<div dir="ltr"><div class="gmail_quote"><div dir="ltr"><div style="font-family:tahoma,sans-serif;font-size:small"><div dir="ltr">I am resending this as the original from Randy was blocked due to graphics.</div><div dir="ltr"><br></div><div dir="ltr">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.<br><br><b>Key Point #1:</b></div><div dir="ltr"><b>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+<br></b></div><div dir="ltr"><div><b><br></b></div><div><b>Key Point #2</b></div><div><b>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.<br></b>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.<br>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</div><div><br></div><div><div dir="ltr"><b>Key Point #3</b></div><div><b>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.</b><br>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)<br>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)</div></div><div><div><br></div></div><div><b>Background Data and Reading: </b><i><b>Raspberry Pi 3B+ power input and voltage regulator system.</b></i></div></div><div>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+)<br></div><div>It would be easy to read this and assume these specs also apply to the 3B+.... Hint: They don't.<br>There have been substantial changes to power consumption between the Model 3 B and the Model 3 B+<br><a href="https://www.raspberrypi.org/documentation/hardware/raspberrypi/power/README.md" target="_blank">https://www.raspberrypi.org/documentation/hardware/raspberrypi/power/README.md</a><br></div><div><br></div><div><b>Pi 3 B+ Power Input Circuitry:</b><br></div><div><div><img src="cid:ii_jqun1o3i1" alt="image.png" class="m_8447419071711730206gmail-CToWUd m_8447419071711730206gmail-a6T" width="542" height="230"></div></div><div><div><br><br></div><div><div><b>Micro USB Connector:</b></div><div>The
Raspberry Pi 3B+ power input schematic indicates use of an Amphenol
Micro USB connector as the power input to the Raspberry Pi 3B+. </div><div>The Amphenol specs for this part indicate a <b><u>maximum current rating of 1.8 amps</u></b> for pin 1 of this connector. <br><a href="https://www.amphenol-icc.com/micro-usb-101035940001lf.html" target="_blank">https://www.amphenol-icc.com/micro-usb-101035940001lf.html</a><br></div><div><br></div><div><div><b>Input Fuse, Protection Diode and Filter Capacitor:</b></div><div>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.<br>The fuse is rated for a holding current of 2.5 Amps, and trip current of 5 Amps. Importantly, <b>the fuse has a resistance of 0.1 Ohms</b>. That doesn't sound like much, but we are working with a nominal 5 Volts. </div><div><b>If you want 5 Volts exactly on the Raspberry Pi power bus, you need about 5.25V on the Micro USB port</b> to achieve that. In fact, the Raspberry Pi official AC power supply is 5.1V DC.</div><div>Question: Where do you buy a 5.1 or 5.25V power supply?</div><div><br>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.<br>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.<br>Fuse: <a href="https://www.bourns.com/pdfs/mfmsmf.pdf" target="_blank">https://www.bourns.com/pdfs/mfmsmf.pdf</a><br></div><div>Diode: <a href="https://www.littelfuse.com/products/tvs-diodes/surface-mount/smbj/smbj5_0a.aspx" target="_blank">https://www.littelfuse.com/products/tvs-diodes/surface-mount/smbj/smbj5_0a.aspx</a></div></div></div><div><br></div><div><b>Pi 3 B+ Voltage Regulator:</b><br></div>On board voltage regulation is done by a new multi-voltage regulator chip, the MXL7704-R3.<br>Previous Pi's used 3 separate voltage regulators. The MXL7704 was created for the Raspberry Pi.<br><a href="https://www.raspberrypi.org/blog/pi-power-supply-chip/" target="_blank">https://www.raspberrypi.org/blog/pi-power-supply-chip/</a></div><div><br></div><div>The MXL7704 will operate from an input voltage of 4.0V to 5.5V.</div><div>At or below 4.63 Volts the regulator sends a low voltage signal over the I2C bus to the CPU.<br>Below 3.9 Volts the regulator locks-out.</div><div>Spec Sheet: <a href="https://www.exar.com/ds/mxl7704.pdf" target="_blank">https://www.exar.com/ds/mxl7704.pdf</a><br><br></div><div dir="ltr"><div><img src="cid:ii_jqumvt3h0" alt="image.png" class="m_8447419071711730206gmail-CToWUd m_8447419071711730206gmail-a6T" width="500" height="441"></div></div><div><br></div><div><br></div><div><br></div><div><br></div><div><div><b>USB Cable Voltage Drop:</b><br></div>Your power supply puts out a nominal 5 Volts, but the cable between your power supply and the Pi will have a voltage drop.</div><div>The voltage drop increases with the larger current through the wire, and the Pi 3 B+ draws more current the previous Pi.<br>Here is a chart I found at <a href="https://goughlui.com/2014/10/01/usb-cable-resistance-why-your-phonetablet-might-be-charging-slow/" target="_blank">https://goughlui.com/2014/10/01/usb-cable-resistance-why-your-phonetablet-might-be-charging-slow/</a></div><div><br></div><div>A voltage drop of nearly 0.5 V is possible with a hefty 22 gauge cable.</div><div><br></div><div><div><img src="cid:ii_jqupxpmu2" alt="image.png" class="m_8447419071711730206gmail-CToWUd" width="459" height="142"><br></div></div><div><br></div><div><div><div><b>Wireless Chip 5V Supply:</b><br></div>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.<br>Acceptable operating voltage range is 3.0V to 5.25V, the optimum voltage is apparently 4.8V</div><div> <br></div><div><div><b>Measure the 5V rail on your Pi:</b><br></div>GPIO Expansion pins 2 & 4 are 5V +, ground is available on pin 6, 14, 20 2etc.<br>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.<br></div></div><div><div><div><br></div><div><br></div><div><div><b>Header Plug as Power Input:</b><br></div>I will be testing powering my 3 B+ via the GPIO Header.<br>I have ordered a 10 pin ribbon cable with female header connector. <br>My current plan is to have the ribbon cable connect to pins 1-10 and extend out the slot in the case.<br>I may use an exacto knife and trim of unnecessary GPIO wires, leaving only 2+4 for +5V and 6+9 for Ground.<br>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.<br><br></div></div></div><div></div><div>Regards,</div><div>Randy W3RWN</div></div></div>
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