aeromaxx
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pi psu with 3 metre cable

Thu Aug 31, 2017 11:35 am

I am in need of putting my pi approx 3 metres away from a power socket, basically on top of some tall bookcase shelves.

I am currently using a raspberry pi 2, and I bought a micro usb extension cable from ebay china, the cable seems to work well, but did have a bit of an initial red flashing power light, but it's now stable.

I did a bit of googling and found these power requirements for the raspberry pi's I potentially would be using.

Raspberry Pi 2 Model B 1.8A
Raspberry Pi 3 Model B 2.5A

I thought that I could get a standard usb plug charger and use a high quality 24awg or 22awg usb to micro usb cable, this would be fine for the raspberry pi 2, but not for the raspberry pi 3 as all the usb plug chargers I have seen have a max 2.4A output per port which is insufficient for the raspberry pi 3.

Does anyone have any advice on what I could do in this instance if I was to move from a pi 2 to a pi 3?

It's a shame the raspberry foundation themselves weren't able to offer a higher quality usb charger with a longer cable.

Edit:

Just stumbled upon this cable: https://www.adafruit.com/product/1995, high quality cable, although shorter than I needed and wrong plug type as I'm in the UK. Am hoping to find something similar with a longer cable and the correct plug.

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rpdom
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Re: pi psu with 3 metre cable

Thu Aug 31, 2017 12:53 pm

Those power requirements are the absolute maximum that a Pi may need with a full set of USB peripherals using 1.2A and a camera and maybe a HAT. The Pi 3 will run on it's own on about half that power. The important thing is to have quality cables to prevent voltage loss.

aeromaxx
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Re: pi psu with 3 metre cable

Thu Aug 31, 2017 1:05 pm

rpdom wrote:
Thu Aug 31, 2017 12:53 pm
Those power requirements are the absolute maximum that a Pi may need with a full set of USB peripherals using 1.2A and a camera and maybe a HAT. The Pi 3 will run on it's own on about half that power. The important thing is to have quality cables to prevent voltage loss.
Oh right so if I am just using a stock pi with no peripherals, no extra addons, ethernet cable, and a usb flash drive, I should be more than fine with the 2.4A.

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karrika
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Re: pi psu with 3 metre cable

Thu Aug 31, 2017 1:24 pm

The voltage drop on 3m cable is big. I would rather use a 3m exrension cord to the wall socket and have a short cable.

beta-tester
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Re: pi psu with 3 metre cable

Thu Aug 31, 2017 1:30 pm

isn't it possible/easyer/cheaper to use a 220V (or 110V) extension cable to bring the mains power closer to the RPi instead of using an expensive USB cable to power up the RPi?

oops, karrika was quicker with his answer...
{ I only give negative feedback }
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aeromaxx
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Re: pi psu with 3 metre cable

Thu Aug 31, 2017 3:49 pm

karrika wrote:
Thu Aug 31, 2017 1:24 pm
The voltage drop on 3m cable is big. I would rather use a 3m exrension cord to the wall socket and have a short cable.
beta-tester wrote:
Thu Aug 31, 2017 1:30 pm
isn't it possible/easyer/cheaper to use a 220V (or 110V) extension cable to bring the mains power closer to the RPi instead of using an expensive USB cable to power up the RPi?
I already have a 10 way extension lead plugged into the wall socket, so to do that I would need a single extension plugged into that 10 way extension lead.

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Imperf3kt
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Re: pi psu with 3 metre cable

Thu Aug 31, 2017 10:42 pm

And? As long as you don't draw more than 2400w you can daisychain 50 extensions if you want.
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davidcoton
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Re: pi psu with 3 metre cable

Thu Aug 31, 2017 11:01 pm

Imperf3kt wrote:
Thu Aug 31, 2017 10:42 pm
And? As long as you don't draw more than 2400w you can daisychain 50 extensions if you want.
Actually, that is not a good idea. But one extra extension for about 10W is not a problem, and will make it far easier to avoid voltage losses.
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hippy
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Re: pi psu with 3 metre cable

Fri Sep 01, 2017 3:38 pm

Rather than use a standard USB PSU and hope the power doesn't drop too much over the cable length -

There are some industrial PSU's which take wires to the target and back again so they can ensure they are delivering the correct voltage at the target.

Other industrial PSU's can have their output voltage manually adjusted to overcome any cable drop.

Some PSU's put out slightly more than the usual 5V to attempt the same.

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karrika
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Re: pi psu with 3 metre cable

Fri Sep 01, 2017 5:25 pm

The problem with 5V DC supplies is that a small voltage drop like 0.3V makes the device malfunction. If you are using a high voltage like 48V DC the 0.3V drop means nothing.

So the moral is: use as high voltage as possible to get rid of problems with voltage drops in cables. The longer the distance, the higher the voltage.

drgeoff
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Re: pi psu with 3 metre cable

Fri Sep 01, 2017 7:58 pm

karrika wrote:
Fri Sep 01, 2017 5:25 pm
The problem with 5V DC supplies is that a small voltage drop like 0.3V makes the device malfunction. If you are using a high voltage like 48V DC the 0.3V drop means nothing.

So the moral is: use as high voltage as possible to get rid of problems with voltage drops in cables. The longer the distance, the higher the voltage.
Yes and it is a square law improvement. For the same power the current goes down as the voltage goes up (inverse proportion). Power lost in the cable is current squared times resistance.

One reason why car makers are working on changing from 12 volts to 48.

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Imperf3kt
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Re: pi psu with 3 metre cable

Sat Sep 02, 2017 4:55 am

davidcoton wrote:
Thu Aug 31, 2017 11:01 pm
Imperf3kt wrote:
Thu Aug 31, 2017 10:42 pm
And? As long as you don't draw more than 2400w you can daisychain 50 extensions if you want.
Actually, that is not a good idea. But one extra extension for about 10W is not a problem, and will make it far easier to avoid voltage losses.
Mind explaining why? My uncle was an electrician and always did stuff like that when needing longer reach from an extension board.
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davidcoton
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Re: pi psu with 3 metre cable

Sat Sep 02, 2017 9:15 am

Imperf3kt wrote:
Sat Sep 02, 2017 4:55 am
davidcoton wrote:
Thu Aug 31, 2017 11:01 pm
Imperf3kt wrote:
Thu Aug 31, 2017 10:42 pm
And? As long as you don't draw more than 2400w you can daisychain 50 extensions if you want.
Actually, that is not a good idea. But one extra extension for about 10W is not a problem, and will make it far easier to avoid voltage losses.
Mind explaining why? My uncle was an electrician and always did stuff like that when needing longer reach from an extension board.
  • Every time you add an extension, you add a little resistance at the plug and socket (actually, three little resistances -- wire to socket, socket to plug, plug to wire). They all add up. If you are drawing a largish current, the total loss through resistance is significant. The lost power ends up as heat, which tends to raise the contact resistance through oxidation, which makes things worse.
  • When you use multiple extensions, you use that slightly dodgy one that normally stays in the box.... (please repair it or throw it out).
  • In general, extension leads should not be used with high loads (heaters in particular) -- certainly check the rating of every extension and do not exceed it, preferably stay below half load for long term use.
  • If your lead is too short, get a longer (and thicker) one rather than connecting multiple leads together.
  • Make sure ALL use of extension leads is from RCD protected sockets (at the consumer unit, socket, or plug in adapter). Test the RCD before use. They reduce both electrocution and electrical fires.
  • Check all extensions and other portable equipment before use. Do not use leads with damaged insulation or with the inner insuation showing at the plug or socket.
  • PAT (Portable Appliance Testing) is a Good Thing, and required by law in UK workplaces.
  • Keep a check on the total load. I know of a case where two 3kW urns were connected to a single 13A (240V) extension -- double the permitted load. It took half an hour before the (correct value) fuse blew. By which time the cable was very warm.
  • If you regularly need extension leads in the same place, look at having permanent sockets fitted properly.
I have no problem with temporary use of daisy-chains of extensions for low current use, but then I have the equipment to check the safety (insulation, continuity, and earth loop impedance) and to ensure that the load is acceptable (both current and voltage drop). I also use 4- and 6-way boards for home entertainment and PC installations -- they provide neat wiring and a single point to turn everything off, while only using low currents.

tl;dr?
Low current -- no problem with extensions in good condition.
High current -- single extension only, check rating and condition.
Always -- use an RCD. Check all equipment before use.
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