Diodes are tricky. They reduce the voltage flowing through them to a lower value, but they don’t really do anything relative to Current. They will pass as many Amps as the power supply can deliver up to the point that the Current burns them out. So you always need a current limiting resistor or the diode will quickly fail.
Gotta come back later on your other questions… Got some other stuff to do.
It’s present, you just can see it. Like the LED in a LED illuminated switch, or that addressable circle of LEDs I posted to above. The current limiting resistor built-in so you don’t need to install one, just hook it up to power.
This is often the case with LEDs specifically intended for lighting, they come with the resistor built-in so it’s less expensive and easier for a manufacturer to package the LEDs in their application.
If you have a 3V LED and you feed it 5V power, you need to go look at what byMH62 said. The calculation is more complicated because the resistor both limits current and “absorbs” the voltage that passes through the LED in excess of the LED’s forward voltage specification.
You don’t want to measure the voltage across the LED - that’s just telling you what the forward voltage drop is. You want to measure between the Anode (input) pin of the LED and electrical ground. You’re probably right that it’s 5V. Any of the exposed gold pads like around screw holes will probably be tied to Ground. Small round exposed pads are test points, so no way to say what they’re connected to.
Not sure why the LED board has 4 leads. Power and Ground should be all that is necessary unless these are also addressable LEDs and they have a “data” and “clock” signal, too. But they don’t look like they’re that type of LED. Is there anything on the back of the board?
Noob attack here 
Ground…that’s new.
So a pcb has a ground. Is that sort of a life line, always with the same amount of voltage?
If you look at picture two (that is the back of the PCB, i see three LEDs, three resistors dedicated to every LED. I can even with the magnifier see that they come in front of the LED (or after the trace of the LED, depending how the current flows. I don’t know what te anode and cathode sides are of SMD LEDs). There are two small pads for thermistors but they are not connected to anything. Are the big round gold holes the ground traces?
Picture three shows the front of the PCB. There are two test pads, TP5 and TP6. When I put my multimeter in diode test mode and touch the testpads, the three LEDS light up a bit slightly. Not bright though. So I guess that’s correct. The left connector goes to the part turbine, the right socket (the open one) connects to the print head body. I’m far far FAR away from knowing what I’m doing with just touching the terminals with my multimeter when connected and seeing what happens. I’m much to afraid the print head will go BOOM. Emeril would have a field day when that happens I guess 
By the way, is it an idea that I remove the paint layer on the front and back of the pcb with acetone, so I can see how the traces run? Or does the paint protects something underneath? The copper traces of oxidizing for instance?
Edit: no of course it’s a bad Idea Timp! The LEDs are made of plastic. It will probably melt them. 
LMGTFY 
If the printer is designed according to generally accepted practice, all the power supplies that provide juice to all the components of the printer share a common “Ground” (negative). So for any circuit carrying power, if you want to measure the voltage, you should be able to put the negative lead of the meter on any exposed ground pad, with the pads around screw holes being the most likely target if you’re not sure about any other pads on the boards.
Given your lack of experience, it might be a good idea to shelve this mod for now. Pick something to learn a bit about electronics on, that won’t cost you a lot of money if you blow it up. Or buy one of those “learn electronics” kits that come with guided experiments to teach you the basics.
Thank you! I googled that also
Hi Randy, thank you for your advice. I’ll do that and proceed with my project also. Doing and learning. Good combo! For the LED I’ll go for the de-soldering option and rewiring new LEDs downwards. The least chance of blowing stuff up. Ill show the finished end result!
Thanks again guys for all the help and explanation!
Ludo,
I had a stray thought while reading the thread: Might it be easier to use some fiber optic strands and a little hot glue on the LEDs to route the light from the LED bar down to the print head instead of messing with soldering et al?
pete
That is a great idea! Thank you for sharing it!
Hi Ludo, did you manage to finish this project somehow?
I really like the idea!
Hi Making Waves.
No, I did not. Yet. I had to put my time in another project that had more priority. But it’s the next project I’ll start.
I’m not going to use the re-wiring and LED re-placement. I had some advice from my friends of BL and as much as they wanted to help me, they advice me not to re-wire or alter anything on the electronics because of short circuit danger or other issues. Since I’m a bit to novice in electronics to exactly know what I’m doing, I’m going for the second best option, which will turn out great to.
I ordered some high end (plastic) fiber which I’ll mount and redirect from the logo LEDs downwards to the hot end area. This way I don’t have to rewire anything or resolder or even replace. Il’l let you know when I start, maybe two minds have a better outcome then one 
That sounds like a great idea. I happen to have a whole bunch of fiber that’s meant to transport light as well. I might give it a try some time but I don’t know how to cut the fibers correctly.
Just throwing the idea out there, but what about using the lights already there? You could remount the LED PCB if you don’t care about the light up logo.
Another option is getting some cheap, clear plastic strands that act like fiber optic wire? They sell the stuff for things like lawn lights and embedding twinkling stars in ceilings, and it’s really cheap. I’m not sure how heat resistant the stuff is, but then you could run that from the LEDs in their current location to the bottom.
In your first picture, with the red wire runs and LED PCB flipped over, cut the surface it’s resting on (the top of the LED encasement), mount the LEDs pointing down towards the fan, leave a small gap if you want the logo to still light up, and then run the fiber as you would the wires in your original plan. It’s a very short distance and SMD LEDs are directional enough that it might actually work.
I have some thin fiber stuff around here somewhere, but haven’t seen it in a few years. I might be able to mess with it a little, but there will still be a lot of unknowns (old, no specs, probably too thin even when bundled, etc.).
I just checked and the stuff I got was 0.75mm diameter, Amazon item B01N0QCAHF. They make stuff at least twice as thick for under $15 per 5m. Make sure to get the “end glow” variety, not the edge glow/neon tube like.
Or maybe use some clear PETG, either a few raw filament strands or a glass-like printed object (super slow, hot, large nozzle, big layer height for best results).
I guess an alternative would be to wrap some aluminum foil around a strip of acrylic that’s heat formed to the right shape. You’d probably have to polish the edges.
Just spit ballin’. I’ll see if I can scrounge that optical stuff up.
Hi xbkingx, welcome to the wonderful world of Bambu Lab, our forum and 3D printing!
Thanks for your thoughts! As I mentioned before, I’m going to proceed with this project in a few weeks. I’m busy now with another project that demands all my attention. When I start with it, I’ll give notice here.
Your thoughts are more then welcome and I already have the optic fiber in my position.
See you soon!
With warm regards, Ludo Timp
Did you happen to see this design where they put the LED down in the cover?
https://www.printables.com/model/467316-bambu-lab-x1x1c-fan-shroud-stock-aftermarket-nozzl
That’s a great way to start modding!
So. Finally time to pick up this project. I’ve decided to go for the fiber optic way. I’ll keep everyone posted of my proces.
The first thing I’m going tot do is build the print head housing LED cover part in 3D and work from there.