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Topic: Several usb cables supplying power? (Read 1 time) previous topic - next topic

optimistx

Is it good practice to take power from several usb cables to connected Arduinos?

The intention is to use a laptop with about 4 Arduinos when bicycling. The laptop battery would be the main source of power for everything (except negative voltages).

I have connected the ground pins of the 4 Arduinos together. I have not connected their +5 V together, but each arduino uses its own usb power cable from one laptop (or one desktop) computer.

I need to measure voltages in millivolt range, so "ground loops" (?) should be avoided. Arduino's might need more power than one usb cable can provide.

So far everything has worked without smoke, including sending data with spi between arduinos and with print commands to the desktop. Perhaps lucky?

What about connecting also 5 Volts from the usb cables together? No?




retrolefty

Quote
What about connecting also 5 Volts from the usb cables together? No?


I would not. Each PC's USB port has current flow protection and wiring all the arduino's +5vdc together would probably cause an imbalance or confusion of current flow and possibly cause the PC to shutdown a USB port(s). So maybe not damage, but who knows for sure, just not something I would do to my laptop.

Lefty

Far-seeker

I agree with retrolefty.  Theoretically the USB ports could be made to handle the proposed situation.  However, you really don't have a way to know if that's true, and trying it could mean seriously damaging your laptop.  Furthermore, given the desire to reduce volume, mass, and production costs; I doubt most laptop manufacturers would make the USB ports that robust in a consumer model (a ruggedized model intended specifically for industrial/military/remote field work could be another matter though).

An alternative would be to use a commercially available back-up power pack made to recharge multiple portable devices simultaneously, like this one (I haven't used this specific model, it's just an example).  Or wire plugs for each of the Arduinos' barrel jacks to a separate battery (perhaps a spare laptop battery so you don't have to worry about a separate charger) and, depending on the battery's nominal voltage, some step-up/step-down circuitry. 

PeterH

Your laptop probably only contains a single powered hub and you may find it gets unhappy if you ask multiple USB ports to go into high current mode at the same time and then try to draw the maximum rated USB current from them all. You can only try, but I wouldn't be too surprised if you find that some of them revert to low current mode resulting in a brownout at your Arduino.

You might also need to take some additional steps to keep the USB ports powered up while you're cycling. You should try to avoid keeping the disk powered up, but I suspect you may need to keep the laptop awake to keep the USB ports in high current mode, and you may need to do some poking around in the power management features to achieve that.

It may not be the most convenient solution, but it seems to me that providing a separate battery capable of meeting the total current requirements of your Arduinos may be the safer bet here.
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optimistx

Very useful viewpoints everyone, thanks.

The laptop is Acer's mini, which has 3 usb connectors. If they give 100 mA each, that is probably enough. I do not know if 500 mA can be taken with certainty; at least one Arduino nano clone is not recognized if powered with op amps connected etc, but works well, when amps are connected afterwards.

The ground level can be connected everywhere, I assume. But I do not take risks with 5 V.

If hard disk can be stopped but display lit and usb communication going  that would be nice. The laptop is in the shopping basket with lots of soft material around, but one cannot be too careful with it.

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