Compatibility using 3.3v external compass from APM2.x on Pixhawk (5v)

I just upgraded from an APM 2.6, flying with an external compass, to pixhawk.
As the PX onboard compass had poor and inconsistent results, I installed the external compass I had on APM.
As I couldn’t install it right away, as my compass works on 3.3v, max 3.7v, and PX’ i2c port works with 5v and would probably ruin my compass, I just looked with a multimeter for a free 3.3v pin on pixhawk, to power it up, and found it.
It is working finely using 3 pins on the I2C port: ground + the two signal pins, NOT using the 5v pin, but connecting the VCC on the second pin from the left to right on SPI port, as it reads 3.3v to ground.

Best regards


The original 3DR GPS/Compass module had a 5 vdc to 3.3 vdc regulator built into it to supply 3.3 vdc to the compass while the GPS used 5 vdc.
The Pixhawk’s processor is actually a 3.3 vdc processor, but is 5 vdc tolerant.
As long as you are supplying 3.3 vdc to the outboard compass it will work fine with the Pixhawk I2C bus as you have found out.