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I bought an old wall plug that outputs 5VDC and maximum 2.5 A and thought I'd make a USB charger. Considering the amperage I figured I might as well put two outputs, but now I'm not sure how to connect them. Will it work if I just wire the + and - connectors of the USB ports in parallel?

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  • \$\begingroup\$ "Just work" and "be in USB specification" are two very different things, FYI. There's a lot of detail behind USB power delivery, even for Type-A ports. \$\endgroup\$ Aug 28 '16 at 15:07
  • \$\begingroup\$ No... there isnt. Even today there is nothing special about usb power delivery other than 5V +- 0.25V and data pins tied together. \$\endgroup\$
    – Passerby
    Aug 28 '16 at 16:15
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A parallel connection should be fine. A serial connection would try to to split the voltage between the two devices, and thus wouldn't charge the devices at 5V.

Keep in mind that with some of the wall plugs there's going to be a set of resistors connecting to the D+/D- lines of the output USB-A. This is so the phone "knows" how much current it can draw from the charger.

Here's a reference as to what I mean by the D+/D- lines. I should mention that the link is specific to one brand, but this is common practice for most others as well.

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  • \$\begingroup\$ In general terms, all dedicated chargers should short-circuit D+ to D-. I know mfgrs don't always "like" following the standards nowadays, but the standard is that shorting the two data pins signals the device that it's connected to a dedicated charger (instead of being connected to, say, a computer which it has to "ask" how fast it's "allowed" to charge from) and can draw as much power as it wants, limited only by the supply being "drawn-down" to the device's threshold voltage (usually around 4.5v). \$\endgroup\$ Aug 28 '16 at 15:07
  • \$\begingroup\$ I don't need to charge some specific brand, it's just a Google Chromecast audio and a USB powered speaker. So I'll go with the standard: connect the data lines together. Thanks! \$\endgroup\$
    – jswetzen
    Aug 28 '16 at 15:52

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