How Much Should I Round Up On My Resistor?

**tiggerrr** · 09-02-2016, 01:48 AM

I'm starting to plan out a Saber build and doing what research I can but I've got a question related to rounding up on resistors.

First, I'll give the math to see if it looks correct to everyone. This build will have a 3.7v 18650 cell powering a Cree Tri XP E2 Red/Red/White. I will be using a NBv3, so the LED's must be wired in parallel.

The red LED's are 2.59v @1000mA, and the white is 3.15v @1000mA. My math works out at 1.1 ohms/1.1W for the red and 0.55 ohms/0.55W for the white.

For the white LED it's an easy choice to round up to a 1ohm/2W resistor. For the red, the first round up choice is a 1.2ohm resistor. However, since a fully charged Li-ion battery can actually provide 4v, should I be rounding up to the 1.5ohm resistor to be safe?

As a bonus question, one of the tutorials on this site recommends that when wiring in parallel, each LED should have its own resistor. But the wiring diagram in the NBv3 manual recommends a resistor for the white, and a single resistor for the two reds, splitting the wiring after the resistor to wire them in parallel. Does it matter much? Space is likely at a premium here.

**Silver Serpent** · 09-02-2016, 04:59 AM

It is not necessary to calculate your resistor based on the 4v of a fully charged li-ion. The 3.7v calculation is fine. Those batteries drop down to 3.7v pretty fast, and your heatsink will keep the LED safe. The 1.2ohm is a good choice for your reds.

I typically recommend that beginners use a resistor for each LED. If you have two identical LEDs (like your two reds), then they can share a single resistor. Space is always at a premium, but you should be able to fit a resistor for each LED in your hilt.

**tiggerrr** · 09-02-2016, 03:58 PM

I'll go with a 1.2 resistor and do one for each red then. I'm generally a fan of doing it right (or at least safely) as I learn. Thanks for the feedback.