osid
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RPi 3 B: Strange behaviour of power LED

Sun May 08, 2016 7:44 pm

I've a Raspberry Pi 3 Model B Rev 1.2
Power supply: I'm using a Samsung USB travel adapter charger, output rated at 5V 2A.
USB peripherals: only a RTL SDR DVB-T stick, with R820T2 tuner.

I'm experiencing power LED constantly off despite RPi up and running.
Measuring current draw and voltage, with a cheap USB voltage display, it shows 500 mA and voltage between 4.95 and 4.98V
It seems to me strange the behaviour of power LED: Raspberry Pi 3 Model B has APX803 as voltage supervisor, so it should be off at or under 4.63V.

Why?

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FTrevorGowen
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Re: RPi 3 B: Strange behaviour of power LED

Sun May 08, 2016 8:00 pm

osid wrote: ...
I'm experiencing power LED constantly off despite RPi up and running.
...
If you've never seen the PoWeR (red) led light up (even faintly) then, maybe, there's a poor solder joint. Inspection with something like a "jeweller's lens" may confirm that. (There have been a few** Pi's reported with missing or partially soldered leds in the past)
Trev.
** IIRC <10
Still running Raspbian Jessie or Stretch on some older Pi's (an A, B1, 2xB2, B+, P2B, 3xP0, P0W, 2xP3A+, P3B+, P3B, B+, and a A+) but Buster on the P4B's. See: https://www.cpmspectrepi.uk/raspberry_pi/raspiidx.htm

osid
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Re: RPi 3 B: Strange behaviour of power LED

Sun May 08, 2016 8:13 pm

FTrevorGowen wrote: If you've never seen the PoWeR (red) led light up (even faintly) then...
No, power LED is on when Pi is halted and blinking under light current load (300 mA).

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FTrevorGowen
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Re: RPi 3 B: Strange behaviour of power LED

Sun May 08, 2016 8:28 pm

osid wrote:
FTrevorGowen wrote: If you've never seen the PoWeR (red) led light up (even faintly) then...
No, power LED is on when Pi is halted and blinking under light current load (300 mA).
O.K. At what point were you measuring the voltage/current? ie. at the power supply end of the uUSB connecting cable or at the Pi/uUSB end. If the former you could have a poor uUSB cable - one with enough internal resistance to reduce the voltage at the Pi end. As well as seeing a "blinking PoWeR led" (one of the low-voltage warnings) have you ever seen a small "rainbow square icon" at the top-right of your display (the other low-voltage warning)?
Trev.
Still running Raspbian Jessie or Stretch on some older Pi's (an A, B1, 2xB2, B+, P2B, 3xP0, P0W, 2xP3A+, P3B+, P3B, B+, and a A+) but Buster on the P4B's. See: https://www.cpmspectrepi.uk/raspberry_pi/raspiidx.htm

osid
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Re: RPi 3 B: Strange behaviour of power LED

Sun May 08, 2016 9:39 pm

FTrevorGowen wrote:[At what point were you measuring the voltage/current? ie. at the power supply end of the uUSB connecting cable
this
you could have a poor uUSB cable
I'll try with another one.
have you ever seen a small "rainbow square icon" at the top-right of your display (the other low-voltage warning)?
I use RPi exclusively headless through SSH. Never connected a monitor to it :D
I'll try to attach a monitor and see.

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Re: RPi 3 B: Strange behaviour of power LED

Thu Jan 05, 2017 4:50 am

The RTL SDR USB devices seem to take a lot of current when in use. On a Pi3 running Raspbian PIXEL the low-voltage lightning-bolt usually stays on and the Pi's red power light will flicker irregularly.

I'm powering the Pi and the RTL stick from a powered USB hub via a 6-foot USB cable to separate the RTL from the Pi. What would be adequate power supply/hub output to stop this behaviour?

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Re: RPi 3 B: Strange behaviour of power LED

Thu Jan 05, 2017 5:16 am

stuzog wrote:The RTL SDR USB devices seem to take a lot of current when in use. On a Pi3 running Raspbian PIXEL the low-voltage lightning-bolt usually stays on and the Pi's red power light will flicker irregularly.

I'm powering the Pi and the RTL stick from a powered USB hub via a 6-foot USB cable to separate the RTL from the Pi. What would be adequate power supply/hub output to stop this behaviour?
1. What is the rating on the PSU for the hub?
2. The spec for the Pi3B is 5V at 2.5A.

Not only do a lot of USB hubs not have PSUs rated that high, but hubs aren't supposed to supply that much current to a single port (though I wouldn't care to assert that none do). Also, just because a PSU says 5v and 2A, that doesn't mean that it will actually provide 5v when approaching max current.

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