# Thread: Maths/Resistor Query/check when using Nano Biscotte V3

1. ## Maths/Resistor Query/check when using Nano Biscotte V3

Using a:
Nano Biscotte v3
Seoul P4 blue LED 3.5W
18650 Li-po cell 3.7v 3000mah
AV switch (led requirements= 3.3v 20mA)

Using that cell and the biscotte my resister needed for the led is
R=(Vsupply-Vled)/ Led currant
R=(3.7-3.5)/1A=0.2ohm
Looking on bicottes manual they recommend using the Drive adjustments for anything less than 0.5ohm

So calculating the drive value:
Drive= 1023x(3.5/3.7)= 967.7 <----should i round up or down for this?

Lastly the Power indicator resistor (for the Lit AV switch)
R= (Vsupply-Vled) / LedCurrent
So inputting my maths

(3.3v-3.3v)/.02=0?

So no resistor required for both the main Led and the Switch Led?
I cant seem to find anyone else with this kind of setup
Thanks for the help in advance

Edit: Even if i used a fully charged Li-po at a rounded value of 4.2v And took the Max forward voltage of the Seoul P4 (4v) I still have the same issue of a required resistor of 0.2ohm

2. Math is valid.

Many would recommend using a 0.1ohm resistor to protect your LED in any case, but you can eschew the resistor. A resistor is cheap and relatively small compared to the work and cost of repairing your saber if the LED burns out.

3. Thanks jbkuma
If i get a 0.1ohm resistor should i still do the Drive value on the Nano?