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

Sun Jan 13 17:10:07 EST 2019

Just wondering if anyone has had any luck using those 2600 mAh USB power
banks that MicroCenter has for around $2 as a UPS, if you will?

DE KC8RYW

On Sun, Jan 13, 2019, 3:35 PM "Doug Crompton via ARM-allstar" <
arm-allstar at hamvoip.org 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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