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schematic

simulate this circuit – Schematic created using CircuitLab

I am trying to test my LT 3083 voltage regulator using a chip resistor to model the load. My input power was 5V and I was trying to create 3.3V using the LT3083 part. I added a green LED with a 1K ohm current limiting 0603 chip resistor. Then I put a 3.3 ohm chip resistor in parallel with the LED and the current limiting resistor without realizing the 3.3 ohm chip resistor was only rated for 0.25W. I was trying to put 1 ampere and 3.3V on the 3.3 ohm resistor which is 3.3W. Obviously, the resistor was failing but it seems the LED was also started to heat as well.

Initially, the LED started green and it started to become yellow before I removed power. I was still getting 3.3V on the net even though the resistor was failing. Once I replaced the 3.3 ohm, 0.25W chip resistor with 3,3 ohm, 50W resistor everything worked. My question is why was the LED burning out even with a 1k chip resistor when the parallel resistor was failing? It seems the LED path had a lot more current than planned when the parallel resistor was failing.

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    \$\begingroup\$ Could you draw a schematic showing where you accidentally put the second resistor? Additionally, was the failing resistor physically close to the LED? \$\endgroup\$
    – Jonathan S.
    Commented Apr 5, 2023 at 15:31
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    \$\begingroup\$ Based on description something was wrong if a green LED damaged with 1k in series to a 3.3V supply. Maybe something was connected incorrectly. Please draw a schematic what you had. \$\endgroup\$
    – Justme
    Commented Apr 5, 2023 at 15:33
  • \$\begingroup\$ Green LEDs do turn yellow-orange when overheated due to some kind of quantum mumbo-jumbo. \$\endgroup\$
    – Criticizing Israel not allowed
    Commented Apr 5, 2023 at 15:38
  • \$\begingroup\$ I don't see how you could break the LED in this circuit even if the resistor failed. Is it possible that the overheated resistor transferred its heat through the circuit board to the LED? How close together were they? \$\endgroup\$
    – Criticizing Israel not allowed
    Commented Apr 5, 2023 at 15:40
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    \$\begingroup\$ Show a picture of the PCB you tried it on! I bet the LED was very close to R3. As R3 was failing, it heated up, and got the LED too hot, and caused that to fail as well. \$\endgroup\$
    – Kuba hasn't forgotten Monica
    Commented Apr 5, 2023 at 16:19

2 Answers 2

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Green LEDs do turn yellow-orange when overheated, due to some kind of quantum mumbo-jumbo.

It seems likely that the LED was not broken, but the 3.3 watts of heat from the 3.3 ohm resistor heated up the whole circuit board around it, including the LED, so it got hot and turned yellow.

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Resistor was pretty close to the LED and the heat from the 0.25W resistor was making the LED hot. When I moved the Resistor to the other side of the perf board I could see the heat was really coming from the resistor. Therefore, there's nothing wrong with the LED current.

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  • \$\begingroup\$ nmr - Hi, You posted this as an answer, but isn't it just confirming what was given in another answer? Therefore the appropriate thing to do would be to upvote and optionally áccept that answer, since you seem to be confirming that it is correct. If you want to add more narrative, it should be a comment on that answer. Thanks. \$\endgroup\$
    – SamGibson ♦
    Commented Apr 5, 2023 at 17:57
  • \$\begingroup\$ I did upvote but It's not letting me choose as the answer \$\endgroup\$
    – nmr
    Commented Apr 5, 2023 at 18:09
  • \$\begingroup\$ nmr - Hi, You said: "It's not letting me choose as the answer" I would need more details about exactly what you mean, before I could assist. As long as you are not trying to choose your own answer (as I explained, that would not be appropriate, since you are just confirming an existing answer) then there should not be a problem to áccept another answer. The process is explained in this FAQ article and here from the Help. \$\endgroup\$
    – SamGibson ♦
    Commented Apr 5, 2023 at 18:16

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