LM2596 Mod
To regulate the 12V supply down to the 5V needed by the Raspberry Pi, I purchased a bunch of LM2596 DC-DC regulator modules. These modules have a small potentiometer that you can dial to set the voltage. While you could easily set this to 5V using the pot and move on, I decided to make it more complicated:
Since I am only using the module at 5V, I wanted to remove the possibility of the module to be set to a different voltage accidentally and damage the Raspberry Pi. As it turns out, doing this is pretty easy. From the datasheet, you can see the reference schematic for the regulator.
The part we are interested in is the R1 and R2 resistors at the top of the schematic. On the regulator, these are located here.
The potentiometer is the R2 resistor. That is the one we want to replace. To start with we need to remove the potentiometer. I used a heat gun to heat up all three leads at the same time, but some fancy work with a soldering iron would probably also work.
Here on the close up view, we can see two things:
- The R1 resistor is 330 ohms (verified with my multimeter, but the 331 on the case is a good clue)
- There is another pad for a fixed R2 resistor. This is a size 603 surface mount pad.
Going back to the schematic or page 17, the equation for the output voltage is
Vout = 1.23*(1+(R2/R1))
Skipping the math, the conclusion is we want a resistor right around 1KOhm at R2 for 5V output. I chose a 1.05K Resistor to bump the output up slightly. This results in an output around 5.15V. Max input voltage on the Raspberry pi is 5.25V, so we should be good.
If you don't fancy surface mount soldering, you could also take a thru hole resistor between the two holes from the potentiometer shown above.
Dont forget to test the regulator to make sure you got it right before plugging it into anything you care about.