What is the minimun supply voltage for the pi3b plus.

The RPi 3b+ has a chip APX803 (which monitors voltage) and triggers at 4.63 ±0.07V according to the Specifications. According to the RPi documentation this warning triggers at 4.63 ±5%.

Luis

It sounds like you are measuring at the source rather than on the RPi itself.  You might be having voltage drop in the power supply cable.  You should measure the voltage at the GPIO connector.  If those are voltages are measured at the at the GPIO connector then you need to look at the voltage on a scope to see whether you see lots of ripple or supply droop when running.  It would be good to measure the supply current also.  You could have a bad cable or possibly a bad RPi.  I've seen a lot of bad cables.

The 4.63V is apparently not measured between the +5V  and GND GPIO pins, otherwise it cannot match my observations. But this https://www.raspberrypi.org/documentation/hardware/raspberrypi/power/README.md , says It must be 5.1 V. So most USB chargers are out.

Bart

I have put the GPIO 5V and 3.3 V pins at the scope. I use audio USB converters to record sound. The result was clearly disturbed by the voltage ripples, because the electret microphone was fed from the USB converter. So I changed to a battery operated preamplifier.

And indeed the supply cable creates a voltage loss, al so more as I used data cables which are to thin for currents around 0.5 A. A supply with cable probably has thicker wires, but alas those are too short.

Bart.

Hi Bart,

5.1V is only a nominal value. The absolute minimum acceptable value will be less than that.

In practice the absolute minimum is going to be slightly different from one Pi to another, because of component tolerance.

If there is 5.0V (as an example - it can be lower) at the +5V and GND pins on the 40-way header, the Pi will function perfectly well. However, if you use a 5.0V USB charger, then it will likely not result in 5.0V at those pins, due to wire resistance. That's why a nominal 5.1V supply is suggested (and a nominal 5.1V supply will have a tolerance too). At that ballpark, then there is confidence that the circuitry will see a voltage within tolerance.

Although the Pi (and lets limit the discussion to Pi 3B+) has a micro USB connector, it is not intended for use with a 5.0V mobile phone charger. It will work sub-optimally at best.

Using the correct supply, the Pi can work at its correct speed (and by 'can' I mean assuming that the processor is cool enough. It may require heatsinking or cooling).

See Ralph's response, voltage should be measured on the Raspberry Pi to measure more accurately what is the "real" voltage that is getting into the device, there is a voltage drop (or loss) caused mostly by the length of the wire (between other different factors). The Raspberry Pi provides some test pads for this kind of measurements in your case is PP2 -see diagram below and a picture where is located.

The GPIO is located after the Circuit protection F1 and is a good place measure the voltage too.

The specs usually target a higher voltage within the specs (5.1V in this case) to compensate easier for the voltage dropped. The RPi 3B+ is power hungry and choosing the right USB Power supply for a project may be difficult at times, one easy way to mitigate this kind of issue is to take a regulated 5V power supply and connect it directly to the 5V GPIO or maybe you can try to put PP2 to good use

Luis

Most "standard" USB cables are unsuitable for use as power cable for the Raspberry Pi. They usually use 28 AWG wire for the data signals and 26 AWG wire (lower AWG number is larger

diameter wire) for power although I have also seen 28 AWG used for both. 26 AWG wire for power is way too small for the 2.5 A current needed by the Pi. 22 - 20 AWG should be used

and I like to keep the cables lengths at 4 - 6 ft. (1.2 to 1.8m) max. Most USB cables, in the US at least, have markings that show the wire gauges used. The markings are usually in the form

of 28AWG/2C+26AWG/2C which says that 2 wires are 28 gauge and 2 wires are 26 gauge. The lower AWG wires should always be the ones used for power. You can find USB cables with

22 - 20 AWG power wires, usually specifically described as Pi power cables but you have to look a little harder for them. And of course the descriptions are not always accurate so buying

from trusted sources helps. Being able to visually verify the gauge markings on the cables before purchase is best. I have been able on occasion to use advertisement pictures of the cables

to read these markings but it is usually very difficult to be sure.

Add on top of that the ones that advertise thick wires and still won't work properly

Some things AROUND the pi require 5V.

USB devices officially need to be provided with 5V. Some might work with less, but some might work with less. If you don't have any USB devices, that's not an issue.

Then, the HDMI connector should provide 5V to "small HDMI devices that might be connected". So for example there are HDMI->VGA converters that use this. If you don't have one of those that's not an issue.

If you don't need any 5V on USB or HDMI, the pi will run correctly down to about 3.3V. At that point the 3.3V regulator will be in "dropout" mode and deliver just less than 3.3V to the stuff that requires 3.3V on the pi. As long as that voltage remains within the allowed tolerance for the 3.3V line everything will continue to run.

In my experiments the LAN9514 chip was the first to run into trouble when I lowered the voltage to my pi. I measured this a really long time ago, not on the 3B+. However I expect the 3B+ to react precisely the same.