Forward voltage?

[buymysoul]

I bought a E2l with the error head.
It has a 5W LED.
It should have the same specs (or is) a Luxeon V portable, right?

According to the specs above, it's typical forward voltage is 6.84V with a maximum of 8.31V (and min of 5.43V)
Does this mean that this is the max forward voltage that one of these LEDs might have, or the maximum forward voltage you would want to apply to it?

I'm not real clear on what some of these specs mean, if someone could help me out, I'd really appreciate it.

(I'm not asking about current b/c it is pretty clear to me on that page that 700ma is what it needs)

I'm sorry if this is common knowledge, but there isn't a sticky (that I saw) about LED terminology and I have not yet figured out how to get the search function to actually search for the words I want it to. /ubbthreads/images/graemlins/wink.gif

 

[Chop]

The forward voltage (Vf) is the voltage needed by the luxeon to run the luxeon at a specified current. If you have a luxeon with a Vf of 7v at 700mA, then it will run at 700mA when 7v is applied to it.

 

[buymysoul]

Ah, that's what I figured. Thanks!

So, according to ohm's law, I can figure out the proper resistor to use with this light at 9V by R=(Vsource-Vled)/I

So the minimum value resistor I could use (greatest amount of light) would be R=(9-8.31)/.7=.98 Ohms
and the maximum value I would use R=(9-5.43)/.7=5.1 Ohms

I guess as long as the voltage I'm supplying this LED with is within it's specified range, I don't need to worry about it.

With that in mind, it seems like the best batteries (for the brightest output) would be some protected R123s, even at a full 8.4V, would only call for a resistance of about .13Ohm and I measure the resistance of the E2l switch to be about .5 Ohms anyway. It should be totally safe to run this head on those batteries.

Right? Or am I missing something?

 

[raggie33]

ty thanks for sying tha in a easy way to understand i was al;ways confused about that

 

[buymysoul]

whichever mod moved this from LED forum, thank you, I wasn't quite sure where to put it

 

[buymysoul]

Woops, doublepost

 

[HarryN]

Your calculation is right, but the error can be in the battery voltage sag. It is useful to include in your calculation the fact that when you draw different amounts of current from a cell (or series of them), the cell voltage drops significantly.

For primary Li 123 cells, you can obtain excellent information from the sticky post above (123 shoot out by Silverfox). Using the 1 amp data, you can estimate the actual voltage available, and might find that 3 x 123 (primary cells) with closer to 2 ohms is a good match.

It is also helpful to use a cell with the flatest voltage vs time performance, such as a Ray O Vac, as this will be the steadiest performer in your setup.

I am not as familiar with the current / voltage curves for R123 cells, but the performance is typically substantially lower, countered of course by the fact that they can be re-used.

 

[buymysoul]

Thanks, I have been slowly checking out that thread, but I can only handle it in small doses.

I think even considering voltage sag, I'll probably be safer with the 2 rechargeables. Plus I don't already have a 3x123 body, so this will be a little cheaper in the long run anyway.

Thanks,

 

[Lynx_Arc]

If you want to cheat, do what I do and get a variable resistor and dial the resistance down with an ammeter inline till you get the right current then disconnect the resistor and measure it, you don't have to worry about battery sag and can try it with both nimh and alkaline to see what resistance is needed in all cases.