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

Sun Jan 13 13:27:55 EST 2019

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
>
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-- 
73 Jim,KI7AY
la ciruela de Panama

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