Dec 072012

As has frequently been written elsewhere, the main source of problems for the Raspberry Pi systems is the power supply. I would even go as far as saying: before debugging anything else, measure the voltage!

On the Raspberry PI two test points are located. TP1 and TP2. With a voltmeter you can measure the voltage between them. According to the Raspberry Pi handbook by Eben Umpton (page 49) it should be between 4,8 V and 5 V.

I am testing several different power supply units (mostly USB chargers …), and will be offering only know good units in the shop. By buying a known good power unit you can avoid the following

Problems / symptoms of low voltage / weak power supplies

  • corrupted SDHC cards (even quality cards will get corrupted / dismounted – eventually the filesystem will not be able to restore itself)
  • frequent USB connects and disconnects
  • some distributions, i.e. Raspbmc / OpenELEC not working correctly / not recognizing mouse and keyboard, while others (Raspbian Wheezy) will work just fine …
  • keyboard (on a powered hub!) working, while mouse (on the same hub) does NOT work

Causes of low voltage

  • weak power supply
  • power supply requires negotiation for more than 500 mA, which the Raspberry PI does not do (Raspberry Pi handbook)
  • greedy USB peripherals attached to the pi, for example illuminated keyboard
  • using rechargeable batteries which are not freshly charged (I see 4,44 V with batteries which are giving 5 + V without load)

Even if your power supply states that it will provide up to 1 A, this will not necassarily be with 4,8 – 5 V – many supplies I’ve tried show this behaviour.

According to Gert van Loo, whom I met at the fair, the Raspberry Pi will take at maximum 1 – 1,5 A, depending on the ambient temperature, before the fuses start kicking in. As such, there will be no point in trying to attach a 4 A powersupply, for instance, in order to provide power to the USB ports.

Known good power supplies

The following adapters or compatible, thoroughly tested, models will be available in our shop:

goobay Model 42438 car adapter (12 V -> 5 V)

Provides between 4,94 V and 5,10 V in our tests. It has a micro USB plug.

D-Parts Mobilphon & Zubehör Gmbh (DP-LGS) Model AC0122-051000 (230 V -> 5 V)

Provides 4,93 V in idle mode. It has a micro USB plug.

Known “bad” power supplies

The power drawn from the Raspi in our tests has been measured at roughly 500 mA – well below the 1 A level specified by the vendors of the following adapters:

LogiLink USB Travel Charger Combo Kit PA0008A v2.0

This (white) mains power supply is quite unstable and NOT suitable to power the Raspberry Pi. It will provide about 4,3 – 4,6 V under load / in idle mode, at about 0,5 A of current being drawn. We’ve tested two supplies of this series. Another downside: the USB plug will not have a good, secure fit in it’s outlet.

We’ve also seen huge oscillations in the voltage with it – ranging from about 3,9 V to 5,1 V.

All in all, the Raspi will display the known symptoms (see above).

The car adapter (which converts 12 V to 5 V) also runs at 4,74 V with the Raspberry Pi in idle moad (no load).

The package states that it will provide 1 A. Maybe. But probably not at 5 V.


FRIWO FW7713 (230 V -> 5 V)

This adapter performs better than the LogiLink one, still voltage levels drop below the magical 4,8 V level. Under load we have measured 4,68 V / 4,62 V for instance. (With a nominal 460 mA load from the Raspi).

The USB plug has a more secure fit than in the Logilink counterpart (see above).

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