Voltage Divider Help

I created a simple voltage divider to step down my cars 12V to 8.5ish. I used a 150 ohm resister and a 330 ohm resistor.

When i feed 12V into it as expected i get about 8.5V between the two resistors. However When i plug it into my arduino with a SIM900 GSM Shield attached it doesn't supply enough power and the voltage drops to around 5V's.

I am assuming this has to do with Current... The Arduino with GSM/cell shield attached draws around 2 amps (not constant, but during start up or cell activity).

What am I missing/ not understanding?

Any guidance is appreciated.

-Brian

Cell Sheild Link: www.elecfreaks.com/3080.html
Using: Arduino Uno V3

You can't use a voltage divider to power something (you can, but don't do that).
You need a voltage regulator (7809 or LM317) or a DC-DC converter.

Good to know.... I actually ordered one yesterday when i couldn't figure this out.

I appreciate your help.

I am assuming this has to do with Current...

**You are on the right track... **
Current through a constant resistance is proportional to the voltage ([u]Ohm's Law[/u]). When you hook-up your device, additional current flows-through the "top" resistor (your 150 Ohm resistor).

That means more voltage is dropped across the "top" resistor. When the voltage is "divided", there is less voltage across the "bottom" resistor and your device.

With a voltage divider, whatever is connected to the output needs a high resistance/impedance relative to the resistors in the voltage divider. That way, whatever is connected has very little effect on the voltage divider.

In theory, you could use lower-value resistors to make your setup work (maybe 1.5 Ohms and 3.3 Ohms). But then the voltage divider would draw more current and waste more power than your device, and you'd need big 'ol high-power resistors to dissipate the heat. That's why it's not practical to use voltage dividers in power circuits!

DVDdoug!

Thanks, this was very helpful!