Hi All
I bought myself one of these which measures instantaneous and peak voltage(low)/current(high).
http://www.maplin.co.uk/p/maplin-usb-power-meter-n55ceMy previous version, was not capable of capturing the peak values, and only gave instantaneous,
This does not capturethe worst case scenario.
I wanted to see different configurations of USB cable on PilotAware, it has confirmed
what I had believed regarding power supply/usage.
The scenarios are as follows
1. USB Auxilliary socket noted as 3.1Amp rating, I bought this
http://www.ebay.co.uk/itm/3-1-Amp-USAMS-Twin-Dual-2-Port-USB-12V-Universal-Car-Socket-Charger-UK-New-/321686700107?hash=item4ae605804b2. Tested against the following 3 cables
2.1
http://www.ebay.co.uk/itm/BLACK-50cm-20AWG-Premium-Fast-Micro-USB-Charger-Cable-for-Android-Phones-Tablets-/271936509656?hash=item3f50add6d82.2
http://www.ebay.co.uk/itm/USB-to-Micro-USB-with-ON-OFF-Switch-Cable-Charger-Power-for-Raspberry-Pi-Android-/3513304157042.3
http://www.aliexpress.com/item/10pcs-1M-Micro-USB-2-0-Male-A-Sync-Data-Charge-Cable-For-HTC-Android-For/1750791723.htmlHere are the results for Peak Voltage (low) and Peak Current (high)
Measured at WiFi Dongle
2.1 PV=
4.83V PC=0.24A
2.2 PV=4.67V PC=0.24A
2.3 PV=
4.20V PC=0.24A
Measured at DVB-T Dongle
2.1 PV=
4.83V PC=0.18A
2.2 PV=4.67V PC=0.18A
2.3 PV=
4.20V PC=0.19A
Measured at Power Source
2.1 PV=5.13V PC=1.16A
2.2 PV=5.13V PC=1.17A
2.3 PV=5.13V PC=1.21A
When the Voltage level drops to around 4.67V, the RPi is approaching a brownout.
so cable 2.3 is really bad, 2.2 is OK and 2.1 has some good margin.
The interesting thing is the range the voltage drop from best(2.1) to worse(2.3) is 0.63v
in this case that is massive 15% difference.
One other thing worth mentioning, the total current draw at the USB is (0.24 + 0.18) 0.42A
There is a (default) cutoff limit of 0.6A on the RPi, this is quite close.
This cutoff point can be doubled in Software to 1.2A, and sounds like a good idea,
especially if future enhancements will be to make use of external GPS etc
Thx
Lee