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Here's the situation, I have an alarm clock that is powered by a barrel plug DC connector that says "5V" next to it. Out of the box, it has an AC plug into a figure-8 connector going into a brick, then from that a DC cable going into the clock. So I got the idea, since it's 5V, I can declutter by powering it via USB-A.

I can't really buy a custom cable because 1) I can't confirm the size of the barrel plug, but more importantly 2) it is a custom L-shaped connector that fits into a molded hole going up into the clock, so if I get a generic plug it won't fit correctly. So buying a ready cable or buying a PD-adapter are not possible.

So then I thought I'll just cut off the DC cable where it goes into the brick and just solder the two wires into a USB-A 5V plug and power it via USB.

So finally my question. The clock is clearly not designed to be powered via USB. Now, the power brick can feed it up to 5V 4A, but that's because the clock has 2 USB charging ports, one can deliver 1A, the other 2.1A - I never use more than one, so it'll never actually need that much current. The clock itself probably uses less than 0.5A.

So now, if I make this cable and plug it into a 2.4A USB-A port, will the clock actually be able to draw 2.4A? Isn't 2.4A a proprietary standard that needs negotiation between the charger and the device? If so, what is the max current that a "dumb" device not designed to be charged with USB will draw?

And final question, does USB include overcurrent protection? That it so say, let's say I use the alarm clock's USB charging port to charge something at 2A, then the clock, not knowing it's powered via USB, may try to draw 3A from the 2.4A port I've connected it to. Will the port just limit it to what it can do or will there be some risk of overcurrent? And if the port limits it, will the clock know to prioritize powering itself vs. powering the USB charging port.

Sorry lots of questions, at this point I doubt anyone will bother to read, but I had to go through everything...

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    \$\begingroup\$ So, if you power the clock by usb, then just don't use the usb ports on the clock to power anything - you obviously have another usb source... \$\endgroup\$
    – Solar Mike
    Commented Jan 14, 2023 at 9:42
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    \$\begingroup\$ It is impossible to guess how the clock is designed to work unless you have the schematics that can be analyzed. \$\endgroup\$
    – Justme
    Commented Jan 14, 2023 at 10:10
  • \$\begingroup\$ The clock has probably been designed knowing it can get up to 4A from the supply so it's unlikely to handle the situation well when it can't. \$\endgroup\$
    – Finbarr
    Commented Jan 14, 2023 at 11:11
  • \$\begingroup\$ Solar Mike - that's the plan, but I'd still like to know what will happen if 1 or 2 things were plugged into the USB port(s) and tried to draw 2-3A. \$\endgroup\$
    – qbccd
    Commented Jan 14, 2023 at 12:49
  • \$\begingroup\$ @qbccd It depends on your USB-A supply. And other parts of the system. If it is rated for less than 3A, it may shut down if you draw 3A. The connectors and wires in the cable may not be rated for 3A either. We cannot possibly know what will happen when you connect things together and if your USB power supply has overcurrent limit. \$\endgroup\$
    – Justme
    Commented Jan 14, 2023 at 18:35

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If the clock doesn't get warm it probably uses less than 500mA

The clock probably does not prioritise running itself over charging but if the slupply limits the current trhe clock will probably operate at a lower voltage than the connected device will charge at, so the clock will still win.

Overcurrent protection seems like a reasonable assumption to make.

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Without power negotiations with the device that has the USB A port you shouldn't get more than 100 mA or 150 mA depending on the port version.

It should have a overcurrent protection, which will likely cause your clock swith off when it kicks in, as the voltage will drop.

It's up to the USB A device designer how they've implemented it though. I know many generic USB power supplies will deliver much more than 150 mA even without negotiating more.

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So I did it, if anyone is curious. It worked like a charm, the USB ports still charge too, but slowly. When plugged into a 2.4A charger, the ports will provide around 0.7A of power, and when plugged into a computer port, about 0.5A. This is guestimating by using the Ampere app, I have ordered a proper USB meter, but I don't expect to see much different results, it's sub-1A for sure. But that's totally fine, I don't really need the USB ports on the clock anymore and it's really nice to get rid of the bulky AC plug and power brick.

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