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I'm going to build a benchtop PSU from an ATX PSU, and I would like to add a USB charging port on it. It should be able to power an Arduino or charge my (non-Apple) phone. (I know Apple devices require additional circuits, but I don't own any so I don't bother with that.)

Since USB uses +5V I could directly connect the +5V of the female USB connector to +5V on the ATX, and USB GND to ATX +0V, leaving the data lines unconnected.

Is this a good idea? Does this allow devices to draw as much current as they need (like a USB phone charger), or are they limited to 500 mA (official USB max current) or even only 100 mA (USB initial max current)?

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    \$\begingroup\$ Nowdays, having D+ and D- pins disconnected is a bad idea, giving you 500ma per port max. Current will be limited internally by most(if not all) modern phones. So we could and should review this extremely important question, as it remains unanswered for now. \$\endgroup\$
    – user149105
    Commented Oct 28, 2017 at 3:03

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The 5V rail will most definitely be able to supply the power that you need in your setup. For most ATX PSUs, the +5v rail has the largest current capacity of all rails on the supply. Many, if not most devices will charge as soon as there is a voltage applied to their USB port, so I'd say that aspect of your setup is also good. I wouldn't worry about a device overdraw; the current control is generally built into the device rather than the charger.

The only thing to really consider is similar to what you mentioned in your question; some devices will begin to charge at say, half of their current capacity, and only ramp up to full current once they've received some data from the charger/host device. But, this is only really an inconvenience, and most devices will charge with at least some initial current. The only device I can think of off the top of my head that won't charge at all without this enumeration is a Sony PlayStation 3 controller (but I'm sure there are more).

The final point is that this really just varies on a device to device basis. Some devices will charge with max current right off the bat, a few will be limited to initial max current, and a couple might not charge at all.

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    \$\begingroup\$ The PS3 Controller won't charge without enumeration. Most devices that require signaling use ""standard"" resistor/voltage level signaling on the d+/d- pins and sometimes id pin. \$\endgroup\$
    – Passerby
    Commented Jun 19, 2013 at 22:41
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    \$\begingroup\$ @Passerby The word was on the tip of my tongue, I swear.... \$\endgroup\$
    – Jay Greco
    Commented Jun 19, 2013 at 22:44
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Your circuit is ok. The USB connector is connected to your ATX PSU so the power limit is determined by the PSU. You can obtain the maximum current available in the 5 V line from the datasheet of your PSU. Usually there is a sticker on the power supply which gives the maximum current for each line.

Remember that modern ATX PSU need to be turned on connecting the PWR_ON cable (usually green) to the ground.

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    \$\begingroup\$ The OP is also asking whether supplying more current than the device needs is an issue. Most ATX PSU's will supply more than 500mA on the 5V rail. While it shouldn't be an issue, I do wonder if certain devices designed for a maximum of 500mA might charge more quickly than expected and generate excess heat. \$\endgroup\$
    – JYelton
    Commented Jun 18, 2013 at 18:02
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    \$\begingroup\$ Usually devices like phones are able to take only the needed current so there aren't problems if the PSU can erogate more than the requested current. If the phones were not able to take only the required current, universal chargers could not exist because not all phones need the same charge current. \$\endgroup\$
    – Oceanic815
    Commented Jun 18, 2013 at 18:06

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