forward voltage, it should be between 3.2 and 3.4V? \$\endgroup\$ – Tom Nov 22 '17 at 15:28
  • \$\begingroup\$ @Tom Ooh, I wouldn't be so sure about that. RS can probably only list one value, not a range. 3.3 is the average of 2.8-3.8, so it might just be that they picked the middle. You could try looking at the exact manufacturer's product code. \$\endgroup\$ – pipe Nov 22 '17 at 15:35
  • \$\begingroup\$ @Tom: Look at the product code. For LEDs, the bin often is the last component of the code. \$\endgroup\$ – Janka Dec 29 '17 at 0:03
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When driving an LED with a wide forward voltage range, you should drive it with a current regulator. For five volts the following is more than adequate.

schematic

simulate this circuit – Schematic created using CircuitLab

For 3.3V it gets trickier since the above circuit needs about a volt of headroom. For that you would need to use something like a PAM2803 driver.

enter image description here

Or if you prefer a capacitive driver TI has a nice part, the LM2750.

enter image description here

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