Quick resistor check

[JeromeV]

It's driving me nuts, I'm in exactly the same situation : nano biscotte, 18650 3.7V battery cell and P4 blue led.
Of course the resistance calculator gives me the same result : 1ohm, 2W.
I don't understand this result : the blue p4 forward voltage is 3.25v, thus resistor voltage is 0.45V, right ?
If I wanna drive my led at 1000mA, R = voltage/current gives me 0.450ohm.
A 1ohm resistor will give 450mA and 200mW.
No doubt I'm wrong, could some point me what I'm missing ?

[Forgetful Jedi Knight]

If you "Do the math" yourself, you will generally get "lower" values. The one thing people tend to miss is that when you "resistor" something, you get HEAT. The "weaker" the resistor, the more HEAT you get. Could you maybe get away with a .83 Ohm 1/2 W resistor? Probably. You would have to run the setup outside of a hilt for quite a while to see how hot the resistor gets, and if you can live with that. Also, those calculators exist for those who hate math, or don't get Ohm's law. If you REALLY want to know how you got the answer they gave you, you would have to ask the calculator's maker.

Personally, I tend to "do the math" myself, but I also have to live with my calculations in my builds. With that said, when we answer questions from "newbies" in subjects like this, we typically rely on our collective experiences, and answer accordingly. If something gets recommended by an experienced member of the community, it's because they have done it (probably over and over), it works and so they share it. This is a hobby and we tend to "play it safe". We don't want people blinding themselves, or hilts blowing up in hands, or tons of parts getting melted, or otherwise borked... we're funny that way.

Hope that helps.

[JeromeV]

No problem about just doing what the calculator or experienced members recommends. It's just for my own understanding for not being a noob forever .

You said :

If you do the math manually it's 1 Ohm-1 Watt, so what you have will work (playing it safe).

I just wanted to know how you do the math to get that result .

Thanks again.

Edit : BTW I read the Nanobiscotte manual and I will probably use the NB "drive" paramer and go without resistor as FenderBender adviced.

[Forgetful Jedi Knight]

No problem about just doing what the calculator or experienced members recommends. It's just for my own understanding for not being a noob forever .

You said :


I just wanted to know how you do the math to get that result .

Thanks again.

Edit : BTW I read the Nanobiscotte manual and I will probably use the NB "drive" paramer and go without resistor as FenderBender adviced.

OK, quickie review of OHM'S LAW:

Required resistor for the FoC circuit :
R = (Vpack – Vf die) / FoC current
R = (3.7 – 3.3) / 1(A) = .4 / 1 = .4 Ohm

Wattage of the resistor :
P = RxI2
P = .4 * 1^2 = .4 W

So based on the above, technically something like a .62 Ohm, .5 Watt resistor (since resistors only come in certain sizes, you usually will have to round up) could have worked, I then rounded up because I don't want to have to deal with the paperwork involved if a padawan gets himself/herself maimed while working on their first saber. Plus some resistors are more common (read: easier to find and cheaper) than others. Safety first!

[JeromeV]

So my math were not so bad .
I'm just missing xp to adjust to more realistic values.

Many thanks

[Forgetful Jedi Knight]

Your math was correct, just the units of measurement were off.