Testing Powered USB Hubs.
by: Jim Hayden, Newark / element 14 Technical Support
Recently I had the opportunity to test 2 powered USB hubs from Digital Innovations. The main purpose of the test was to see if the voltage supplied by the hub to the Raspberry pi would stay above the recommended minimum for stable operation when the hub was loaded down with peripherals.
To do this, I started off by plugging the powered USB hub in question to the micro USB power port on the Raspberry Pi using a USB to Micro USB (in this case I used the Samsung brand cable that came supplied with my phone) and powered on the device and measured the voltage between TP1 and TP2 (with nothing connected to the USB type A ports on the Raspberry Pi). I then connected the same powered USB hub to one of the USB type A ports and measured the voltage at the same test points. Finally, one at a time I plugged in a wireless keyboard and mouse dongle, wireless N dongle, 1 Gb USB stick into the USB hub, and a cat 5e cable connected to the local lan measuring the voltage between TP1 and TP2 after each addition. For the USB stick I measured the voltage while the Raspberry PI was accessing the device. For the Wireless N module I measured the voltage when the raspberry pi was idle and while it was downloading a webpage. For the cat 5e cable I measured the voltage while the device was downloading a webpage. In each instance all the previous peripherals stayed plugged into the module. My results are below.
4 port USB Hub: (Newark part number: 32T0469)
· Power supply for hub is rated for 5 Volts 2.0 Amps
· Initial startup, nothing connected: 4.913V
· With powered hub plugged in to data port and power port: 4.923V
· With wireless keyboard module plugged into hub: 4.910V
· The above with the USB wireless adapter plugged in: 4.87V (idle) 4.85V (downloading)
· The above with a USB memory stick plugged in: 4.83V (when reading)
· The above with an Ethernet cable installed: 4.79V (downloading)
7 port USB hub: (Manufacture model: 4390300)
· Power supply for hub is rated for 5 Volts 2.5 Amps
· Initial startup nothing plugged in: 4.89V
· With the Hub & wireless keyboard & mouse plugged in: 4.88V
· The above with the wireless adapter plugged in: 4.87V (idle)
· The above with a USB memory stick plugged in: 4.83V (when reading)
· The above with Ethernet plugged in: 4.79V (downloading), 4.83V (idle)
The driver for the wireless N adapter failed when I was testing the 7 port hub which prevented me from getting the voltage measurement during a download. Throughout both experiments the Raspberry Pi stayed stable (I was using the GUI supplied with 10-28-2012 Rasbian image). Your results may vary depending on the losses in the power cable (USB to micro USB cable) you use for your Raspberry Pi.
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