USB Power Supplies

Contents

USB Power Supplies

General Background

Test Kit/Method

As can be seen above, testing was performed with a breadboard configured to allow a number of resistors to be switched in parallel across the supply under test via a DIL switch package. Four (56Ω) resistors were mounted on the top-side of the board and the other four (47Ω) were mounted underneath. Virtually all of the components were items purchased, maybe, 20-30 years ago - the exception being the USB socket. Ideally I would have liked to use eight resistors of the same value but I didn't have such "in stock". The calculated and measured parallel resistances are shown in the table below - for a (constant) 5V supply this leads to expected nominal load currents of 100-800mA in 100mA steps.

Switches "On"	Calculated Ω	Measured Ω
1xxxxxxx	47.0	45.9
12xxxxxx	25.6	25.4
123xxxxx	16.6	16.4
1234xxxx	12.8	12.7
12345xxx	10.0	10.0
123456xx	8.5	8.6
1234567x	7.2	7.3
12345678	6.4	6.4

Tested Devices

Test Results


Mean slope resistance: 0.27 Ω¹, Mean Vmax: 4.93 V²	Slope resistance: 0.25 Ω¹, Vmax: 4.97 V²

Mean slope resistance: 0.11 Ω¹, Mean Vmax: 4.84 V³	Mean slope resistance: 16.1 Ω⁴, Mean Vmax: 1.35 V³
Sony Ericsson 850ma "Re-visited" (2018)⁵

Good Enough for the Pi (Models A|B1|B2)?

The power budget of the original model B Pi is associated with a nominal "self-current" of 700mA plus 200-300mA for attached USB devices. Hence a 5 Watt (5V at 1000mA) PSU should be fine. The "FX" and "TCB" devices both meet that specification and the samples measured confirm their suitability. Whilst the Sony Ericsson chargers have a "below spec." maximum current, they are the most stable of the devices tested and one has been used to power a first generation model B Pi running "headless" ie. without monitor, keyboard or mouse but with one or two USB flash drives plugged in. This is the Pi shown on the front page and on which this wiki ran for nearly three years. Whilst the "Generic" devices were specified as "5V 1000mA" they do not strictly meet this, or rather the "1000mA" appears to be their short-circuit current limit, and hence these are not recommended. Since the above measurements were performed (2012) a number of other PSU's have become available (see here for more info.), of these two are noteworthy: the Pluscom Hub PSU and the FX Factory TR9202 Dual Charger. The first is the best PSU supplied with a hub tested so far, and the second can power both a Pi and an isolated (non-back-powering) hub.