Is a resistor even necessary if you're straight up using only drive parameters?
Is a resistor even necessary if you're straight up using only drive parameters?
You use one or the other.
The "drive" parameter tweaks the voltage to the led, so you'll generally use that when the forward voltage of the LED is close to the battery voltage.
If your calculated resistor value is less than 0.5 ohms, then adjust the drive level with the formula "drive = 1023 * (Vled / Vcell)" and omit the resistor. If the forward voltage of the LED is much smaller than the battery voltage, then use a physical resistor using the formula “Resistor = (Vcell – Vled) / LedCurrent”.
So, if I’m using a 3.7V battery powering a LED with a Vforward of 2.3V and forward current of 0.7amps, my required resistor is 2 ohms. I leave “drive” at 1023 and install the resistor. If instead I want to use a LED with a Vforward of 3.4V, I forgo the resistor and set "drive" to 940.
I get that, In your example it looks like you're using a Rebel Red (3.7-2.3) / 0.7 = 2ohm (Resistor For Rebel Red @ 700mA)Originally Posted by Xevious
I was wondering if it were possible to forgo the 2 ohm resistor completely and just set the Drive Parameter to 636. (1023 * (2.3/3.7) = 636) Using the same Rebel Red as an example.)
I would not do that because in that example the resistor is dissipating 1 Watt. Power dissipated = Voltage drop x Led Current. So instead of dissipating that Watt across a resistor, you'd be asking the precious NB to do it instead. (In fact, I'd derate that resistor by 50%: use a 2 Watt resistor and never, ever have to worry about it.)
The "drive" setting trick works because the voltage drop is small. To return to the above example of a 3.4V LED, it would probably be on the blue side, so let's say it burns 1A. That's only 0.3 Watts, which appears to be in the neighborhood of whatever Erv has designed it for. The NB manual says to use the "drive" setting if you're using 0.5 ohms or less -- assuming a 1A led current, that means he'd rated the thing at 0.5 Watts max.
I've got it together and it works! I went with the resistor. Learned a lot here about sabers from you guys and I learned a whole lot about soldering and wiring. Thanks a bunch! As it is right now, I am using with a dual battery holder using a one 14500 Lion cell and one dummy cell. I looked that the NB installation and it asked for one cell, so...since I have a dual cell holder, will it be more than the board can handle if I fill it with two 14500 Lion cells?
Last edited by whitmers; 11-02-2012 at 07:22 AM.
Too high a forward voltage youll make it crispy! Why not wire the 2 batteries in parallel to keep the 3.7 volts and double your run time? it's not to hard to do with a double battery holder ( I have an NB as well and a good soldering station (makes a ton difference) and am currently planning on installing it in a katana build with 2 14500 in parallel
I think that is what I initially wanted thought would happen, then the NB asked for one. What you mention is exactly what I want to do. What should I do to make this dream reality?
I know people say not to dothis with a battery holder( I normally just make up a 2 cell pack by soldering a lead to the 2 hot poles, and a lead to the 2 ground poles, from there you have 1 lead soldered to each end giving you 3.7 volts with lotsa runtime some 1" heatshrink and voila! a pack(I put 4 18650's in parallel in my protosaber, with math that adds up to 4x 2400MA - 9600 ma which as mentioned before is just your runtime, think of a car battery they come in many diiferent CCA or cold cranking amp ratings....same idea) for a 2 place battery holder it takes a bit of cutting and modding,that last one I reworked, I just very carefully removed the spring from the top, soldered a black lead wire to it, and cut off the black lead from the holder, and then a small rivet and washer to secure it to the existing hole/lead on the bottom of the holder, now the batteries can only be installed with the + up, now that both springs are on the bottom, this now makes it a parallel battery holder in that the 2 grounds are now connected as well as the 2 positives with a dedicated wire for each.(hope that made sense)
Last edited by DarthAlice; 11-05-2012 at 03:21 AM.
That looks and sounds pretty neat. I might give that a try. I'll try it on the next beast I attempt to make, probably for my son's seventh birthday.Thanks!
I'm also wondering because I will be using a P4 green and a single 18650 or 14500 whichever is better...
"Your move!" -Obi-wan kenobi-
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