[TriLUG] Raspberry PI power requirements

[Tadd Torborg via TriLUG]

I ran into a hardware problem that I thought was an over-sensitivity on the part of Raspbian 9-stretch.  It wasn’t, but it got me looking into power consumption and voltage sensitivity on the part of the Raspberry PI.

The Raspberry PI wants > 4.65v all the time, no exceptions.  It draws almost an amp intermittently with no daughter cards and no USB peripherals.  More current if you have peripherals. Some of the USB sources which can power a battery device like a cellphone are not adequate to run a Raspberry PI.  The Raspberry PI doesn’t have a battery to fall back on during high current moments.  

The easiest thing to work at for your Raspberry PI’s power situation is cables.  USB cables are not all made the same, and the longer the USB cable between the supply and the Raspberry PI the more voltage drop you will get across the wire.  USB cables are commonly specified with power conductors having  24 gauge 26 gauge, or 28 gauge wire, i.e 24AWG, 26AWG and 28AWG.  (American Wire Gauge)   28 gauge is really really thin.  I suspect that some of the USB cables have even thinner wire but those don’t come with specifications.  A 28 gauge wire USB cable at 2 meters long is available.  If you try to run a Raspberry PI on one of these it will go badly. 
According to http://www.calculator.net/voltage-drop-calculator.html <http://www.calculator.net/voltage-drop-calculator.html> 
1amp load, 5 volts, DC, copper, single set of conductors.  Here is the volts at the Raspberry PI end of the wire. 
24AWG is 4.69v at 6 feet     4.85v at 3 feet
26AWG is 4.51v at 6 feet     4.76v at 3 feet
28AWG is 4.22v at 6 feet     4.61v at 3 feet 
30AWG is 3.76v at 6 feet     4.38v at 3 feet

Raspberry PIs can complain if the voltage goes to 4.65v or lower.  They will put a graphic icon in the upper right of the screen.  Modern firmware, after late 2016, will show a lightning bolt.  Earlier firmware will show a colored square.  If you get a colored square, upgrade your Raspberry PI’s firmware.   The danger is that your SDcard will be corrupted during operation or you can corrupt internal FLASH during a firmware update or maybe during a pin remapping.   
To update your firmware do these commands
Don’t do this if your power source isn’t great!  A power glitch or below-volts operation can brick your raspberry pi.  
sudo apt-get install rpi-update
sudo rpi-update

Since most of the 3’ wires are 28AWG (i expect), there is your source of low-voltage warnings!  

You can buy a 6inch USB A to USB-micro-B cable having 26AWG (gauge) wire for $3 on Amazon.  that’s the ticket.   
https://www.amazon.com/s/ref=nb_sb_noss?field-keywords=B00D0XUKIQ <https://www.amazon.com/s/ref=nb_sb_noss?field-keywords=B00D0XUKIQ>      This is 5 cables for $10 or so.  

Get your self a "USB safety power monitor” for about $4 on ebay, $11 on Amazon.  This will display the voltage and current passing through.  I would suggest getting one with a single USB A female socket.  Any unit with more than one socket must have a bridge chip and you don’t need another of those plugged into your devices.  On Amazon you can get one of those for about $11 in 2 days so don’t spend that on an eBay unit.  

https://www.amazon.com/s/ref=nb_sb_noss?field-keywords=B01D9Y6ZFW <https://www.amazon.com/s/ref=nb_sb_noss?field-keywords=B01D9Y6ZFW>

Use your power monitor to measure the voltage on the Raspberry PI’s USB-A sockets.  That’ll give you an idea of the voltage actually at the Raspberry PI.  You can also use a voltmeter on the GPIO expansion header between pins 2 and 6.  See https://i.stack.imgur.com/sVvsB.jpg   for pin-out of the header.  Don’t short the header!  Shorting 5v to a 3.3v pin or signal pin will kill your Raspberry PI!  

Tadd / KA2DEW
tadd at mac.com
Raleigh NC  FM05pv

“Packet networking over ham radio": http://tarpn.net/t/packet_radio_networking.html <http://tarpn.net/t/packet_radio_networking.html>
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