NB and resistors questions

[Kagetora]

After pawing through the NB owner's manual, and noticing the part about external resistors, I started hunting around these boards, and came across this, from Forgetful Jedi Knight:

"They normally do, but since Greens can handle a fairly high Vf (almost 3.7V) it isn't that necessary, it can be controlled with the drive parameter. For LEDs with a lower Vf, I personally recommend resistors. Everyone's mileage may vary."

For the internal guts of the hilt, I was planning on using:

Nano Biscotte (3.3V-5.5V, 2A - no current regulator)
Green Rebel Star LED w/MHS heatsink module (3.4V @ 1000mA)
Li-Ion 18650 3.7V 2600mAh PCB protected rechargeable battery
16mm AV momentary green ring switch (3.3v @ 20mA)
Premium 28mm speaker
MHS V6 speaker mount (holds speaker, NB, and battery all in one nice tube)
Switchcraft 2.1mm recharging power jack

According to the NB manual, it seems I should be putting a 3.9 ohm 5W resistor in the main LED circuit (which should then be running at 3.7V and roughly 950 mA), and a 185 ohm 1/4W resistor (or as close as I can get to it, probably just go a smidge higher, like a 220 ohm) in the circuit to the AV switch to run it at 3.7V/20 mA (17mA with a 220 ohm resistor). I don't think I need a resistor in the speaker circuit, if I read the manual correctly.

However, after reading the quote above in another thread, I am wondering if I need the resistors at all with the NB board, or if I can control it with "drive parameters," although I admit I don't actually know what that means. I am guessing it means the outputs from the NB can be altered or programmed in some fashion?

Or should I order the resistors I mentioned above to put in the circuits? I may have completely misunderstood what FJK was talking about...it was in reference to a Tri-Rebel Star, so it may not apply to the LED I am using.

Thanks muchly.

[Forgetful Jedi Knight]

If you are going to be using the setup you posted, I normally recommend external resistors. You would need at the bare minimum .25 Ohm, 1/4W or you can use .5 Ohm, .5W for the Main LED. You can use the drive parameter, but I have come to find that it can cause a little heat on the board, which can trigger sounds unexpectedly.

For the AV switch, if you attach it to the 3.3V pad, you can use an 18 Ohm, 1/4 watt resistor and you will be fine.

[Kagetora]

If you are going to be using the setup you posted, I normally recommend external resistors. You would need at the bare minimum .25 Ohm, 1/4W or you can use .5 Ohm, .5W for the Main LED. You can use the drive parameter, but I have come to find that it can cause a little heat on the board, which can trigger sounds unexpectedly.

For the AV switch, if you attach it to the 3.3V pad, you can use an 18 Ohm, 1/4 watt resistor and you will be fine.

Hmmm...for the AV switch wouldn't that be running it at 180mA (3.3V/18ohms)? The store page says 20mA, so I mathed it out to 185 ohms off of 3.7 volts, or 165 ohms off of 3.3 volts.

As for the main LED, I'm apparently pretty confused... pushing 3.7V out of the battery, trying to get 1A (1000mA) for the green LED, I come up with a 3.7 ohm resistor, at 3.7 Watts (the closest in the store was the 3.9 ohm 5 W resistor). Wouldn't a .5 ohm resistor make it push 7.4 amps (3.7/0.5)? Or is the board doing something I am unaware of (and missed or isn't spelled out in the manual)?

So...confusing....used to be...good at this... arghhhh...

Thanks.

[Forgetful Jedi Knight]

Hmmm...for the AV switch wouldn't that be running it at 180mA (3.3V/18ohms)? The store page says 20mA, so I mathed it out to 185 ohms off of 3.7 volts, or 165 ohms off of 3.3 volts.

As for the main LED, I'm apparently pretty confused... pushing 3.7V out of the battery, trying to get 1A (1000mA) for the green LED, I come up with a 3.7 ohm resistor, at 3.7 Watts (the closest in the store was the 3.9 ohm 5 W resistor). Wouldn't a .5 ohm resistor make it push 7.4 amps (3.7/0.5)? Or is the board doing something I am unaware of (and missed or isn't spelled out in the manual)?

So...confusing....used to be...good at this... arghhhh...

Thanks.

The 3.3V pad puts out 3.3V @ 20mA. You probably could get away without a resistor entirely, but it is always safe to put a small resistor on it - think of it as insurance.

As for the main LED, you really need to brush up on Ohm's Law. I have no idea how you came up with your answers (You appear to have the formulas wrong), and the board only puts out 2A. ;)

[Silver Serpent]

R = (VSupply - VfLED) / LedCurrent

It looks like you're calculating the resistor with just the VfLED. Subtract the Vf of your LED from the source voltage of your power supply, then divide by current.

[Kagetora]

The 3.3V pad puts out 3.3V @ 20mA. You probably could get away without a resistor entirely, but it is always safe to put a small resistor on it - think of it as insurance.

As for the main LED, you really need to brush up on Ohm's Law. I have no idea how you came up with your answers (I'm going to guess you have the formula's wrong, or using wrong figures in your calculations), and the board only puts out 2A. ;)

LOL. Yeah, I could be off. I was just using the most basic electronics I could think of.

Ohms law: I = V/R

So, if I wanted 1000mA (1A), and had a 3.7V source (the Li-Ion battery), I would need a 3.7 ohm resistor. 3.7/3.7 = 1.

Since the NB manual said 2A unregulated, I figured if there was no resistor, it would just try and output 2A in the green LED at 3.7V, twice what it can handle. I know it wouldn't make it to 7.4A (since the board can only put out 2). :D But I figured I better cut down the current to the LED.

What I think you are telling me is that I need a resistor to cut the 2A unregulated output down to 1A for the LED. So, if the NB is pushing out 3.7V and 2 A (indicating it has a built-in resistance of 1.85 ohms), thats 7.4 watts. I need to cut that down to 3.7V, 1A, and 3.7W. Which would mean doubling the resistance to 3.7 ohms, or adding another 1.85 ohm resistor in series.

Essentially, I need 3.7 ohms of resistance from somewhere to cut the 3.7V from the battery down to 1 A. I know this is what I would need if I simply wired up the battery, a switch, a resistor, and the LED into a circuit. I must be COMPLETELY misunderstanding what the NB board is doing or pushing out.

[Kagetora]

R = (VSupply - VfLED) / LedCurrent

It looks like you're calculating the resistor with just the VfLED. Subtract the Vf of your LED from the source voltage of your power supply, then divide by current.

Ok...so what you are saying is take the 3.7v battery, subtract the 3.4Vf of the green LED, and I am left with .3V. If I put in a .33 ohm resistor, I would come up with 1A?

[Silver Serpent]

Reasonably close. 0.3V / 1A = 0.3 ohm resistor. You can use a 1 ohm, and it'll work fine. The Rebel Greens can get by just fine without a resistor with this setup though.

[Forgetful Jedi Knight]

Ok...so what you are saying is take the 3.7v battery, subtract the 3.4Vf of the green LED, and I am left with .3V. If I put in a .33 ohm resistor, I would come up with 1A?

You divide the .3V by the current you want to have in the end - 1A like SS pointed out. ;)

[Kagetora]

Reasonably close. 0.3V / 1A = 0.3 ohm resistor. You can use a 1 ohm, and it'll work fine. The Rebel Greens can get by just fine without a resistor with this setup though.

Right. .33 ohms would have give .9A. I'm tired, AND stupid it would seem.

Ok, so for my setup, I would ideally toss in a .3 ohm, .5 watt resistor, but since those are probably hard to find, I could use the .5/.5 that FJK suggested, the 1 ohm you mentioned, or nothing at all.

The speaker is fine coming off the speaker pad, and the AV switch should have some small resistor also, but could probably get away with nothing.

Thanks. Again, I appreciate the help. :D

[Forgetful Jedi Knight]

They don't likely make .3 Ohm .5W resistors, which is why I recommended the .5Ohm, .5W, which I do know is made. You can also use a 1 Ohm, .5W resistor.

[Kagetora]

They don't likely make .3 Ohm .5W resistors, which is why I recommended the .5Ohm, .5W, which I do know is made. You can also use a 1 Ohm, .5W resistor.

Right...I'd have to put resistors in parallel to get to .3 ohms, most likely.

Would running it with the .5 ohm resistor dim it significantly in your experience? My faulty math give me 1A with a .3 ohm resistor (.3/.3), but only 600mA with a .5 ohm resistor (.3/.5).

Also, on a COMPLETELY unrelated note, I was wondering if either of you had a suggestion for which charger to grab from the store for the Li-Ion battery. I was just going to grab the 3.7V Li-Ion Smart Charger with 2.1mm plug for $10, since its going to be dedicated to the saber anyway (didn't figure I needed all the modularity of the other chargers).

[Forgetful Jedi Knight]

Right...I'd have to put resistors in parallel to get to .3 ohms, most likely.

Would running it with the .5 ohm resistor dim it significantly in your experience? My faulty math give me 1A with a .3 ohm resistor (.3/.3), but only 600mA with a .5 ohm resistor (.3/.5).

Also, on a COMPLETELY unrelated note, I was wondering if either of you had a suggestion for which charger to grab from the store for the Li-Ion battery. I was just going to grab the 3.7V Li-Ion Smart Charger with 2.1mm plug for $10, since its going to be dedicated to the saber anyway (didn't figure I needed all the modularity of the other chargers).

No, It won't dim the LED much if at all, and it'll get 1A.

The 3.7V smart charger should be fine for you.

[Kagetora]

Sweet. Thanks again, both of you, for all the help. :D

[JDuffPhoto]

Can I jump on this discussion? I'm having some trouble figuring out which resistor to go with on my build. I had a PC1.6 in my saber until last night. Didn't like the way it fit into the ribbed section of my hilt, so I am going with the NB board instead. I have a Lux Rebel Amber, drawing 3.2V at 700mA. I'm using a 18500 3.7V power source. Doing the math, the best I can come up with is this

(3.7-3.2) / .7 = 0.714 ohm

Calculating the wattage:
(3.7-3.2) x .7 = .35Watt

I'm looking over the general wiring diagram on the NB manual and it shows a 3-5 Watt resistor in line with the LED. I'm going basic with this build - I'm not even lighting up the LED ring on my switch, so the main LED is it. Can someone look over my math and tell me where I'm going wrong here? Any recommendation on a resistor?

Thanks
Jon

[Forgetful Jedi Knight]

Your math looks pretty fine. You would need a .75 Ohm 1/2W, but since those are hard (but not impossible) to find, I would just go with a 1 Ohm, 1W resistor.

[JDuffPhoto]

Thanks FJK - I found some resistors online and have parts in transit. I'll post an update on my other thread when I have some more pictures and this time I'll include the chassis and wiring.