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I am trying to Charge a USB connected device through a computer without transferring data.

An USB 2.0 cable should have 4 wires: Vcc, Data+, Data- and Ground. If I only cut the Data wires and let the Vcc and Ground wires connected, will it still be able to transfer power or will the power cables be shut off because of the disconnected Data wires?

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  • \$\begingroup\$ A USB device has to start somehow... \$\endgroup\$ – AaronD Jan 7 '16 at 2:51
  • \$\begingroup\$ ...but with that said, the spec says that a device must announce itself via the data lines shortly after drawing power. \$\endgroup\$ – AaronD Jan 7 '16 at 2:52
  • \$\begingroup\$ How that's enforced, if at all, is up to the specific port driver. Given that USB fans and keyboard lights exist, I'd expect that most ports don't care, even if it technically violates the spec. \$\endgroup\$ – AaronD Jan 7 '16 at 2:53
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    \$\begingroup\$ Possible duplicate of Will cutting only the Power Lines damage the Data Lines or the Device? \$\endgroup\$ – uint128_t Jan 7 '16 at 3:57
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    \$\begingroup\$ @uint128_t it's a direct opposite of that question. Related, sure, dupe, no way. \$\endgroup\$ – Passerby Jan 7 '16 at 6:39
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It depends on the device. And on the source.

Usb spec states that a device should only use 1 block of power until it enumerates with the host. Then it can request up to 5 blocks. Each block is 100 mA. It should not pull more than 5 blocks.

In practice, very few, if any hosts limit a usb port to 100 mA at all. Some do limit ports to 500 mA (Apple computers for one, though they do allow iDevices to request more in a Apple only standard, so they are physically capable just not normally allowed). Some USB ports have very little power protection and may be hooked up straight to the 5V power source.

Now usb peripherals also have requirements for power. Dumb devices may power directly from any power source, 5V or different. These dumb devices don't even have the data lines connected. Older cell phones also allowed dumb charging. As time passed, more and more restrictions come into play. Some devices monitor the power pins for regulation, and won't work on anything outside of usb spec power (4.75 to 5.25 volts).

Some devices won't work unless a resistor pairing setup is on the data pins. Apple started this, and others followed, and there are competing standards. You can't plug a dumb 5V power supply into an iPhone and get it to charge. It will state unauthorized charger. Some devices require enumeration before charging, but this is rare. The PS4 Six axis controller does this.

And some devices will treat a dumb charger as unsafe, and limit charging to 500 mA or less. Most modern cell phones do this. If the data lines do not have a resistor indicating it is a high power charger.

In short, a usb cable plugged into a computer without the data lines will either not work, work slowly, or work fine, depending on your computer and your device.

As a side note, there are commercial usb cables that have a data or charging switch, allowing you to disconnect the data lines to prevent data communication. They typically switch in the resistor setup, but it varies.

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yes, but you are breaking the rules if you take more than 100mA

the device may not like recognise the computer as a charging source with the lines cut, so charging may be much slower.

there are adaptors that make the compute pose as a 2A charger which will give a fast charge.

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    \$\begingroup\$ I'm sure the computer is thrilled with those adaptors. The smart ones may shut off that port while the cheap ones are hardwired to a constant 5V supply that is probably shared with other stuff. If you drag that down by excessive load... \$\endgroup\$ – AaronD Jan 7 '16 at 2:59
  • \$\begingroup\$ there's possibility of melting the wires from the motherboard to the front panel, or even burning up traces on the motherboard. A 20A 5V supply is hard to drag down. OTOH 2A is probably not enough to do that, but a short-circuit could. \$\endgroup\$ – Jasen Jan 8 '16 at 4:14
  • \$\begingroup\$ I was thinking more like small, cheap devices that simply assume that certain rules won't be broken so they don't have to design an enforcement mechanism. Probably phones, tablets, and a fair number of laptops. \$\endgroup\$ – AaronD Jan 8 '16 at 4:42
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Cutting the data lines will let the device charge in most circumstances. Some devices are too smart and want to negotiate with the host device to determine maximum current available, but even these I suspect will default to a low value if comms isn't available.

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