'Over-volting' an LED.

[Thom2022]

Hey guys. So I've been doing some research and I'm a touch curious as to whether I've interpreted the info correctly.
It's regarding direct driving an XHP50.2
I will break it down into points as I understand them and then if anything seems wrong please point out which and why.

1. An LED is an open circuit until it's forwards voltage is reached at which point it becomes a near perfect conductor with minimal resistance.

2. The current through an LED is largely dictated by the resistance in the circuit limiting current.

3. As voltage and current are intimately linked then as voltage through a fixed resistance increases so does current.

4. When Vs is higher than the Vf of an LED then there is an increased current, this then causes the LED to heat up as it tries to dissipate the extra power. This extra heat can cause damage to the LED causing it to fail.

5. An LED is not damaged by the voltage directly, it is damaged by the extra current (a byproduct of extra voltage) in the form of heat.

6. If there is sufficient thermal management (heat sinking), an LED can have a higher voltage supplied to it. This will create a higher current and increased lumen output, however the Lm/W increase at the higher current will start to diminish exponentially.

If this is true then am I correct in Thinking an xhp50.2 (Vf 5.6-6.2v)can be directly driven by 8.4v (most likely 7.8-8v due to voltage drop) provided it is prevent from reaching damaging temperature levels?

Also why do xhp50.2's struggle, when an xml2 which a Vf of 2.8v can withstand direct drive from 4.2V (both Vf and Vs are exactly half that of XHP50.2)

Thanks in advance

Thom.

 

[staticx57]

XHP50.2 in my experience works fine with 8.4v. I built a C8 with a zener fet driver over a month and a half ago and it still works fine. Runs 2X18350.

 

[Thom2022]

Awesome, what sort of current are you getting at the tail cap? I've got an extension tube so will be running 2 18650's.

 

[Thom2022]

So just thought I would report my finding now my build is done.
2x18650 VTC5's
8.4v static 7.88v under load.
8.2A at the tail cap (very short thick wires to multimeter)
The C8 host handles the heat relatively well. On turbo it gets hot fast but it does also cool very quickly when the output is lowered.
In a smooth reflector this combo works brilliantly. There's a slight bit of donut hole but not very noticeable. Beam is nicely focused with good hotspot intensity and ALOT of spill but this is perfect for me.
I have the 3000k 80+ CRI variant and it's just superb! I'm toying with the idea of picking up a second XHP50.2 the same as this one to see how it fair after having ~1/3 of the dome sliced off as there seems to be no definitive answer as to what it does (or if there is I've not seen it).
In summation this LED WILL handle direct drive from an 8.4v source with mediocre at best heat sinking. Put it in something like a convoy L2 and you will have a beast!

 

[staticx57]

Hey in answer to all your questions here and in PM. When I first built it the best comparison I had was against a Convoy L6 to which I thought it was remarkably similar. I never did think it had a heat problem and it didn't get that hot, then again two 18350 is a far cry from two 18650. Nor did I ever leave it on turbo for that long. I also never measured the tailcap current.

I bought it when 5700k was the only available on MTN, a bit cool for my tastes. 3000k 80cri is next on my list though.

A good indication of excessive heat is your hand. If it gets too hot to hold turn it off, the LED can easily survive more, but if it is never hot to the touch you're in the clear. Assuming there is a clear heat path that you can feel of course. A much larger L2 for example should probably be fine indefinitely.

 

[Capolini]

XHP50.2 in my experience works fine with 8.4v. I built a C8 with a zener fet driver over a month and a half ago and it still works fine. Runs 2X18350.

Fet driver,,,Boring!!!

 

[staticx57]

I agree, it was cheap and it works, but it is very unglamorous. OEMs such as olight and acebeam and making some pretty great drivers that are much more feature rich.

Or did you want to go there. I don't care for that certain someone on BLF and I still don't care for that certain someone here.

 

[Thom2022]

See I'm just stuck to modding convoys at the moment as I'm testing LED's to see what fits my preference. I'm quite happy with the mtn17dd FET I'm using at the moment. UI is a touch clunky with the gruppydrv Rev.1 firmware but it does the job.