Forward voltage of different emitters.

Flashlight Modding and DIY Parts
1

Hey guys, I have kind of a wierd question. If I use a emitter with a rather high forward voltage, would this be enough to protect from over discharging the battery?
For excample a cree XP-L (I know its ancient) starts conducting current at around 2.6V. This should be fine as a discharge voltage for a normal Li-Ion cell. I guess there are emitters with higher forward voltage which would be even better suited for this. Has anyone tried this before? I didnt find anything online.

Thanks for your help

1 Thank
2

I quests you have no lab supply to check it out?

3

No i dont. Info have a variable step down converter and a Multimeter though, so I might try that.

What would be some emitters to try this with?

4

I know the Convoy S2+ with a UV emitter doesn’t have LVP because the vF of the UV emitter is >3v.

But I’ve wondered if white high vF emitters could get away without LVP.

5

I just tried a luxeon V at 2.5V (just had it lying on my desk). It draws around 2mA. So it will go a little under 2.5V but not really far. It is a F bin for voltage which should be the lowest. So using a higher voltage bin would make this even more possible in my opinion. Now I only have to find a good emitter for this.

6

I recall that old XP-G2s used to have crazy high forward voltage, mine wouldn’t draw more than 4 amps in direct drive with a 30Q. First-gen XPLs should also be ok. Modern emitters are evolving to have lower and lower forward voltage.

7

Good to know, I’ll take a look at the datasheet

8

I just looked at the datasheet and both the xp-g2 and g3 are rated for 2.7V @350mA. So this sounds great. They are produce a fairly high amount of light there (up to 200 lm depending on the bin).

9

The Samsung lh351d has a pretty high Vf, you might have more success with that

10

That’s a very good suggestion! It’s also a mid-to-high CRI emitter. Think the SST20s also have decently high Vf if I recall correctly.

11

Figure an LED has about a 3V drop. Take a 9V battery. 9V-3V=6V. Figure 50mA just to test, so R=E/I=6/0.05=120Ω. Measure the voltage across the LED. Done.

That measures the current for your LED, and not off a generic curve from a datasheet.

Plus, you set the current, so your “cutoff” can vary between “let me know the voltage is dropping pitifully low while I’m using it” to “oh crap I forgot to turn off the light and left it on for days” and see what the “cutoff” voltage would be for that level.

12

I will take a look.

13

good idea, I ordered some first gen xp-l, according to @QReciprocity42 suggestion. I will try your method.

14

As a result of being curious about direct driving LEDs using 2xAA NiMH in series I lit up XP-G2, XP-L Hi, XM-L2 (old version), XHP50.2(3V), SST-20, SST-40, E17A, CSLPM1, and LH351D LEDs using my bench PSU and to my surprise they are all the same at low voltage, they all start lighting up under 2.2V (barely visible in the dark) and increase in brightness in a similar way up to the 2.7V i went up to where they were still only drawing mA.

Either there’s something weird about my PSU or the differences in vF only kick in too far up the voltage curve to prevent the over-disharge you’re looking to avoid.

Just checked a 7C XP-G2 and it draws 8mA and 2.6V.

They’re all significantly dimmer at 2.7V then at 2.8V, and significantly dimmer again at 2.6V though, so if you’re running them with any power and without a boost circuit you would get a very visible low voltage indication by way of the LED dimming.

2 Thanks
15

Thanks, I will take another look when the other emitters arrive. As I only have a single emitter (the above mentioned luxeon V) i cant really test a lot