Resistor and accent LED questions...because I'm stupid and can't remember.

[Kagetora]

Hey gang.

Just about got my purchase parts list complete, but, to save a bit on shipping (so I don't have to order twice), I'd like to get it filled up. Need some resistors.

If I am going to use an NBv2, with a 3.7v Li-Ion 18650 battery, what resistor should I be getting for a Cree Royal Blue LED? My calculations are: need 1000mA (1 amp), so I should get a (R=V/I is R=3.7/1 or 3.7) 3.7-ohm 5-watt resistor? Or, from the store, a 3.9-ohm 5W (slightly underdriving the LED)?

Also, is there any reason I can't drive TWO accent LED's off the pad on the NBv2? Seems like I could just parallel them off the pad with different resistors (one for the 20mA AV switch, and the other with a different resistor depending on what the LED needed)? Also, the 16mm AV switch (20mA) needs a 185-ohm resistor? Closest in the store is either a 150-ohm 1/4W or a 220-ohm 1/4W.

Why does this make me feel stupid? Must be the rum talking...

[Greenie]

Hello Kagetora, the led on your a/v switch needs a 20mA dynaohm resistor (in store)
Yes you can run more than one accent off the NB, I've just done it. One on the switch and one for a crystal chamber
As for the main led check the specs for the forward voltage of the RB Cree, subtract that from you battery voltage and that will be the value of V in ohms law. I've not checked or calculated it but I suspect your resistor will be closer to 1ohm
Hope this helps

[Silver Serpent]

Your calculation for the main LED is incorrect. R = (Vsource - Vled) / I.

Read through this post for a more detailed version: http://forums.thecustomsabershop.com...e-to-Ohm-s-Law

[Kagetora]

Yeah...yeah. That's right. I remember now. (Vs-Vl)/I. Sorry, it was getting late, and I was groggy. Thanks, both of you. For the Cree it would be (3.7-3.41)/1 or .29, with a wattage of 1^2*.29 or, again, .29W. Nothing like that in the store, although there is a .47-ohm 1/2W resistor. That would only give about .62A for the LED (.29/.47), though. What resistors are other people using with the 3.7v battery packs and Cree LED's? And where are you getting them?

[Silver Serpent]

The .47 ohm is what is generally recommended. You'll get plenty of brightness, don't worry.

[Kagetora]

Ok. Again, thanks. I also found you can get literally any resistor you can imagine at Mouser. They have, for example, a .3 ohm 1W metal film 2% tolerance resistor for something like $1.80. Could always split the difference and get some .4 ohm ones, but if the .47 ohm get the LED bright enough, I'll just get them from Tim. Less current should equal longer battery and LED life anyway.

[Miraluka]

I usually try to avoid mouser. I know other people have had good experiences with them, but the only purchase I made from them was a battery holder. It took somewhere in the 5 month range, I think, for it to ship to me. It may have been longer than that. I would have canceled the order long before that if it wasn't so cheap. It was only a few dollars so I wasn't obsessing over the money. Just frustrated by the wait.

[jt_trouble]

I use this, makes it so much easier http://led.linear1.org/1led.wiz

Keep in mind the DynaOhm actually have a IN and OUT on them .. go figure.

[Bark]

Yes, DynaOhm - for small 20mA lights only, ex. AV switch lights and accents:

1. Positive side of circuit only.
2. IN/+ connected to power source
3. OUT/-/<blank> connected to LED+ terminal

[JunieBass]

The led on your a/v switch needs a 20mA dynaohm resistor.
Also you can run more than one accent off the NB, I've just done it. One on the switch and one for a crystal chamber.
As for the main led check the specs for the forward voltage of the RB Cree, subtract that from you battery voltage and that will be the value of V in ohms law.
Your resistor will be closer to 1ohm