MikeDunn
Posts: 83
Joined: Wed Jul 01, 2015 6:58 pm

Use a 2-gang USB port to power a Pi ?

Sun May 05, 2019 7:49 pm

Hi all,

Have had a search on the fora & not found anything useful on this topic, so thought I'd try asking direct - just in case my Google-fu is failing me :o

I've recently been doing occastional 2-gang replacements around the house to have USB ports available in various rooms; this is the one I've been using : 2-gang with USB I've finally got around to doing one of those in my study & thought I'd use it to power a Pi while I'm working on it (ie before I push it into service), but am a trifle puzzled with what I'm seeing ...

According to the 2-gang specs, this is supposed to have a 5v supply of shared 3.1A to the USB ports, but shared only if both sockets are used - ie only use a single device and you get 3.1A if the device wants to pull that much. To my recollection, (first question) this should be fine for a Pi3 B+, yes ? What I'm seeing, however, is the yellow spark symbol top-right of the Pi's display so it's not happy with what's being delivered.

My second question - can I (easily) find out from the Pi how much voltage / amperage it's actually pulling / getting ? The Pi works fine, of course, if I plug in a PSU or phone charger - but I'm trying to get awy from these in the various rooms.

Cheers,

Mike

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rpdom
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Location: Chelmsford, Essex, UK

Re: Use a 2-gang USB port to power a Pi ?

Sun May 05, 2019 8:10 pm

Those sockets are classed as "Chargers". The Pi requires a proper "Power Supply".

The requirements for the output for a charger is not very strict. They may be able to provide a constant 5V at low current, but the voltage will drop as the current goes up. This doesn't matter for charging as most devices will be able to charge their batteries from about 4V upwards and fluctuations in voltage are unimportant.

The Pi requires a stable voltage supply at a strict minimum of 4.75V, preferably above 4.85V and below 5.25V. Many chargers cannot reliably provide this at the current needed.
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davidcoton
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Re: Use a 2-gang USB port to power a Pi ?

Sun May 05, 2019 9:00 pm

The other thing to check is that the USB cable is short and thick. Some cables will have a specification for the wtre size printed on them, something like

"28AWG/1P +28AWG/2C"

"28AWG" is a thin wire, suitable for signals and low current only. "1P" means one pair, the signal part of a USB cable. "2C" means two cores, the power part of a USB cable. For carrying up to 2.5A, which is the normal maximum for a Pi, you want cores of at least 22AWG, preferably 18AWG (lower numbers are thicker).
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MikeDunn
Posts: 83
Joined: Wed Jul 01, 2015 6:58 pm

Re: Use a 2-gang USB port to power a Pi ?

Sun May 05, 2019 10:45 pm

davidcoton wrote:
Sun May 05, 2019 9:00 pm
The other thing to check is that the USB cable is short and thick. [...] For carrying up to 2.5A, which is the normal maximum for a Pi, you want cores of at least 22AWG, preferably 18AWG (lower numbers are thicker).
Interesting ... I wasn't aware of this specific - thanks, David :)

As I have several cables handy (I bought a pile for use on phones etc) I re-tried my 'old standby' that gave the symbol plugged in the port, but swapped to a phone charger - same issue. Replaced the cable with one of the new ones - and had the symbol intermittent while on the charger ... moved the cable to the port - stable 8-) So it seems it was my older cable at fault; I'll keep that only for the phones ! But good to know I have a non-PSU supply handy for my playing & testing, so I can re-use the proper supply on another Pi (or this one, elsewhere), and I don't have a faulty 2-gang !.

In case anyone is interested in what I had success with : it's one out of a set from RavPower I bought from Amazon (one of the 3 foot ones).

Mike

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