I have a 10 W 308-385 nm UV LED which is rated for 3.4 V to 3.8 V at 2800 mA. I want to power it using a Li-po/Li-ion battery so that would be 4.2 V at the peak when the battery is full.
Do I need to add protection here, and if so what kind of protection, just a simple cement resistor of a very low value or some electric circuit?
When the voltage is in the range of 3.4 V to 3.8 V (or lower,) do I still need to add something to limit the current then or will at that point the LED automatically do that?
Most people talk about using LEDs in cases where the supply voltage is a great deal above what the LED can handle, but in this case it isn't far above that.
I have seen commercial high power LED floodlights also directly connect to Li-pos without current limiting (8.4 V 2S Li-po, so it might be that they use 12 V power LEDs.)
I have looked into the LM338 but that one seems to only regulate voltage.
I have also thought about using two transistors and two resistors or such to make a current limiter.
About just directly using a 1.5 ohm cement/high power resistor.
I also considered the possibility that it is so close in range that it either might not be needed or that something else much simpler or better might be possible or that I might choose the wrong parts for it.
EDIT_ I did some testing and compared it to the data in the datasheets. turns out the datasheets do in some cases show a relative correlation between voltage and current, during my tests this also showed to be pretty much right.
For many LEDs, there is actually a pretty big range of voltage offset where it will limit the current by itself thus not needing extra hardware (as long as you are okay with the LED brightnes changing with voltage.)
When using it with a powersuply with a roughly stable voltage this will make it pretty much not needed to add extra components if you look good at the datasheet or test the voltage-current relation yourself for example using a component like a Xl4015 version with voltage and current meter (for rapid simple testing.)
In my case, the speciffic 10W UV LED had a range of around 1V which is pretty much for a sub 4V LED. Actually, the 2800mA was somewhere around 4V, but it worked just drawing less current and being less bright down to around 2.6V.
This means that yes, you can power an LED directly without a current limiter if you make sure it is in the right rated range or slightly below.
In my case, however, since I had set up a screenless XL4015 board already to limit to just below 2800ma I used that anyway so I could hook up a much broader voltage range.
As my original reason for asking this question was to see if it was safe or unsafe to use an LED without a current limiter, as in if a LED will actually also limit current based on voltage, then YES LEDs do have an analog property of limiting the current based on input voltage.
Just make sure to either test it or measure the current at certain voltages or look in the datasheet.
This also explains why many commercial LED lights actually do not use current limiters.