Why is my red LED dimmer without a resistor than with a 100 Ohm resistor?

Why is my red LED dimmer without a resistor than with a 100 Ohm resistor?

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Complete newb here; sorry this may be a basic question but trying to learn. I made a simple circuit with nothing except an Arduino power source a 100 Ohm resistor in series with a 10mm red LED. Weirdly it gets dimmer if I remove the resistor and my question is why is that occurring? I thought reducing resistors would brighten the LED. The datasheet is attached. The max current is 80mA and the LED worked fine after and this phenomena is repeatable so I don’t think I burned the LED out. This phenomena doesn’t happen if the power source is 3.3V in which case I don’t notice much brightness difference between the two. This phenomena also doesn’t occur with a same sized blue LED by the same manufacturer. FYI, there’s no code, I am running the power direct from the Arduino Uno’s 5V and 3.3V pin and gnd pin. I read the Sparkfun tutorial on resistors and am confused.



simulate this circuit – Schematic created using CircuitLabledShareEditFollowFlagedited 6 hours agoasked 7 hours agorfii10111 bronze badge New contributor

  • You’ve probably already damaged the LED. Not broken it completely. Just damaged it. If you have a second one laying around, try hooking both up separately with the same circuit and turn out the lights and compare them. (Use perhaps 68 Ohms each instead of 100?) If I’m right about the more permanent nature of the damage, the fresh LED will shine a little bit brighter. Worth an experiment. – jonk 6 hours ago
  • (1) I suspect your LED is sort of super bright with a built in resistor, so that you can directly apply 5V and get say, 70mA without any problem. If you supply 3V3 the current might be smaller, but might not be much smaller. (2) the built in resistor might be much bigger than 100R, so with or without the series resistor does not make much difference to human eyes. – tlfong01 4 hours ago   
  • References: (1) Red LED 5mm with Resistor (2V ~ 5V, 2000 ~ 3000 MCD, 620 ~ 625NM) Product Sheet – SparkFun $9/25pcs sparkfun.com/products/14560, (2) YSL-R531R3c-5V Red LED 5mm with Resistor Datasheet – YunSun cdn.sparkfun.com/assets/c/9/b/9/c/YSL-R531R3C-5V.pdf, (3) YSL-R531R3c-5V Red LED 5mm with Resistor Spec – YunSun imgur.com/gallery/GkKqwEo. – tlfong01 4 hours ago   

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1 Answer


No resistor = way too much current

Way too much current = LED is running way too hot

When an LED gets too hot, it’s ability to emit light goes down.

Touch it, bet it’s really warm if not hot.

Another thing, when the temp of an LED changes, it’s output wavelength changes. It gets longer. So your red LED may have shifted to infrared


The LEDs are overloading the power supply and its responding by folding back it’s output voltage

Either way,, blue behaves differently because it has a higher voltage requirement than red. This is probably limiting the over current condition.

They behave similar at 3V instead of 5 because there is less difference compared to Vf, so they aren’t getting as much over current.


Got an ammeter??? Measure the current you’re putting through them.ShareEditFollowFlagedited 6 hours agoanswered 7 hours agoKyle B2,71344 silver badges1818 bronze badges

  • 1I don’t think the wavelength shift is that pronounced that it’d get to infrared before the LED melts. – Hearth 6 hours ago

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