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I'm trying to power my Raspberry from my TV's USB port (originally intended to show pictures from an usb stick).

It mostly works, but I get some random reboots, while it works perfectly with an external USB power supply. Usually it will work correctly for a couple of minutes, so I suspect that particular USB port gives enough power for most of the time but not enough for some spikes in power usage.

It is a Model B (not by choice, it's just what I have lying around, I bought it on launch day) with a WiFi dongle and a TSOP32238 on GPIO. I control it using an IR remote and I already shut it down before turning off the TV (even if the Raspi is running on a separate power source).

I don't have a space problem (behind a typical TV you have plenty) but a cabling / power outlets one (in this particular location I don't have space to hide power bricks and the like).

I'm thinking about adding a large capacitor to help it survive those spikes:

  • Does it makes sense at all?
  • How large could be large enough?
  • Instead of building a custom USB cable, it would be much cleaner to plug it on the Pi's pins. Is this going to cause any problem? I'm pretty sure it could not overvolt the Pi (typical risk when trying to power a Pi from those pins using an external supply) but I'm not sure about other kinds of problems.

I already do a clean shutdown every time I turn off the TV (either before or after it) so having the requirement to remember shutting it down is not a problem.

Plan B would be to use some UPS-like (like this or a similar idea ) thing also taking power from the TV's usb port.

Community
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Luke404
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5 Answers5

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You should definitely get an official RasPi power supply from any of the distributors, otherwise, you'll corrupt your file system (that usually results in data loss and the need to reimage the system), or break your SD card (that usually results in trip to the nearest store for another SD card).

Whatever you plan to do with the capacitors will not work for many reasons, and would definitely cost much more than a decent power supply.

lenik
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    I do have (lots of) working power supplies, and reimaging OpenELEC or throwing away an old 2gb SD won't be a problem. Could you please elaborate on what the many reasons are? – Luke404 Oct 20 '13 at 13:54
  • usual capacitors won't be able to store enough energy to power your RasPi longer than a few milliseconds. you will need to use ionistors, that are expensive and still would not be able to provide the power longer than a few seconds -- not long enough for any serious purpose, because high-energy consumption intervals may easily go into the minutes. if you have a working power supply -- USE IT! =) – lenik Oct 20 '13 at 23:36
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In a similar situation I decided to go with your "plan B": the RPi is powered via a power bank which is in turn powered by the USB port. A cheap power bank is cheaper than supercaps and provides power for much longer, covering much longer periods when power the from USB port is insufficient.

If you need to shut down when the USB port goes off (like I do), pick a power bank with a "charging" or "external power" LED indicator. I managed to hook up a signal from my LED to a GPIO pin, so that the RPi knows when it has to shut down and does so properly. I have a trivial script which checks the GPIO in question every few seconds and executes shutdown if the "external power" signal is not present.

Dmitry Grigoryev
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I did this, powered my XBMC Raspberry from my TV. It worked OK, but when I turned off the TV, it cut power to the Raspberry. The Raspberry don't like to loose power, it likes a shut down. So now I use a 5V USB adapter to power it. So reconsider this if your TV cut power to the USB when in standby.

user10128
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  • If your TV supports CEC, you can configure XBMC to shut down as soon as you Turn off the TV. At least on my Sony it then shuts down and after 40 seconds the tv cuts the power. – Gotschi May 29 '14 at 11:36
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If this is a Model B, you may want to switch to a Model A ($25) which uses much less power.

The disadvantage (and reason why it uses less power) is that it has only one USB port and no ethernet. If you communicate with a WiFi dongle, you won't have a port for keyboard and mouse input. (You can add a hub, but I am guessing you want to have everything as stripped-down as possible, otherwise you would just add a power supply.)

But check to see whether the USB power goes away when the TV turns off. If so then you really want to just power it from a power supply or you will corrupt your system about every N power cycles.

DMPalmer
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  • I added some details to the question to clarify the points you're talking about. No keyboard/mouse are used or were harmed :) – Luke404 Oct 21 '13 at 14:53
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Generally speaking such USB on TVs are not really designed to offer higher currents to loads connected to it. That said what you need is an external power supply with a decent USB cable (yes, that is important USB Cables... Why are some of them bad for Raspberry Pi? ).

Clóvis Fritzen
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