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I have a Raspberry Pi 3B+, running Raspbian.

Linux raspberrypi 4.19.97-v7+ #1294 SMP Thu Jan 30 13:15:58 GMT 2020 armv7l GNU/Linux

It server primarily as a MagicMirror2 machine.

I can see the little yellow lightning symbol on the screen always, it flickers a few times during boot, but then practically stays on the whole time. The RPi is connected to a 60W, 6-port USB PSU. (2x 2.4A, 2x2A, 2x1.4A ports, RPi is on one of the 2.4A ports).

I initially though it was a case of bad power supply and decided to order a new one, but while waiting for the new PSU to arrive I decided to test it with an inline USB voltage/amp meter.

The meter reports the RPi is drawing anywhere from 0.64 to 0.88 amps (EDIT: was "watts", my bad) of power and the PSU is supplying it with 5.13 to 5.16 volts.

Surely that isn't supposed to trigger a low voltage signal at all, let alone be constantly on?

ps. there is nothing connected to the Pi other than a regular PC monitor, not even keyboard and/or mouse.

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  • 6
    May be a poor quality usb cable causing too much voltage drop.
    – CoderMike
    Commented Apr 24, 2020 at 15:59
  • 3
    Try testing the voltage right at the Pi's 5v GPIO pin. It must be at least 5v. If it's less, then you need to thicken your power wire. Or shorten it.
    – Botspot
    Commented Apr 24, 2020 at 20:02
  • 1
    Where is your meter plugged in- which ends of the cable?
    – 300D7309EF17
    Commented Apr 25, 2020 at 4:54
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    I would question the quality of a meter which reports 0.88W for a RPi. It should be 5-10W.
    – vidarlo
    Commented Apr 25, 2020 at 7:16
  • 1
    Does this answer your question? RPi3 show under voltage warning while input voltage seems good
    – Dmitry Grigoryev
    Commented Apr 25, 2020 at 17:35

3 Answers 3

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Get a better cable. Most PSUs guarantee the voltage only on the USB-Port.
If you use the official Rasperry Pi PSU on the other hand, it guarantees your the voltage on the Micro USB plug.
Depending on the used cable this can go way below the USB specifications. I've had cables that dropped 5.25V to as low as 4.7V which already under medium load showed the undervoltage warning.

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  • Note that USB 2.0 allows a drop till 4.75V according to the spec, and that is only at the max USB 2.0 current. An USB cable dropping to 4.7V with a Rasberry PI doesn't mean its really bad cable, just that its unsuiteable for the Rasberry PI
    – Ferrybig
    Commented Apr 25, 2020 at 11:01
  • I know. I forgot to add that I tested several cables, at least one which is powering a Lenovo Tab with 2A/5V without problems. It shouldn't be at least that cable.
    – Anna Vahtera
    Commented Apr 25, 2020 at 19:25
  • "If you use the official Raspberry Pi PSU on the other hand, it guarantees your the voltage on the Micro USB plug" I can't see anything in the spec that says that it senses the voltage at the plug, only that it's rated 5.1V 2.5A.
    – Pete Kirkham
    Commented Apr 25, 2020 at 20:30
  • @Pete Kirkham. Each power supply states its output voltage and current. On the official RPi power supply the output IS on the micro-USB plug, as it is an integral part of the power supply. All losses along the wire are taken into account. If your power supply has a USB socket, then the output voltage and current refers to that USB socket. Losses in any cable you attach to it cannot be accounted for.
    – kwasmich
    Commented Apr 27, 2020 at 15:33
  • @kwasmich for it to stabilise the voltage at the plug, it needs to sense at the plug. None of the literature says it does have a such remote sensing - generally it only is true for power supplies costing tens of times that of the official Pi one community.keysight.com/community/keysight-blogs/…
    – Pete Kirkham
    Commented Apr 27, 2020 at 16:46
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The low voltage indicator is shown when the 5V rail drops below 4.65V.

You have to decide whether you trust the Pi's circuitry or the inline USB voltage/amp meter circuitry to correctly see and display transient voltages.

The Pi is telling you that you need a better power supply.

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2

Resistance is impeding the flow of power.

If possible, consider powering your Pi through the 5v and ground pins (2 of each) on the GPIO header. This will have a considerably lower resistance than the MicroUSB connector and consequently less voltage drop. A better USB cable might help as well.

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