Resistor question

**TrypWyr** · 05-03-2014, 02:19 PM

Hey guys, never tried running multiple LEDs off a single resistor, want to get some feedback before I do. I'm running the Tri-Rebel RRW with the two reds in parallel and the white FOC, on a NB2.0. Here's my calculations:

(3.7 - 2.3) / 0.7 = R = 2 ohm
(3.7 - 2.3) * 0.7 = P = 0.98 watts

So a 2 ohm/1 watt resistor for each LED. But I don't have one, so I'm trying to be a little creative. What if I wire up a single resistor for both parallel LEDs? Should be the same voltage, but twice the current:

(3.7 - 2.3) / 1.4 = R = 1 ohm
(3.7 - 2.3) * 1.4 = P = 1.96 watt

So a single 1 ohm/2 watt resistor should work, yes? I have plenty of those. I understand the risk of a single LED failing and then the other getting popped by the current, I'm okay with that. If one die were to fail, I would probably replace the LED or upgrade to the newest tech anyways. I just want to make sure my calculations are sound, and that I won't risk my NB2.0 in any way.

Thanks!
Tryp

**Forgetful Jedi Knight** · 05-03-2014, 02:42 PM

The best feedback is: Don't do it. Or: Take the time and get the right resistors to do it right the first time.

Based on the numbers you provided, your math seems ok.

Will it work? Maybe. For how long? Unknown and I don't know if or how soon you'll damage the LEDs.
I don't think you'd damage the NB. The RRW, however, probably won't last too long.

But like the old saying goes, "There's never enough time to do it right, but always enough time (and money) to do it over.

Good Luck!

**TrypWyr** · 05-03-2014, 02:55 PM

Yeah, that seems to be the general consensus online from what I can find on various websites... Listen to FJK, I will.

Off to find some 2ohm/1W resistors. Thanks!

**dgdve** · 05-03-2014, 03:23 PM

I'm no pro with the Nano v2 but.. I know there are some settings called...

Maybe "drive" and "fdrive" <- something like that

If you look in the Nano manual it explains that if your using an led with a really close vF as your battery then you can use those setting specifically to forgo the need for external resistance)the example in the book is GREEN..

And brings the Nano overall output from 3.7v to the 3.4 necessary to keep the green safe (the actual SAFE numbers are in the manual so go read what I'm saying to be sure for yourself)

Anyway the RRW can handle 2.4.. So if you adjust the "drive" to 3.4v output (completely safe and recommended).. Guess what resistors you need then...

CORRECTED= 1.5ohm 2w (to run the Reds @ 700)

____

Remember I'm not pro but as I understand it that should be perfectly acceptable and within the safe zone on the Nano (as far as drive settings are concerned)

I hope that helps.

**TrypWyr** · 05-03-2014, 05:02 PM

Huh... now that's an intriguing idea. I wonder what Erv would say to this solution?

**Forgetful Jedi Knight** · 05-03-2014, 05:04 PM

Originally Posted by TrypWyr

Huh... now that's an intriguing idea. I wonder what Erv would say to this solution?

*FJK gets the "Padawan Accident Report" ready to be filled out, and will deal with the Sith-let later*

**TrypWyr** · 05-03-2014, 05:12 PM

Actually, according to my calculations (and by "my", I mean the online calculator), it would need a 1.5ohm resistor at 2.4v...

Anyways, it is an intriguing thought, but ultimately I think I will just order some proper resistors (or hope my local den of geek has what I need somewhere in the multitudes of little dusty boxes). This build has been in progress for over a year now, it can wait a little longer.

Can't wait to show it off though, it's a matched pair with a lot of custom acrylic work by Tim. Seriously, that dude is magic.

**Forgetful Jedi Knight** · 05-03-2014, 05:24 PM

Originally Posted by TrypWyr

Actually, according to my calculations (and by "my", I mean the online calculator), it would need a 1.5ohm resistor at 2.4v...

Anyways, it is an intriguing thought, but ultimately I think I will just order some proper resistors (or hope my local den of geek has what I need somewhere in the multitudes of little dusty boxes). This build has been in progress for over a year now, it can wait a little longer.

The drive feature of a NB is really good for only Greens and high voltage Blues (where you are talking under .5 Ohms) , when you use the drive function in lieu of resistors, the board does build up a bit of heat (because it acts like a resistor), and can trigger unintended swings and such, as Yoshi found out when he came to me with that issue last week. Ideally you want to use external resistors wherever possible.

Remember the NB doesn't have a voltage chopper OR a current regulator.