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I'm trying to figure out how I can charge my wall mounted tablet using a power supply that's about 40 feet from the tablet. I could move the power supply closer, and I may do that if this proves too hard/impossible, but ideally I'd like to keep the power supply in this location as it makes things easier for me in the future.

The current setup I have:

  • A 12V/5A wall wart power supply
  • To that I have a cheap buck converter stepping the voltage down
  • From the buck converter to the tablet I have 50 feet of 18 gauge stranded core speaker wire (no particular reason for this choice, it was just around)
  • The tablet is connected via a USB-C cable using only the power leads (there's no option to connect any data lines)

Originally I hooked up my voltmeter to the ends of the speaker wire on the tablet side and dialled to buck converter in to 5V (when I did this I had the meter connected in-place of the tablet, which I later realized my have been wrong). When I hooked this up to the tablet, it recognized it was getting power, but didn't charge. I eventually remembered when you test voltage it's supposed to be in parallel, when I tried that, connecting the meter where the speaker wires connect to the USB cable, I noticed I was getting something 4.7V which I could see being the problem.

My questions are:

  • Can I just increase the voltage from the buck converter so that the tablet is getting 5V?
  • If so, how should I be testing the voltage? Meter connected where the speaker wires join to the USB cable?
  • If it's not a voltage problem, could it be a current problem? How would I even check that? If it is, is there any way to force the tablet to draw more current (E.g. Could I add a resistor somewhere or would that just increase the current draw on the converter but not into the table)
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    \$\begingroup\$ Probably your tablet will not charge unless the voltage source identifies itself as USB-PD, USB-BC, etc and since you didn't connect the data or CC lines, it doesn't know what the source is and won't charge. Try plugging it directly into the buck converter without the long wires. I bet it still doesn't charge. \$\endgroup\$
    – user1850479
    Commented Nov 6, 2023 at 17:34
  • \$\begingroup\$ you could dial in 5.25v, which is still in spec, and will sag to about 5.00v when charging, without over-volting when not charging. \$\endgroup\$
    – dandavis
    Commented Nov 6, 2023 at 23:07

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I would put a dedicated DC-in USB supply instead of a buck converter close to the tablet and run 12V from your wall wart to the USB supply. The issue with your setup now is that you are seeing voltage drop from the resistance of the long cable run which is current dependent so if your tablet reduces current consumption you could end up overvolting it. If you use a relatively cheap cigarette lighter USB supply, it should be able to deal with a bit of voltage sag and keep powering the tablet safely.

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  • \$\begingroup\$ That's an awesome idea I hadn't considered, checks pretty much all the boxes I had, I'll be giving that a try when I have a chance, thanks for the idea! \$\endgroup\$
    – amura.cxg
    Commented Nov 6, 2023 at 22:03

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