Partlist Wednesday: LEDs

Every Wednesday we highlight a component from the updated partlist. This week: LEDs.

Surface mount LEDs feel a bit smaller than capacitors and resistors in the same package. Despite moving to 0603 C and Rs, we still use 0805 LEDs. It’s not just because we’re stuck with a reel of them, we find 0603 LEDs to be more difficult to work with.

As a rule of thumb on a quick prototype we use a 2K resistor with SMD LEDs at 3.3volts, and 4.7K or 10K at 5.0volts. Both are common values we have in bulk. The exact current at those values depends on the exact LED, but usually that’s less then 1mA.

Also on the partlist are two through-hole infrared LEDs. One is fairly narrow (25degrees) one is a little wider (50degrees). Both have a operating current of 100mA, and a pulse up to 1A.

Lots going on behind the scenes today, so the partlist post had to be short. What are your thoughts on LEDs?

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1 Comment

  1. Personally, I find it frustratingly annoying to achieve the desired brightness from an LED, especially with a random LED from a bin and no part number. Even with the data sheet in hand, there is usually only a rough idea of the maximum DC current and AC (PWM) current. A few LED brands have beam angle and lumens, but too few have this level of detail.

    I have found that 100 Ω is the most common resistance I end up calculating for an LED circuit. Not surprisingly, when I take apart LED-based products to repair, I generally find 100 Ω resistors there, too.

    The problem with 100 Ω is that you end up with lots of current, and the LED can be too bright to look at. I’ve had some modern LED products put out blinding levels of light even with only 2 mA or 1 mA! For that reason, I think your starting points of 2000 Ω, 4700 Ω or 10000 Ω are a really good idea. It’s always possible to put a minimum resistor like 100 Ω in series with a variable resistor, say 10 kΩ. Then you can dial in the brightness, switch off the circuit, and measure the variable resistor to see what you should be using for the desired output.

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