[arm-allstar] Powering Your Pi (Lessons learned from Pi 3B+)

stanley stanukinos ka5iid at swbell.net
Mon Jan 14 08:01:19 EST 2019

Doug, has the Pie foundation been made aware of the issue, I hope they address it in the next release. 


Sent from my iPhone

> On Jan 13, 2019, at 10:49 PM, Doug Crompton via ARM-allstar <arm-allstar at hamvoip.org> wrote:
> I know there has been a lot of noise about Pi power today and I want the
> larger group that is not experiencing any problems to know that there is
> absolutely no reason to start messing with things if you are not
> experiencing problems with your Pi. There are literally thousands of Pi's
> being run by users of this email list and when problems occasionally crop
> up like this everyone goes bananas thinking they might have a problem when
> the larger audience does not.
> The bottom line is if you have a wall wart supply (the most common power
> source for a Pi) that is rated for the Pi you probably will not have any
> problems and should not be alarmed by all this talk of different supplies
> and connection schemes. FOB's draw little power and the same for USB
> sticks. If you plug in anything heavy into the USB ports like hard drives
> or other power hungry devices then, yes you need to be aware of the current
> drawn. In that case you always have the option of using a powered USB hub.
> *73 Doug*
> *WA3DSP*
> *http://www.crompton.com/hamradio <http://www.crompton.com/hamradio>*
> On Sun, Jan 13, 2019 at 10:34 PM "Jim Kinter Jr. via ARM-allstar" <
> arm-allstar at hamvoip.org> wrote:
>> Since we Hams usually have 12VDC handy, I like using UBEC's to run my
>> Ham Pi stuff.
>> https://hobbyking.com/en_us/power-systems/power-distribution/vr-ube-bec-sbec.html
>> I have had really good luck with the HobbyKing and Turnigy brands,
>> and they seem to have enough amperage that a Pi never complains.
>> I just scarf the microUSB off a USB-A ->uUSB cable and tie to the
>> output of the UBEC.
>> I did learn (the hard way) to always go in the uUSB port on the pi
>> and NOT direct to a 5VDC line on the 40-pin, as the uUSB has extra
>> regulation on that port.
>> 73
>> K5KTF
>> At 02:34 PM 1/13/2019, \"Doug Crompton via ARM-allstar\" wrote:
>>> I find that the wall warts designed specifically for the Pi work very well
>>> in most cases. I usually buy from Microcenter and get the 5V 2.5A or 3A
>>> models. They actually are 5.2 or 5.25 volt output. One of the problems is
>>> people using charger type supplies not designed for running the Pi. Also
>>> poor USB cables which are only designed to carry 1/2A.
>>> Here are some good articles on Pi 3B+ power design.
>>> https://www.raspberrypi.org/blog/pi-power-supply-chip/
>>> https://www.datenreise.de/en/raspberry-pi-power-supply-recommendation/
>>> *73 Doug*
>>> *WA3DSP*
>>> *http://www.crompton.com/hamradio <http://www.crompton.com/hamradio>*
>>> On Sun, Jan 13, 2019 at 2:09 PM "Jim Darrough via ARM-allstar" <
>>> arm-allstar at hamvoip.org> wrote:
>>>> I am currently using a PiJuice HAT device to power my RPi. This device
>>>> has a charge port (conveniently a micro USB) and my Pi power supply
>>>> plugs into that. Since the Pi Juice acts as a UPS and provides power to
>>>> the RPi, it should alleviate any voltage drops since it plugs into the
>>>> expansion (IO) headers.
>>>> Nice device. It will run the RPi for over 3 hours. The producers are
>>>> also selling large capacity (12,000 mah) LiPo cells that can be
>>>> connected to the PiJuice external power terminals and will be charged
>> by
>>>> the HAT.
>>>> 73 Jim KI7AY
>>>>> On 1/13/2019 09:17, "Doug Crompton via ARM-allstar" wrote:
>>>>> I am resending this as the original from Randy was blocked due to
>>>>> graphics.
>>>>> Writing this email with the intent to share some knowledge I learned
>>>>> this week, and with the hope that you can avoid a problem that I'm
>>>>> currently experiencing at my remote repeater site - an unhappy Pi and
>>>>> a repeater that's mostly off the air.
>>>>> *Key Point #1:*
>>>>> *The Pi Model 3B+ is a power hungry beast. The power supply that
>>>>> previously ran your Pi 2 B, or Pi 3 B hardware may be inadequate for
>> a
>>>>> Pi 3B+
>>>>> *
>>>>> *
>>>>> *
>>>>> *Key Point #2*
>>>>> *Voltage Drop in cables and internal input circuit to the Raspberry
>> Pi
>>>>> is material. If you start with a 5V supply, the chips in your Pi will
>>>>> have less than 5V.
>>>>> *Starting with a 5V power supply, voltage drop in your power cable,
>>>>> and a further voltage drop through the internal protection fuse of
>> the
>>>>> Pi can easily result in the 4.63V Low Voltage Signal from the
>> regulator.
>>>>> Micro USB cable losses can easily be 0.3V, and combined with the 0.1V
>>>>> drop across the Pi's internal fuse, we're at 4.6V
>>>>> *Key Point #3*
>>>>> *The Micro USB connector is rated for 1.8A, this will not support the
>>>>> full power that the Pi 3 B+ and usual peripherals can draw.*
>>>>> It may be time to consider powering Pi 3 B+ via the header pins,
>>>>> possibly with some sort of Pi Power Hat having a 12V to 5V converter
>>>>> located close to the Pi. (for those that run their Pi's at repeater
>>>>> sites with DC power)
>>>>> This bypasses the 1.8A limit of the Micro USB connector, and the 0.1V
>>>>> drop across the input fuse. (The Pi Power Hat should have appropriate
>>>>> current limiting)
>>>>> *Background Data and Reading: */*Raspberry Pi 3B+ power input and
>>>>> voltage regulator system.*/
>>>>> The Raspberry Pi website provides hardware documentation re power
>>>>> input, however it is one version of hardware behind - It describes
>>>>> power requirement for a Pi Model 3. (not 3 B+)
>>>>> It would be easy to read this and assume these specs also apply to
>> the
>>>>> 3B+.... Hint: They don't.
>>>>> There have been substantial changes to power consumption between the
>>>>> Model 3 B and the Model 3 B+
>> https://www.raspberrypi.org/documentation/hardware/raspberrypi/power/README.md
>>>>> *Pi 3 B+ Power Input Circuitry:*
>>>>> image.png
>>>>> *Micro USB Connector:*
>>>>> The Raspberry Pi 3B+ power input schematic indicates use of an
>>>>> Amphenol Micro USB connector as the power input to the Raspberry Pi
>> 3B+.
>>>>> The Amphenol specs for this part indicate a *_maximum current rating
>>>>> of 1.8 amps_* for pin 1 of this connector.
>>>>> https://www.amphenol-icc.com/micro-usb-101035940001lf.html
>>>>> *Input Fuse, Protection Diode and Filter Capacitor:*
>>>>> The power input circuitry of the Pi 3B+ has a fuse, diode and filter
>>>>> capacitor to protect the Pi from reversed power leads, too high a
>>>>> voltage, and noise transients on the power supply.
>>>>> The fuse is rated for a holding current of 2.5 Amps, and trip current
>>>>> of 5 Amps. Importantly, *the fuse has a resistance of 0.1 Ohms*. That
>>>>> doesn't sound like much, but we are working with a nominal 5 Volts.
>>>>> *If you want 5 Volts exactly on the Raspberry Pi power bus, you need
>>>>> about 5.25V on the Micro USB port* to achieve that. In fact, the
>>>>> Raspberry Pi official AC power supply is 5.1V DC.
>>>>> Question: Where do you buy a 5.1 or 5.25V power supply?
>>>>> The transient voltage suppression diode has a breakdown voltage of
>> 6.4
>>>>> Volts. Exceeding this voltage will cause the diode to short the 5
>> volt
>>>>> bus to ground, resulting in high current tripping the the fuse.
>>>>> Should you wire up the power to your Pi with reversed polarity, the
>>>>> diode will conduct, shorting the 5V bus to ground and blowing the
>> fuse.
>>>>> Fuse: https://www.bourns.com/pdfs/mfmsmf.pdf
>>>>> Diode:
>> https://www.littelfuse.com/products/tvs-diodes/surface-mount/smbj/smbj5_0a.aspx
>>>>> *Pi 3 B+ Voltage Regulator:*
>>>>> On board voltage regulation is done by a new multi-voltage regulator
>>>>> chip, the MXL7704-R3.
>>>>> Previous Pi's used 3 separate voltage regulators. The MXL7704 was
>>>>> created for the Raspberry Pi.
>>>>> https://www.raspberrypi.org/blog/pi-power-supply-chip/
>>>>> The MXL7704 will operate from an input voltage of 4.0V to 5.5V.
>>>>> At or below 4.63 Volts the regulator sends a low voltage signal over
>>>>> the I2C bus to the CPU.
>>>>> Below 3.9 Volts the regulator locks-out.
>>>>> Spec Sheet: https://www.exar.com/ds/mxl7704.pdf
>>>>> image.png
>>>>> *USB Cable Voltage Drop:*
>>>>> Your power supply puts out a nominal 5 Volts, but the cable between
>>>>> your power supply and the Pi will have a voltage drop.
>>>>> The voltage drop increases with the larger current through the wire,
>>>>> and the Pi 3 B+ draws more current the previous Pi.
>>>>> Here is a chart I found at
>> https://goughlui.com/2014/10/01/usb-cable-resistance-why-your-phonetablet-might-be-charging-slow/
>>>>> A voltage drop of nearly 0.5 V is possible with a hefty 22 gauge
>> cable.
>>>>> image.png
>>>>> *Wireless Chip 5V Supply:*
>>>>> While the Cypress Wireless chip on the 3B+ uses lower voltages from
>>>>> the regulator for most of its operation, the WiFi power amplifier has
>>>>> a separate 5V supply.
>>>>> Acceptable operating voltage range is 3.0V to 5.25V, the optimum
>>>>> voltage is apparently 4.8V
>>>>> *Measure the 5V rail on your Pi:*
>>>>> GPIO Expansion pins 2 & 4 are 5V +, ground is available on pin 6, 14,
>>>>> 20 2etc.
>>>>> Ideally this should be 5.0 Volts DC. It's likely to be low if you're
>>>>> powering your Pi via the Micro USB port.
>>>>> *Header Plug as Power Input:*
>>>>> I will be testing powering my 3 B+ via the GPIO Header.
>>>>> I have ordered a 10 pin ribbon cable with female header connector.
>>>>> My current plan is to have the ribbon cable connect to pins 1-10 and
>>>>> extend out the slot in the case.
>>>>> I may use an exacto knife and trim of unnecessary GPIO wires, leaving
>>>>> only 2+4 for +5V and 6+9 for Ground.
>>>>> I plan to use a 5V, 5A power supply that is not potted in epoxy so
>>>>> that I can see the component values and ensure it has good filtering.
>>>>> Regards,
>>>>> Randy W3RWN
>>>>> _______________________________________________
>>>>> ARM-allstar mailing list
>>>>> ARM-allstar at hamvoip.org
>>>>> http://lists.hamvoip.org/cgi-bin/mailman/listinfo/arm-allstar
>>>>> Visit the BBB and RPi2/3 web page - http://hamvoip.org
>>>> --
>>>> 73 Jim,KI7AY
>>>> la ciruela de Panama
>>>> ---
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