How to calculate resistor value.

[aburkhardt]

Ok, so maybe I'm making this more complicated than it needs to be, but I want to build some blades and do it right.
I'm a little confused about how I should be calculating the resistor value(s). I have played around with the online LED resistor calculators, I have read many of the threads on this forum but I still have questions. I have asked individuals and they have been very helpful, so thanks to all! But here goes

1. Since I am using Lipo do I use 4.2 V or 3.7 V? Maybe somewhere in between? (I think I know the answer to this.)

2. Many bulbs (0805 smd) are rated at 30 mA but tested at 20 mA, what current should I do my calculations at?

3. Many bulbs have a range of forward voltages, do I use the high value or the low value?

4. It seems that green tends to be significantly brighter than other colors. Should I use a larger resistor to dim it down?

5. It seems most calculators recommend 1/8 - 1/4 Watt resistors. Does it really matter?

I understand the math, it's just the application I am unsure about.
I have noticed post where people say use this resistor for these colors etc.
I want to understand WHY I am using the values that I plug into my equations.

Example: I have done the calculations for amber Led at various voltages and currents and get several resistor values.
Forward voltage Vf = 1.8 - 2.6
Forward current IF = 20 - 30 mA
Source voltage Vs = 3.7 - 4.2 V

Resistor calculation ranges from 68 - 150 Ohms




Sent using Otto correct.

[thanasi]

I just orderd all my stuff today for my first night rig
well this is what i came up with ur values, i would emagine this resistor is for the one led :

[aburkhardt]

Thanks! It seems that you are using the higher value for forward voltage and 4.2 V for source.


Sent using Otto correct.

[thanasi]

i have read you can use 4.1 would probbably make sense cause there will be a quick voltage drop under load and then drop slowly as lipo depletes .

[Elgyn]

i use this
http://www.sengpielaudio.com/calculator-ohm.htm

[tracernz]

I would calculate resistor using typical LED values and 3.8 V for your battery, check that I_f,max will still be less than 30 mA with 4.2 V and typical forward voltage and you should be safe. i.e.
R = (V_batt,typ - V_f,typ) / I_f,typ
V_batt,typ ~ 3.8 V, V_f,typ depends on colour, I_f,typ ~ 20 mA
and to check:
I_f,max = (V_batt,max - V_f,typ) / R < 30 mA

[skunkworx]

Some of them are tested to 5V, so be sure to look up your data sheets per the manufacturer. While I haven't experimented with more than a couple brands, they definitely vary a lot and shouldn't be used user a blanket color/voltage/current principle (although they do tend to be within similar limits). Some can take as high as 120mA and not really seem affected. Others fried on me in less than 80mA. Just pay attention to your forward bias voltage and work your way up from there. If you are doing a one and done calculation I'd go with 3.9V as a constant and pick out a resistance based on how much current you want. You won't be able to see the difference unless you can actively change the voltage, plus it'll look a lot better once you get them into the dark (especially the reds and oranges).