When I use my tablet on my balcony I would like to have it connected to the charger, but using a long (3m) USB cable seems to drop the voltage so that it doesn't charge.

I am wondering whether I can modify a wall-wart to give (say) 5.5V, thus compensating the voltage drop. I reckon I can open a wall-wart with a hacksaw (and a great deal of care), and later glue/tyrap it shut, but is there any chance of adjusting the voltage when I am inside?

I have an electronics background, can (de)solder regular components (but not SMD), and can use a multimeter. I am also aware that playing silly-buggers with mains voltages carries the death penalty; ie I shan't do anything dangerously stupid. Is it worth my while to spend half an hour opening a wall-wart (obviously a 2A unit so that the available current is sufficient) in the hope of finding a twiddle-pot or a location where I can add a 0.7V diode, or is there an alternative?

  • \$\begingroup\$ Buy a USB plug matching each end of the cable, and wire up your own USB cable with 3m of 4mm^2 for the power pins and regular wire for the data lines. \$\endgroup\$ – Brian Drummond Sep 6 '15 at 21:33
  • \$\begingroup\$ Eh, consider using a PSU with kelvin taps and a short USB adapter instead. \$\endgroup\$ – Ignacio Vazquez-Abrams Sep 6 '15 at 21:33
  • 3
    \$\begingroup\$ Buy a 3m 120/240v ext lead and shorten your usb lead. ie move the charger closer to your tablet. \$\endgroup\$ – BenG Sep 6 '15 at 21:37
  • \$\begingroup\$ @Ignacio what is a kelvin tap? Google didn't help me much. \$\endgroup\$ – NL_Derek Sep 6 '15 at 21:45
  • \$\begingroup\$ Instead of having the voltage sense connection at the near end of the wire/cable, you put it at the far end. This compensates for the voltage drop over the wires. Four-terminal sensing \$\endgroup\$ – Ignacio Vazquez-Abrams Sep 6 '15 at 21:52

Honestly, I would not bother trying to open up the wallwart. Why?

  • it's likely to be in a snap-fit or ultrasonically welded plastic case, which will take some opening and not want to go back together
  • the circuitry inside will be cost optimised - not optimised for easy modification or repair

Your quickest bet is to either create a custom USB extension by splicing in some heavy gauge wire to lessen the voltage drops or to extend the mains voltage (into a weatherproof enclosure / socket if you're looking to make a permanent solution).

Failing that, you could look at finding a variable output power supply module and then splicing on a USB connector at the end, but you might as well just go the "bigger cable splice" route if you're going to bother cutting and splicing anything.

  • \$\begingroup\$ @NL_Derek: Having opened quite a few, the run of the mill SMPS wall-wart is based on a TL431. Very cheap but very hard to design with properly. See onsemi.com/pub_link/Collateral/TND381-D.PDF or switchingpowermagazine.com/downloads/… to get a idea what you'd be getting yourself into. \$\endgroup\$ – Fizz Sep 7 '15 at 3:47
  • \$\begingroup\$ As both you and @Brian Drummond suggest, a custom cable sounds like the best bet. \$\endgroup\$ – NL_Derek Sep 14 '15 at 20:55
  • \$\begingroup\$ True. Battery packs also work great. \$\endgroup\$ – mikezter Feb 27 '16 at 13:07

Buy a 3m 120/240v extion lead and shorten your usb lead. ie move the charger closer to your tablet.


Not the answer you're looking for? Browse other questions tagged or ask your own question.