Raspberry Pi 1 B+, 2, 3: APX803 chip state via GPIO (Rpi 1,2) or GPU (Rpi 3) flashes the power LED for voltage input less than about 4.65 V. A yellow lightning bolt also is superimposed on the Raspberry Pi display output for low voltage.
It’s not good to have low voltage to the Raspi, you can scramble the SD card, have random malfunctions, and corrupt data. When you see the low voltage icon, stop and get a properly rated power supply and cord. 2.5 Amps is the current recommendation, with a quality micro-USB cable.
The Raspberry Pi 3+ finally brought Power over Ethernet (PoE) to the Raspberry Pi with gigabit Ethernet, particularly good for high-bandwidth software defined radio applications.
Measure DC input
There is no built-in capability to measure the Raspberry Pi DC input voltage, without adding an external ADC. You can read the APX803 state (OK/low) from the Terminal:
- when compiling programs with
make -j2to avoid over-temperature and under-voltage warnings (this uses 2 of the 4 CPU cores).
- Detailed Raspberry Pi voltage/current measurements
- Power recommendations for each Raspberry Pi model.
- Pi power alarm LED controlled with software since it’s on GPIO.