# Correct way to choose resistors for load

I've come across two ways to determine which resistor value is needed for a load. The first one, which is always titled 'How to determine resistor value for an LED' says that you can use the formula:

$$V_{in} = input\ voltage$$ $$V_{l} = LED\ voltage$$ $$I_{l} = LED\ current$$ $$R = \frac{V_{in} - V_{l}}{I_{l}}$$

And I've also come across one that says to use resistors in series to add a voltage drop like this:

simulate this circuit – Schematic created using CircuitLab

And then use the formula:

$$V_{out} = V_{in} \cdot \frac{R2}{R1 + R1}$$

My final goal is an attempt to power the Raspberry Pi (5v 2.1A) with a 12v battery pack. So, could I just use a 3.3R resistor:

$$R = \frac{12 - 5}{2.1}$$ $$R = 3.333$$

Or do I need to use the second equation to drop the voltage? I'm just confused as to which method to use

• The first formula works only for LEDs. The second formula works only for high-impedance loads. The Pi is neither. – CL. Nov 1 '15 at 12:16