Two USB ports provide 1amp - Page 1
Forum section: electronics / projects

author: arun2118
author_rank: Newbie
timestamp: May 25, 2015, 06:30:19 am
content: I'm going abroad in a few days and will like to charge my phone and powerbank while on the plane.I know these planes will have USB ports which probably provide 500ma.I want to use two USB ports and wire two cables together to provide 1amp would this be possible?

author: Richard Crowley
author_rank: Super Contributor
timestamp: May 25, 2015, 06:37:38 am
content: It would seem unwise to fiddle with the electrical system of a plane that you are depending on for your life.Most devices simply charge more slowly if the full maximum current is not available.For example, my phone pops up a message saying "slow charge" but then proceeds to charge at a slower rate.

author: arun2118
author_rank: Newbie
timestamp: May 25, 2015, 07:05:43 am
content: ....really?probably that's what happened to flight 370?

author: German_EE
author_rank: Super Contributor
timestamp: May 25, 2015, 07:27:25 am
content: After the Swissair crash which was caused by the in-flight entertainment system you can be reasonably certain that nothing you do to a USB port on an aircraft will result in an incident. Just don't let out the magic smoke or you'll be in a world of hurt as these days the passengers fight back.

author: rs20
author_rank: Super Contributor
timestamp: May 25, 2015, 07:44:23 am
content: I'm ignoring the legality and airplane safety aspect here, although I'd be surprised if the end user could cause a problem with a couple of USB plugs... I mean, the SwissAir flight was a short circuit in the plane's wiring, the OP is talking about something that in the worst case could short out the USB lines. That's exactly like comparing shorting out mains wiring in your house versus shorting out the USB port in your laptop. Also, fwiw, what the OP is suggesting used to be standard practice with portable hard drives.As Richard indicated, typical phones will draw as much current as they can*, subject to the constraint that the supply voltage doesn't sag too much. Forms a simple but effective contract between the charger and the phone so that the phone takes as much current as the supply can supply, but no more. So if you have two isolated supplies that each supply 5V/500mA max, and have a CC mode at that point; then yes, connecting the two in parallel will give you a 5V/1A supply and your phone will charge faster. Interesting failure modes include when the supplies have slightly different open circuit voltages, and you end up with current flowingintothe lower voltage supply, which is... not something it's designed for.Addendum: if you wanted to do this properly/by the book, you'd have full-on isolated DC-DC converters, one dedicated per port (and each one behaving in a USB-compliant way), that conveyed the power over to the phone side.* If the D+/D- lines are shorted out, as they are in phone chargers.**** Apple excluded.

author: Fungus
author_rank: Super Contributor
timestamp: May 25, 2015, 09:47:27 am
content: Quote from: arun2118 on May 25, 2015, 06:30:19 amYou could always charge it before you get on.Quote from: arun2118 on May 25, 2015, 06:30:19 amIt's generally not a good idea to connect two power supplies together. There's all sorts of ways it can go wrong, eg. you can easily create a ground loop with 0.5A current and cheap-ass cables. This will feed power from one port into the other.Maybe the ports will give more than 500mA. This is 2015, many ports do (and I'm sure the people who designed the aircraft probably anticipated everybody instantly going "Whoah, dude!! I can charge my phone/tablet/whateverfor free!!!").

author: m98
author_rank: Frequent Contributor
timestamp: May 25, 2015, 11:07:29 am
content: Quote from: Richard Crowley on May 25, 2015, 06:37:38 amSo terrorists only need to short out an usb-port in the plane to crash it? Dont't let Al-Quaida see this threadNo, serious: the usb-ports of the entertainment-system should be designed to withstand all possible abuses, as you can't rely on people connecting only compliant devices to the port. A chinese fake iPad for 29€ doesn't care about USB or safety standards.

author: Richard Crowley
author_rank: Super Contributor
timestamp: May 25, 2015, 01:30:40 pm
content: I am saying that pushing the limits of "conventional" behavior during airborne mass-transit is provocative and could make cabin crew and even fellow-passengers nervous. "Trust me, I'm an electronics hacker." seems like inadequate justification.Furthermore, as @arun2118 observes, hard-wiring two power supplies of unknown design together will produce undefined results, by definition. It is not even clear exactly how that mode of operation works on the computer you are reading this on. Is this "kludgy" mode even allowed in the formal USB specs? It is considered bad design to hard-wire multiple stages in parallel without some "relief" (typically in for form of a low-value resistor) for any slight variation in the parallel paths to prevent current-hogging.

author: rs20
author_rank: Super Contributor
timestamp: May 25, 2015, 02:03:17 pm
content: Quote from: Richard Crowley on May 25, 2015, 01:30:40 pmCurrent hogging isn't an issue if you have a CC power supply, or even anything close to it. But it would be pure speculation on my part that plane USB ports behave this way. Carry-on programmable dummy load, anyone?

author: cyr
author_rank: Frequent Contributor
timestamp: May 25, 2015, 02:14:56 pm
content: Cables like this used to come standard with USB harddisks, not sure if they still do, or ever did any good.http://www.amazon.com/StarTech-com-USB2HABMY3-Cable-External-Drive/dp/B003HHK576

author: Richard Crowley
author_rank: Super Contributor
timestamp: May 25, 2015, 02:48:56 pm
content: Quote from: rs20 on May 25, 2015, 02:03:17 pmBut of course, USB does not resemble a Constant Current mode in any way.It is a Constant Voltage source of a somewhat wide tolerance of acceptable voltage.And that is the PERFECT STORM for the problem of current hogging or worse.

author: Fungus
author_rank: Super Contributor
timestamp: May 25, 2015, 03:17:20 pm
content: Quote from: cyr on May 25, 2015, 02:14:56 pm

author: Richard Crowley
author_rank: Super Contributor
timestamp: May 25, 2015, 04:19:18 pm
content: Quote

author: mushroom
author_rank: Regular Contributor
timestamp: May 25, 2015, 05:30:17 pm
content: What company ? AFAIK, charging devices onboard is just prohibited (and advertised before taking off). You have to get a "power bank" if you want to charge something on board. But be aware that most companies won't allow flat batteries on board. You can charge in airports (for free or not...).Redad the instructions about USB port on your company's website. For example :http://www.virgin-atlantic.com/eu/en/the-virgin-experience/in-flight-connectivity/laptop-power.htmlI have a laptop PSU with EmPower, but charging is disabled when in use !

author: m98
author_rank: Frequent Contributor
timestamp: May 25, 2015, 09:07:29 pm
content: How do they distinguish between charging and operating? If the battery is not replaceable, you can't stop the device charging when plugged in...Anyway, it's easier to charge the device with one of those powerpacks who take normal primary AA-cells. No charging required and you can get new energy almost everywhere in no time. Not very environmental friendly, but compared to the flight...

author: mushroom
author_rank: Regular Contributor
timestamp: May 25, 2015, 11:58:31 pm
content: For laptops, some asked the battery was removed...But laptops are not phones... After some googling, it seems to depend on companies, planes, class.Some allow USB as source for the personal screen (USB stick), or for downloading flight informations or other contents. (At least for Virgin Atlantic and Air France). Others allow charging... It depends...With Google, type your company name and USB, and visit seatguru.com.A few years ago, I probably fried an USB port with a Y adapter on a Asus P5P55D mobo. But I am using Y adapters every day, all day long, on other computers (Dell, Asus, Acer...) for about 10 years without a problem (external 3"1/2 HDDs and DVD burners).USBPower DeliverySpecification Revision 2.0, Version 1, page 35 :The USB Power Delivery Specification is guided by the following principles:1) Works seamlessly with legacy USB Devices2) Compatible with existing spec-compliant USB cables3) Minimizes potential damage from non-compliant cables (e.g. ‘Y’ cables etc.)4) Optimized for low-cost implementationsSo, if the USB ports are Power Delivery compliants, risks are minimized...Could this plane charge your phone from its USB port, and be used on board ?http://www.poweruptoys.com/products/powerup-v3

author: Electro Fan
author_rank: Super Contributor
timestamp: May 26, 2015, 07:08:43 am
content: I think it would be better to get one of these:http://www.amazon.com/gp/product/B009USAJCC/ref=oh_aui_detailpage_o00_s00?ie=UTF8&psc=1- $22, and over 7,000 reviews indicate it probably works pretty well and offers a decent valueIt will help you keep your gear charged on the plane and in the airport and while traveling anywhere else you don't have access to AC.Also, it's probably best not make adjustments to aircraft as a passenger. And for good measure it's probably good to eat the nuts out of the bag rather making a big deal about having them served in a dish.Sometimes it's best to go with the flow and take the conventional route.

author: Kjelt
author_rank: Super Contributor
timestamp: May 26, 2015, 07:25:52 am
content: Buy a phone that does not need charging every few hours, problem solved.

author: NiHaoMike
author_rank: Super Contributor
timestamp: May 27, 2015, 05:01:21 am
content: Would a "Y" cable be allowed if it ended in a proprietary connector that kept the two power rails separate? What about a standard B connector (or mini/micro B) plus a DC plug? Or what if it just had a single B type connector but had some active circuits inside to balance power and prevent backfeeding?The limitation of a USB 2 device being limited to 500mA on a USB 3 port is silly and absurd. As long as it had some way to tell the difference (e.g. checking for the USB 3 ground), it should be OK.

author: Jeroen3
author_rank: Super Contributor
timestamp: May 27, 2015, 06:24:07 am
content: Quote from: NiHaoMike on May 27, 2015, 05:01:21 amWhen a USB device plugs in. It is guaranteed 1 unit load of 100 mA. Then while enumerating, the slave can request more power. The host can deny this (because of the hub not capable of delivering more) and the slave should not draw more than 100 mA. If it does, it risks the hub to shutdown due to overload.Some PC's actually have a warning for this. But it requires an expensive USB hub implementation.

author: Fungus
author_rank: Super Contributor
timestamp: May 27, 2015, 01:34:55 pm
content: Quote from: NiHaoMike on May 27, 2015, 05:01:21 amSure, but not on an aircraft. Flight attendants can't be expected to know the difference.