[arm-allstar] Pi Power Supply Information

[Doug Crompton]

Because we often have power supply issues with the Pi's mainly due to poor
supply voltage and/or current here is a little technical information on the
Pi's internal power system. Many of us are now using the 3B+ which has been
out for over a year. It has several upgraded power features. All of the
Pi's have a polyfuse on the incoming 5V line. This is NOT in the circuit
when you feed power directly via the GPIO connector. There is also a
reverse polarity protector that is also NOT in the circuit when you feed
power via the GPIO connector. Both of these items have a small amount of
voltage drop and this is why the official Pi wall-warts supply slightly
more than 5V, usually about 5.2V. The polyfuse is rated at 2A so the
sustained maximum that the Pi and any connected devices can draw is 2A. The
polyfuse is (in most cases) a recoverable device. If it blows it should
recover with several hours or more of NO voltage applied. So more caution
is in order in feeding power directly to the GPIO connector and bypassing
the USB connector input. You will not have fuse or reverse polarity
protection.

The Pi's also have an over voltage Littlefuse transorb which would conduct
and cause this fuse to open thus protecting the Pi. Feeding 5V to the GPIO
would still have the over voltage protection but not the fuse so the
transorb or something in its path would probably smoke if you attached a
voltage higher than its rated voltage.

So if you are feeding power to the Pi via the micro USB connector be sure
to use a supply that provides about 5.2V at least 2A and preferably 2.5A.
If you are feeding power directly to the GPIO connector 5V is acceptable
but remember feeding at the point has no reverse polarity or fuse
protection.

Here is a good article that describes the difference in the various Pi
power supplies -

https://learn.adafruit.com/introducing-the-raspberry-pi-model-b-plus-plus-differences-vs-model-b/power-supply

This is a schematic for the Pi 3B+  -

https://www.raspberrypi.org/documentation/hardware/raspberrypi/schematics/rpi_SCH_1bplus_1p2_reduced.pdf

Technical information on the design of the Pi power supply chip -

https://www.raspberrypi.org/blog/pi-power-supply-chip/

The Pi 3B+ under voltage detector data sheet -

http://www.mouser.com/ds/2/115/APX803_D-86185.pdf

Data sheet on the Pi Power protection chip feeding HDMI -

https://www.diodes.com/assets/Datasheets/AP2331.pdf


*73 Doug*

*WA3DSP*

*http://www.crompton.com/hamradio <http://www.crompton.com/hamradio>*