Friday, August 14, 2015

USB Cable Resistance

Just a quick post today.  I've been having trouble getting a Raspberry Pi with an attached webcam to take still images reliably.  I think it's a driver issue, but to make sure it wasn't a power problem I decided to sacrifice one of the cables that powers the Pi to make sure it was up to the job.  My concern was that the cable inside was so small it had a high resistance and when the current required for my application passed through, it would cause a voltage drop, resetting the Pi or causing it to lock up.  The cable tested was a 15cm cheap USB to micro USB cable with a braided fabric cover.

USB Cable
Micro USB cable
I initially did a quick measurement of the shield resistance.  Turns out the shields on both ends weren't connected to anything.  Not a good start.  The woven cover was then removed to reveal a plastic cable that contained 4 cores.  The ends of one of these cores was removed to allow its resistance to be measured.

USB Cable
USB cable core
I don't have a multimeter for measuring resistance values this low (accurately), so I ran 500 mA from a power supply through a single core and measured a 38mV voltage drop across the wire equating to a resistance of 76 mOhm.  That's not too bad.  I'm not considering the effects of contact resistance this is just to get a ball park figure to see if this cable is the source of my problem, and from what I can see here it most likely isn't.  The load draws about 600mA continuously which wouldn't cause a large enough voltage drop to cause the problems I'm seeing.  There's the chance that switching currents I'm not measuring are causing large drops, but I think it's unlikely.  To be sure, I tested a different high quality cable to power the Pi and got the same problems.

USB plug
USB plug solder connections

It thought the way the cable was constructed was unusual.  I'd expected some sort of crimp attachment, but I don't manufacture cables for a living, so maybe solder is the way to go.

USB cable
Wires inside USB cable

Looking inot the section of cable that I cut you can see that the middle two wires are closer together.  I assume this is because they are the differential pair of the USB cable and have a defined characteristic impedance.  In all, these cables aren't too bad.  They're fine for charging devices and supplying power to medium current draw devices.  As a side note I changed the webcam attached to my Pi and the problems seem to have gone away, which is a shame I really wanted to use the original camera.

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