External power supply Servo PI and seperate supply for RPi
322 Views - Created 19/04/2022
I have a Raspberry Pi Zero W using a PiJuice and a Servo PWM Controller. The Raspberry Pi Zero W is powered by the PiJuice using a small 1S LiPo. The Servo PWM controller is powered by a different 2S LiPo. I connected the grounds together as described in the datasheet. When I first connect the Pi and measure the voltage on the external supply pins, for the Servo PWM Controller that should be connected to 2S LiPo, I measure a voltage of 1.4V. I expected 0V but probably due to the I2C connected (which needs 3.3V) voltage is somwhow passed to the Servo PWM controller. When I connect the Servo PWM with the external supply I now measure the expected 2S voltage. Everything seems to work ok.
My question is:
Is this normal?
And which should be powered first, the PI or Servo PWM controller?
The 1.4V you measured on the Servo Pi is normal.
The MOSFET used for the I2C level translator on the Servo Pi contains a diode which allows current to flow from the Raspberry Pi to the Servo Pi. When the Servo Pi is unpowered the current will flow through the I2C pull-up resistors, into the I2C port on the PWM controller and out of the Vdd pin to the external supply pins. The current flow should be limited to less than 2mA so this shouldn't cause any problems with servos or any other connected devices.
I would recommend powering the Raspberry Pi before the Servo Pi. When the Raspberry Pi powers up it takes a short amount of time for the voltage regulators to stabilise and during this time you may get voltage ripples on the I2C pins. There is a remote possibility that the Servo Pi would see these ripples as a command so by powering the Servo Pi after the Raspberry Pi voltage has stabilised you eliminate this risk.
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