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Power Supply for a USB Charger

Discussion in 'Power Electronics' started by Cueball, Nov 18, 2020.

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  1. Cueball


    Nov 18, 2020
    Howdy! Noob here, but long in the tooth with little experience in electronics and still trying to learn.
    I picked up a multi-port USB hub. I want to modify it to use it as a charger for USB devices including smart devices (phones, pads) and regular 'dumb' USB devices. I've been reading articles on the 'Net but haven't found what I can reconcile as a definitive answer.

    Here's the short of it. The hub has 13 ports. To stop any power going to data circuits, I've read that I should short the data terminals on each USB port. I plan to remove the USB cable that would attach to a computer.

    The load will probably include no more than 6 smart devices at once and up to 7 'dumb' devices, total at once. My reading says that the smart devices will draw 10 volts each and the dumb devices will draw 2.5 volts each, all DC. Further, I read that the smart devices are rated at 2100 mA and the others 500 mA. The power supply that came with the hub is rated at 120-240 VDC at 50-60 Hz, 0.5 Amps and outputs 5.0 VDC 2500 mA. I don't know, but I suspect that this is not adequate for a fully loaded charger.

    The question is what should the specs be for a power supply to handle the load; however, what follows is an explanation of my thinking that might demonstrate the fallacies. You will see that I'm not afraid to show my ignorance.

    When fully loaded there would be a load of (6 x 10 VDC) + (7 x 2.5 VDC ) or 77.5 VDC, if the information I found is correct; however, I also found a reference saying that to quick charge an iPhone would require a 20 W or higher power supply.

    I've been taught to think that batteries are rated in volts as in 1.5, 9, in small form batteries and 6 volt and 12 volt auto batteries which are meant to be used up then recycled. I have no idea about rechargeable batteries, but I would guess that the USB devices have some way to absorb as much power (voltage) as they require with some means to prevent over-charging. That leads me to believe that the batteries take in their charges over time. I also infer that the hub itself cannot tolerate too much input voltage but I don't know anything about the components on the PCB in the hub.

    From Apple, I need a supply with at least 20 W for fast charging - but I'm not hung up on that as I can always charge overnight. Would I need more wattage to cover multiple devices? What about amperage? IOW, would the charger need to consider total wattage of all devices and total amperage?

    I can't go any further with my limited education on the subject. I'd really appreciate it if someone could tell me how to figure it out - so I'd learn something worthwhile - and give me the end goal which is the specs for a replacement power supply, assuming one is needed.
  2. Harald Kapp

    Harald Kapp Moderator Moderator

    Nov 17, 2011
    Imho a bad idea. Where did you read this? A short circuit always poses the risk of damaging a driver IC.
    If you want to prevent data transfer, cut the data lines (make them open circuit).

    That makes no sense at all. For one, devices do not "draw volts". They are supplied with a voltage and "draw" amperes. Secondly, USB operates on 5 V, neither 2.5 V nor 10 V. Where did you get that "info" from?
    That is even outside the specs of power charge via USB and makes no sense. Voltages do not add up like this. Otherwise what would be the supply voltage of mains in your appartement adding up all the 120 V (or 230 V) appliances connected to mains?

    It is not only power that matters but for quick charge to work you need an active power supply that can negotiate the required voltage and current requirements between power supply and charged device (phone). See here for a few more details. Incidentally the German version of the page I linked states that the so called quick charge cables where the USB data lines are short circuited are not suitable for the modern quick charge methods as bot data lines are required for negotiating. Could it be that this is where your info about shorting the data lines comes from?
    You cannot, as far as I know, use quick charge vi a a hub unless the hub provides a quick chare method by itself (I know of no hub that does this). The higher voltage from a quick charge charger would destroy the hub (which is designed for 5 V operation only).

    My advice: Instead of tinkering with the hub and in the end risking loss of devices due to overvoltage, use dedicated chargers. These don't have to come from Apple if you wan to save money, but be aware that the use of non-Apple chargers is not undisputed.
    That advice is valid for other smart devices, too.
    davenn likes this.
  3. Cueball


    Nov 18, 2020
    Thanks for the response. I warned you that I was about as dumb as a whole box of hammers when it comes to this topic. I've always been confused about what is consumed from electric current - volts, amps or watts. I think I understand impedance, sort of like drag on a boat or airplane.

    As for shorting D+ and D-, I read about it originally in a DIY article but searched further and found this (from a USB standard, Battery Charging Specification, Rev 1.1, 4/15/2009 (BC1.1),):
    The ultimate question is whether or not I can turn this hub into a Dedicated Charging Port array as defined in the spec. Tell me if I'm wrong, but doesn't this come down to load and power supply?

    I searched again today and found information on a ZDNet page stating that the iPhone X "charges at the usual 4.85V/0.95A (give or take)." I also read (from a different source) that USB 3.0 calls for charging downstream and dedicated charging ports provide up to 1,500mA (1.5A). Because the USB 1.0, 2.0 and 3.0 spec all call for 5.0 VDC, I've inferred that this applies to the smartdevice as well. Starting with a given that RAPID CHARGING IS NOT A REQUIREMENT, how would one use the information above to determine what spec the input power source should have?

    Again, being willing to demonstrate my ignorance for the sake of learning, I posit the following: If voltage is a measure of pressure (at 5.0 V constant Direct Current, or DC, for USB), and amperage is a measure of power consumed (by the batteries being charged, presumably) at a given voltage here 5.0VDC - , would I not add the amperage that would be consumed if all ports were in use at once to determine load? (I know I confused this earlier by adding volts.) Or, conversely, is the load a function of each devices' amperage needs delivered over time but not exceeding a fixed limit, such as 1500 mA (1.5A) or 2500 mA (2.5A)? I'm seriously trying to learn here. I used the 1.5A number from the specs above and the 2.5A from the spec on the power supply that came with the hub.

    I really appreciate your taking the time to respond to a complete tyro's questions. If you would like to point me to where I can educate myself, I shall certainly understand.

    Thank you, again.
  4. Harald Kapp

    Harald Kapp Moderator Moderator

    Nov 17, 2011
    Yes, but in your original post you added voltages.
    Yes. A device will draw as much amperes as it requires as long as the power supply is rated for that current or higher. Thus e.g. a load rated at 0.5 A connected to a power supply rated at max. 1.5 A will still draw only 0.5 A.
    You need to match voltages (easy here as we deal with a 5 V system if we disregard any modern quick charge options) and the max. current of the power supply needs to be rated to as much or more as the total of the max. currents of all devices attached.

    That said in theory your 13 port hub could supply 13 devices with 1.5 A each if the power supply to the port can deliver 13 × 1.5 A.
    There's acatch, however: The hub has definitely not been designed with 20.5 A on the power supply rail in mind. If you're lucky, there's a fuse that will trigger under this load. If there's no fuse (guess how many hubs are fused?) chances are that the traces on the pcb cannot carry that much current and will burn.
    Worse if you reach out for 2.5 A charge current ...
  5. Cueball


    Nov 18, 2020
    Thanks, again, Harald!!
  6. dave9


    Mar 5, 2017
    No offense intended, but you should just buy a ready made, multi-port USB charger. It's not just a matter of having enough volts and amps, rather modern devices have quick charging tech that benefits from an identification of the charger capability. Plus, it may already have polyfuses, 0-ohm resistors, or thin traces to act as a poor-man's fuse so they blow if you attempt much past 500mA.

    I could see it as a learning exercise, but what you will learn when finished was that it's not worth the bother to reinvent the wheel when purpose specific USB charging widgets already exist. A sufficient power supply is not much cheaper than the entire charging widget with the PSU built in, PLUS, your hub barrel jack (assuming you'd try to use it, when the PSU input would "probably" need soldered after it instead) probably isn't rated for the current anyway. A better learning exercise is to start out with constructing circuits that you're not going to attach your expensive electronics to. ;)
    Harald Kapp likes this.
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