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The Arduino Uno manual (from the kit) instructs to connect a resistor to GND when lighting an LED on the breadboard with the 5V (socket?). The red, green, and yellow LEDs don't light up so brightly... actually they are barely bright enough to detect. Is it safe to apply all 5 volts by connecting the cathode to the ground with a wire? I tried it for a second and it worked... Just don't try it on the blue, I assume that beyond 5V it will explode, it burnt so bright..

Optional: Why does only the blue LED light visibly?

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  • So you want to know if you should use a resitor along with the LED?
    – RS2322016
    Aug 31, 2016 at 22:46
  • The brigtness of an LED depends on the voltage acorss it and the resistor value that used in series when it connect to a power supply. You will find varying the voltage and resistor values will cause the LED to have different brightness.
    – RS2322016
    Sep 1, 2016 at 2:42
  • I don't understand why people downvote (and I don't want you to upvote). I'm just asking a beginner question that I cannot find answer to on Google. Should I put "WARNING: BEGINNER QUESTION" at the beginning of my question?
    – Roman
    Sep 2, 2016 at 3:59
  • @Roman I see that someone has flagged this question as "off-topic" I think it's actually a very relevant question (though may fit better on electronics.se, but they'll probably dislike your question anyway). I must say that your question is also not very clear, you seem to ask a lot of different questions at once.
    – aaa
    Sep 27, 2016 at 13:43
  • I'll vote to leave your question open, since it already has a lot of good answers. You may want to try to cleanup the question (make it clearer) for future reference.
    – aaa
    Sep 27, 2016 at 13:46

4 Answers 4

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Different color LEDs are not the same. The are made with different impurities. And some colors (blue) were really hard to figure out. So much so that the blue LED inventor was awarded the Nobel Prize!

Consequently, it should not be assume all LEDs operate at the same voltage and current. As such, when driving an LED with a constant voltage source, derive the proper series resistance for each case.

Example 1:

Given a red LED which may have 1.8 volt drop and may need 20mAmps to operate driven by a 5 volt source, we find the proper series resistor to be...

V = I * R

5 - 1.8 = 20mA * R

R = (5 - 1.8) / 0.020

R = 160 ohms

Example 2:

Given a blue LED which may have a 3.3 volt drop and may need 20mAmps to operate driven by a 5 volt source, we find the proper series resistor to be...

V = I * R

5 - 3.3 = 20mA * R

R = (5 - 3.3) / 0.020

R = 85 ohms

Your specific LEDs may have different specifications. The above are only examples.

Go here for more about calculating the proper LED series resistance.

In general, LEDs that are over driven will grow dimmer and not recover.

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No, you cannot safely power an LED with 5V without a resistor. The resistor is absolutely 100% required.

The resistor isn't put there purely on a whim, it's required to set the current based on the supply voltage minus the LED forward voltage and the resistance of the resistor.

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  • What would happen if the LED recieved 5V for longer than a second?
    – Roman
    Aug 31, 2016 at 23:27
  • It will probably survive, since the internal resistance of the supply will limit the available current. You should be more worried about what is providing the 5V - that won't be too happy.
    – Majenko
    Aug 31, 2016 at 23:28
  • Looks like the 5V pin providing the 5V
    – Roman
    Aug 31, 2016 at 23:39
  • An IO pin? Then you will be stressing that IO pin considerably and sustained usage will result in the death of that IO pin as the high-side drive MOSFET burns out through excess current.
    – Majenko
    Aug 31, 2016 at 23:40
  • @Roman You can get LED's with integrated resistors if you dont want to use your own resistor
    – RS2322016
    Sep 1, 2016 at 0:01
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Ah. I found it. From the manual:

Without the resistor, the LED would be brighter for a few moments, but quickly burn out.

Since 5V is HIGH on this thing, it looks like 5V would be too much for an LED.

Question 2: You were using the wrong resistor (too much resistance). But even with the incorrect resistor, the blue one shined! Strange.

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  • Blue LEDs tend to be very efficient and will even give a little bit of light when the current is in the micro amps range. PS running too much current through LEDs can be fun. They sometimes give a different color light for a while before dying. They won't explode. They can release a bit of smoke though.
    – Gerben
    Sep 1, 2016 at 13:08
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Something else to take into consideration (as if this is not complex enough) is that if you're comparing LEDs visually you're not always going to be able to tell if your math is correct.

The human eye is about 3x more sensitive to blue light (and green light for that matter) than it is to red light.

So, presume that you had 3 absolutely identically manufactured LEDs: 1 Red spectrum, 1 Blue spectrum, and 1 White ("full spectrum"). If they are all hooked up with proper current, the Blue LED will appear to be approximately 3x the brightness of the Red LED (to your human eyeballs) and the White LED will appear to be some large factor brighter than the Red LED as well, and likely some factor brighter than the Blue even as it contains more 'spectrum'.

TL;DR: Check your layouts with a volt meter and math, not with your crummy eyeballs.

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