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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?

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closed as off-topic by Leon Heller, Null, Nick Alexeev Sep 7 '15 at 3:36

This question appears to be off-topic. The users who voted to close gave this specific reason:

  • "Questions on the use of electronic devices are off-topic as this site is intended specifically for questions on electronics design." – Leon Heller, Null, Nick Alexeev
If this question can be reworded to fit the rules in the help center, please edit the question.

  • \$\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
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    \$\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
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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.

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

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