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I have 20 identical devices (GPS trackers- QStarz BT-1300ST) which are charged by mini-USB cables. I have no need for data transfer, which is done by bluetooth.

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I have bought two USB hubs that are powered from the wall and split to 10 USB charging cables.

But I want to build a custom wall-charger that is smaller, and doesn't have 20 ugly cables sticking out of it. Ideally, I'd like either a short, rigid cable which can support the weight of the device (just 22 grams), or an array of 20 mini-USB ports, like this pic, that are embedded or glued on a (3D-printed) structure.

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(This is sort of similar to the iPhone Speaker systems, where the phone clips onto the adapter - although I'm looking for 20 smaller devices and no speakers)

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My question: If I want to build a 20-device array charger, and each device needs 3-5 Volts of DC, can I take a 5V AC transformer with ~10 Amps, and split the cables 20 times, to get 0.5 Amps per device? I would then solder the power pins on the 20 mini-USB-ports.

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Is this in any way dangerous to me or the device, or impracticle because of readily available consumer products?

I see things like this for DC power supplies, but nothing for (mini)USB, which is probably because most people need to also transfer data.

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update 1 Device is USB 2.0. Here's the info from the manufactorer.

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  • \$\begingroup\$ split the cables 20 times, to get 0.5 Amps per device - How will you make sure some device won't draw 1A or 2? Or 5? \$\endgroup\$
    – Eugene Sh.
    Mar 3, 2017 at 14:49
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    \$\begingroup\$ @eugene if they abide to USB spec the they won't. Besides I think we can consider them low-power devices given how small their batteries might be. \$\endgroup\$
    – user1890202
    Mar 3, 2017 at 14:54
  • \$\begingroup\$ @user1890202 Which USB spec? 1.0? 2.0? 3.0? \$\endgroup\$
    – Eugene Sh.
    Mar 3, 2017 at 14:56
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    \$\begingroup\$ @EugeneSh. USB 1 and 2 dictates a maximum current of 500 mA. Cellphone chargers typically provide a lot more - but they also signal that power is available, but that is outside of USB 1 and 2 specifications. Charger incompatibilities are due to absence of a universal spec, this is however not the case when it comes to USB3 that defines a much more complex and intelligent power scheme that allows for higher voltages and currents. \$\endgroup\$
    – user1890202
    Mar 3, 2017 at 15:10
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    \$\begingroup\$ @philshem the spec does not tell how much power they draw but trust me - given their size it's not much. \$\endgroup\$
    – user1890202
    Mar 3, 2017 at 15:46

2 Answers 2

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If I want to build a 20-device array charger, and each device needs 3-5 Volts of DC, can I take a 5V AC transformer with ~10 Amps, and split the cables 20 times, to get 0.5 Amps per device? I would then solder the power pins on the 20 mini-USB-ports.

Yes. The devices will draw the current they need, not the max current the supply can provide.

Only exception to this would be a perverse device which would use the USB current limit as a constant current source to charge the battery, but that would be a very poor design. It would also probably not work with computer USB ports.

If you're in doubt, get a USB cable, cut the +5V wire, insert amp-meter, and measure current from a standard 5V 1A cellphone charger.

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    \$\begingroup\$ It's important to note for the OP that providing 20 device cables from a simple supply as imagined DOES NOT split the current. All 10 Amps would be available from any cable. That said the devices likely draw a very limited current, but if there is any fault then the current is uncontrolled. The OP should either put a series resistor (primitive I know) or some form of current limiting for each of the cables. 10 Amps is more than enough to burn up cable should there be a fault in the connection. \$\endgroup\$
    – Jack Creasey
    Mar 3, 2017 at 18:30
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    \$\begingroup\$ You're right. A little polyswitch or even a fuse should work well. A more elaborate solution would be a current-limited 5V switch like this: ti.com/product/TPS2061C \$\endgroup\$
    – bobflux
    Mar 3, 2017 at 18:52
  • \$\begingroup\$ The TPS2061C would be ideal... good suggestion. It's best for him now to put a 5 V regulator in the powers supply since the current limiter does have input voltage limits itself. Now it starts to look like something that would work fairly well. Though now the OP might well be better of buying instead of building: alibaba.com/product-detail/… \$\endgroup\$
    – Jack Creasey
    Mar 3, 2017 at 19:21
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Your avatar has two giraffes... Maybe we are relatives!

The question about building this hipotetical device has two sides: electrical and mechanical.

Electrically, it can be built, but to be reliable it would need current limiters, polyswitches and fuses, as peufeu and Jack Creasey has said. This would add much complexity.

Mechanically, you could build one array of mini-usb male connectors, but the force applied in this connectors in each operation would render some of then unreliable after some cycles of operation. Maybe you could 3D print something, but this is not so simple: who would project this 3D part, and who would create the 3D file?

Definitely, this is not so easy. If you think that is ugly to have 20 usb cables from the hub, await to see how this would-be device can be really ugly that you'll return to the USB hubs very fast.

If you still don't want the USB hubs, and don't want to buy a ready made product, I think that the simplest alternative would start from a cable-adapter like this one enter image description here Cut the original plug from each output, and solder a mini-USB connector to each wire.

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