Arduino voltage input vs voltage divider

[Lucky-Luka]

Hi all
I am trying to power my Arduino Uno board using a 12V battery. I decided to use a voltage divider to supply around 7V to the Vin pin so that the Arduino voltage regulator isn't stressed too much with 12V. I used a voltage divider with 2.2k and 3.3k resistors that gives 7.2V when the Arduino Vin pin is disconnected. When the pin is connected the voltage drops to 2.5V.
Is it related to the Vin pin impedance? How much is it? I haven't found the value in the forums.
Can somebody help me to clarify this issue and suggest some values for the voltage divider?
It is not the final solution: I will use a step down DC-DC voltage converter but right now I am intersted in this solution
Thanks

[garethw]

You cannot use a voltage divider to power a load, as you have found out!
Why? When you attach a load to the divider, the load draws current. This current flows through the top resistor and causes a voltage drop.
You say that you have around 2.5V when the Arduino is connected. This means that the voltage drop across the top resistor is 12-2.5 = 9.5V
Ohms law shows that:
9.5V / 2200 Ohms = 4.3mA
This is the combined current that is drawn by the Arduino and the bottom resistor.

Without the Arduino connected we have:
12V / (3300 + 2200) Ohms = 2.18mA
2.18mA x 3300 Ohms = 7.2V
This is the 7.2V you were expecting

Hopefully this makes sense

[Lucky-Luka]

It makes sense.
Not even changing the values of the resistors the voltage divider can be used as a solution to lower the input voltage?
What can I do to power Vin using a 12V battery?

[garethw]

The Arduino Uno's Voltage regulator can be supplied with 12V without any problems (according to their specifications online). The absolute maximum value is 20V but this is not advised by Arduino themselves.

[Seekonk]

You can always put a number of diodes in series or a zener. Battery often indicates an engine type application. Always suggest some resistor and filter cap in addition to any other voltage dropping to absorb spikes. I use those small cheap $1.50 buck regulators to drop to 9V.  I'm often powering several other devices too.  Avoid pulling much 5V from this board to power other things.

[ozcar]

It makes sense.
Not even changing the values of the resistors the voltage divider can be used as a solution to lower the input voltage?
What can I do to power Vin using a 12V battery?

There is noting wrong with the basic idea, it is just that as garethw indicated, you calculated the resistance values ignoring the current drawn by the Arduino. Measure how much current the Arduino draws. Calculate what value of resistor would be required to drop 5V (12V-7V) at that current, and check what power would be dissipated by this resistor. Insert the resistor in the Arduino supply line, in effect creating a voltage divider using the Arduino as one of the resistors. If you have something connected to the Arduino that causes the current it draws to vary, well, you can see where it could get a bit tricky.

[phil from seattle]

It makes sense.
Not even changing the values of the resistors the voltage divider can be used as a solution to lower the input voltage?
What can I do to power Vin using a 12V battery?

There is noting wrong with the basic idea, it is just that as garethw indicated, you calculated the resistance values ignoring the current drawn by the Arduino. Measure how much current the Arduino draws. Calculate what value of resistor would be required to drop 5V (12V-7V) at that current, and check what power would be dissipated by this resistor. Insert the resistor in the Arduino supply line, in effect creating a voltage divider using the Arduino as one of the resistors. If you have something connected to the Arduino that causes the current it draws to vary, well, you can see where it could get a bit tricky.

I couldn't leave this alone.  The last little bit is where it will go completely awry.  So, I have to disagree with the "nothing wrong" part.  I think 5 or 6 cheapo diodes in series is a better solution but why not just use a cheap VReg, like a 78xx series? It is fairly resilient to current draw and source voltage.

[Lucky-Luka]

In the end of the day I used a step down buck converter, not a linear one, due to power losses.