Further resistor frustrations.

[SirRawThunderMan]

Yeah, I was surprised by that one as well. But (3.95-2.95)/1= 1. ergo, 1ohm.

No, actually it makes sense. we initially went by specs (4.2V-2,6V).. Using actual readings could lead to mod builds..

Though I would test that naked res under 1V to see if it gets warm at that crazy rate.

And I would use P=UxI instead of P=V2/R. It pulls R out of the equation.
You tested R and it gives 1 Ohm, no prob. But is it stable when getting hot? Not sure.
U and I, you may read them "life" (ok, if you have 2 testers..) 

I mean, okay. P=U*I. so, P=(Vs-Vf)/I=(3.95-2.95)*1= 1W

my math seems to be holding up.

Somehow.

...It never did in school.

[Nemezis]

...It never did in school.

  I know the feeling ..

[K-2SO]

FJK’s resistor tutorial... an oldie but a goodie.

Tutorial - FJK's "Down and Dirty" guide to Ohm's Law

[SirRawThunderMan]

Back to step one. 

I found a spare 1ohm, 3W carbon film resistor in m box of resistors. Soldered it into the red die, powered it up for 30 seconds and it overheated enough to crack a chip off the ceramic coating when it cooled down.

Well, that's it. I'm done. nothing makes sense. All the numbers agree with me, but this sodding saber hasn't had the memo.

[K-2SO]

The 2.95V sounds very high for a Cree Red. Usually Reds need at least 1.2 Ohm (usually 1.5 Ohm) or more. I would recommend you use a 2 Ohm, 3W resistor.

[Nemezis]

or just try another cree 
Your maths are good, the readings strange.
It's not the sound card, it's not the alim, you tried multiple resistors..
One thing left is the HPL

[SirRawThunderMan]

or just try another cree 
Your maths are good, the readings strange.
It's not the sound card, it's not the alim, you tried multiple resistors..
One thing left is the HPL

Yeah, I'm starting to suspect it as well. I haven't got a spare on hand to play with.

I've gotta say, I'm not usually a fan of pixel blades. They're cool, but they're just not for me. This saber, though? Really making me think twice about that.

[SirRawThunderMan]

Definitely something up with the tri-cree. all the Vfs at 1A are different from what they were last time I measured them. R is now 2.78, G 3.6, rB 3.56.

New Tri-cree time, I think.

[SirRawThunderMan]

New Tri-cree time, I think.

Or not, I've just taken a look at my bank balance and I've gone so far over budget on this saber. Okay then, plan D. I have an old Pixel saber that I built to test the technology out on but never was really satisfied with, I think I'm going to rip the battery out of that and convert this one into a pixel saber. Not my first choice, but I have all the parts on hand for it already.

And I've got an old PRIZM I can put in the old pixel saber later, so I guess that works out?

[Nemezis]

So.. you’ll be fustrated.. and so will we be cause we’ll never know what was actually wrong 

[SirRawThunderMan]

So.. you’ll be fustrated.. and so will we be cause we’ll never know what was actually wrong 

I think the most reasonable explanation is that there's something wrong with the LED, and call it at that-the change in Vfs is pointing me in that direction (it changed again when I measured it earlier this afternoon). I'm going to hang onto it and have a play around, try and see if I can figure out either what's wrong with it, or why it's acting like this, maybe get some temperature readings on the resistors when my new multimeter arrives.

At this point, I've been done with this build for a good few weeks. I just want it to be over and move onto something else.

[SirRawThunderMan]

So, just for the heck of it, I thought I'd show this thread to a friend of mine who teaches engineering at the university level. Not electrical engineering, but he has enough grounding in it to deal with my level of questions.

In short, my math is correct, but only on an ideal level. I'm not taking into account things like the resistance of the wires, the kind of power that the proffie is supplying, thermal breakdown, and a couple of other factors. Ideally, I should be troubleshooting this with an oscilloscope, but as I don't have one or access to one (right now, at least) any answers I get to aren't fully going to resolve the issue, and any math I come up with is going to be very simplified. Although he does also suspect the LED might be dodgy, so, that's something.

He also ran up the datasheet for the crees, and thinks that there isn't much point running them much past 0.7A, because the brightness gained for the power consumed isn't going to change enough to be perceptible. Also, he doesn't get why everyone here suggests carbon film resistors as the only thing to use, and would like it explained better than "Well, 'Erv said to use them a decade and a bit ago, and it's kinda stuck."  So, if anyone's got one, I'm all ears.