fiend
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Help with powering RPI 3b+ and 7" touch screen

Thu Nov 01, 2018 7:52 am

Hi,

I'm working on a project that I plan on making wireless down the line, I was looking into having a power supply that I can use the RPI and screen while charging the battery when it is docked.

I have found the "Adafruit PowerBoost 1000 Charger - Rechargeable 5V Lipo USB Boost @ 1A - 1000C" and I was wondering if it could supply enough power to the Rasberry Pi and 7" touchscreen. Really the only concerning thing is the 1A output. The RPi is not under very much load so I don't think it will be using 1A.

Thanks

Screen:
https://www.amazon.com/Raspberry-Pi-7-T ... B0153R2A9I

Adafruit PowerBoost 1000 Charger - Rechargeable 5V Lipo USB Boost @ 1A - 1000C:
https://www.amazon.com/PowerBoost-1000- ... B01BMRBTH2

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DougieLawson
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Re: Help with powering RPI 3b+ and 7" touch screen

Thu Nov 01, 2018 12:25 pm

Best supply is the Official RPF 2.5A 5.1V power supply. I've got that running my 7" display with a USB cable between the display and the microUSB on the RPi 3B.
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giuino
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Re: Help with powering RPI 3b+ and 7" touch screen

Tue Nov 13, 2018 4:05 pm

I have several similar setups (basically CarPc and dashboards), sorry to say you can't get away with 1amp.

a Pi3 uses around 400mA @idle, 800mA on full load, then add Wi-Fi and BT, which draw 100mA and 240mA each when searching;

You need at least [email protected] only to power the Pi3
ad any peripheral your amperage count sums up.

My waveshare 7"inch displays use about 1A each, drawn from the usb port, so add to the Pi request and you are at [email protected]

Your options are: original Raspberry power supply or some kind of "power hat" like this: http://kck.st/2z4LBer , or a mausberry.

It's simpler to find a 12v power supply than a 5.2V one (5.2v is odd, because standard USB is generally 4.85V, so any generic "5v" adapter will result in an undervolt Pi).

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Re: Help with powering RPI 3b+ and 7" touch screen

Tue Nov 13, 2018 4:36 pm

giuino wrote:
Tue Nov 13, 2018 4:05 pm
I have several similar setups (basically CarPc and dashboards), sorry to say you can't get away with 1amp.

a Pi3 uses around 400mA @idle, 800mA on full load, then add Wi-Fi and BT, which draw 100mA and 240mA each when searching;

You need at least [email protected] only to power the Pi3
ad any peripheral your amperage count sums up.

My waveshare 7"inch displays use about 1A each, drawn from the usb port, so add to the Pi request and you are at [email protected]

Your options are: original Raspberry power supply or some kind of "power hat" like this: http://kck.st/2z4LBer , or a mausberry.

It's simpler to find a 12v power supply than a 5.2V one (5.2v is odd, because standard USB is generally 4.85V, so any generic "5v" adapter will result in an undervolt Pi).
Standard USB is (or should be) 5v.

The Pi power supply is 5.2v to ensure that after voltage drop accross the cable the Pi is still supplied with 5v.
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mahjongg
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Re: Help with powering RPI 3b+ and 7" touch screen

Tue Nov 13, 2018 5:10 pm

giuino wrote:
Tue Nov 13, 2018 4:05 pm
It's simpler to find a 12v power supply than a 5.2V one (5.2v is odd, because standard USB is generally 4.85V, so any generic "5v" adapter will result in an undervolt Pi).
"standard USB is generally 4.85V", is nonsense the standard is 5.00V, not 4.85V. But yes, all power supplies do have an percentage that voltage can deviate, but nominally they should deliver 5.00V

The problem is not that, actually, but when you use such thin USB cables that the voltage drop over them is more than [5.00 (nominal USB output voltage) - 4.65V (low voltage threshold) = 0.35V], then you get in trouble!
That means that any (USB) cable, used with a 2A current running trough it, and a resistance larger than R=V/I = 0.35V/2A = 0.175 Ohm, (=175 milli Ohm) will cause a problem.

As the currents runs trough twice through the length of the cable, then, for say a cable of typically a meter long, the copper length is 2 meter, and so we need a copper wire gauge where two meters of it has less than 175 mOhm, or one meter (the typical length wire gauges are expressed in) must be better than half that or 87.5 mOhm. so according to wikipedia (https://en.wikipedia.org/wiki/American_wire_gauge) we need a copper wire gauge equal to or better than 24AWG (for 80 mΩ/m) , that means we need wire with a copper diameter of half a millimeter.

Half a millimeter copper wire (for both power wires) you will find only in really thick cables, some cheaper cables use much thinner cables, some don't even use copper wires.
I have opened up some cheap Chinese USB cables to find that the "copper wires" consisted of small strips of electro plated (with aluminium) plastic, to keep the cost down, and increase flexibility of the cable.

To keep the copper thickness of the USB power cable low, one solution we can choose is to compensate the voltage drop a bit by starting with a slightly higher voltage to begin with, so with 5.2 Volt, the cable may drop 5.2 - 4.65 = 0.55V before the under-voltage detector is triggered.

The official supply, not only can provide a stable output voltage of 5.2V at 2.5A, it also uses a wire gauge that is sufficient for the task.

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