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

Doug Crompton wa3dsp at gmail.com
Sun Jan 13 10:17:24 EST 2019

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

*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+

*Pi 3 B+ Power Input Circuitry:*
[image: 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.

*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

*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.

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: 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

A voltage drop of nearly 0.5 V is possible with a hefty 22 gauge cable.

[image: 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
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.

Randy W3RWN
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