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I have a diode where forward voltage is 3.5V and forward current is 25 mA. I supply it with 5V so I calculate a value of resistor in the following way:

R = (5V - 3.5V)/25mA = 60 Ohm

However I see that most times bigger resistors are used, like 220 Ohm or 330 Ohm. Why is it better to use bigger resistors then to stick with the calculated values?

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    \$\begingroup\$ It is common practice to run a LED at less than its rated maximum current. \$\endgroup\$
    – user173271
    May 28, 2020 at 14:30
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    \$\begingroup\$ @James Also common practice to answer questions in the answer section, not in the common section. \$\endgroup\$
    – pipe
    May 28, 2020 at 14:31
  • \$\begingroup\$ When you look at the numbers on the data sheet for your part, some of them will be under the heading, "Absolute maximum ratings." The "25mA" for your LED probably is one of those. It does not mean that your circuit is required to supply that amount of current, and it does not mean that it is normal under some set conditions for the LED to draw that amount of current. What it means is, if you allow more than that much current to flow, then you have voided the warranty. The manufacturer will no longer stand by any of their promises. \$\endgroup\$
    – Solomon Slow
    May 28, 2020 at 16:52
  • \$\begingroup\$ for the same reason they sell different wattage light bulbs; control brightness and consumption. \$\endgroup\$
    – dandavis
    May 28, 2020 at 20:30
  • \$\begingroup\$ The 330 Ω may date back when only red 20 mA max. LEDs with about 1.65 V forward voltage were affordable, and logic supply was 5V. \$\endgroup\$
    – greybeard
    Feb 23 at 16:56

2 Answers 2

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Because your LED has an unusually high forward voltage. It is a blue or white LED, while a "normal" LED is red or green. A red or green LED has a forward voltage of roughly 2 volts and a standard forward current of about 10 mA.

Another reason is that you simply don't want to run the LED at the maximum brightness. Maybe it's annoyingly bright, and you will get a longer life out of it if you run it at a lower current.

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  • \$\begingroup\$ So is the forward current is basically the maximum current we can pass through a diode? \$\endgroup\$
    – michalt38
    May 28, 2020 at 14:45
  • \$\begingroup\$ I have encountered a bit of equipment with a blue LED that must have been driven at its maximum current. And it flashed whenever the equipment transferred data. It was so annoying, I ended up sticking a bit of black tape over it. \$\endgroup\$
    – Simon B
    May 28, 2020 at 15:38
  • \$\begingroup\$ @SimonB Same here. Glad you reminded me, have to replace the LED in my computer case.. \$\endgroup\$
    – pipe
    May 28, 2020 at 18:51
  • \$\begingroup\$ @michalt38 The forward current is typically the maximum current that's "comfortable" for the LED, and the current used for testing at the factory. It's what you should take as a maximum unless there is a more detailed datasheet. \$\endgroup\$
    – pipe
    May 28, 2020 at 18:54
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Several reasons.

  1. In most cases you don't actually need the maximum power of the LED.
  2. The LED may be able to take 25mA, but LEDs are often driven directly off logic outputs which can't supply that kind of current.
  3. Your LED has a high forward voltage, probablly because it's a blue or white LED. Typical red/green/yellow LEDs tend to be more like 2V.
  4. The closer to the line you run things, the more careful you have to be. Particularly if you are designing kits or tutorials for those new to electronics, it makes a lot of sense to err on the side of safety.

With a typical LED 330 ohms will give you around 9mA on a 5V supply which is fine for most LEDs, on a 3.3V supply that drops to around 4mA, a bit on the low side but probablly still sufficient for most modern LEDs. With a blue/whote LED on 5V you will also get around 4mA which again is probablly fine.

With a 3.3V supply and a blue/white LED, the supply voltage is about the same as the nominal forward voltage of the LED, so it's difficult to predict the current. There will be current, because the forward voltage of the LED does reduce at lower currents and there likely will be visible illumination, but at this point the resistor value is not the main thing setting the current, the characteristics of individual LEDs are.

In summary 330 ohms is a decent "general purpose" LED resistor value if you just want to light a LED from a digital output and don't want to think too hard.

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