USB power (Master vs Slave)

[jono11]

USB uses a master/slave star architecture. Master supplies power.

Question:
If I plug in a usb wallwart (think cellphone charger) into my pc usb port. And the PC is a default master (supplies power to usb connected slave devices, like your cellphone). What would happen? Both the PC and Charger are trying to supply the power. Would the PC pop? Is there power protection built into the circuitry? :\

 

[pcgeek11]

They would null out. Nothing would happen.

 

[Rakehellion]

You can't connect a wall charger to your PC's USB port since they're both female plugs, and for good reason.

Is this hypothetical or something you're wanting to attempt?

 

[pcgeek11]

They make adapters and nothing would happen. The voltage would cancel out.

 

[PottedMeat]

generally connecting power outputs with no protection together is a bad idea - the voltages on opposite ends are never exactly the same. large currents could flow between them ( damaging one or both of the regulators ) given even a very low voltage differential and low resistance. many motherboards i've seen have a fuse on the +5V line though. some are even those resettable polymer fuses. sometimes they implement overcurrent & overvoltage detection.

 

[serpretetsky]

Generally, when two devices are trying to drive a line to a certain voltage, the current induced in the cable is dictated by the difference in voltage.

So if both are driving to 5v, nothing will happen (difference of 0v). It's like two pumps pumping a tank up to 30PSI. They'll work just fine at the same time as if one is completely removed and the hole is plugged.

However, devices never match voltage exactly. So if one is driving 5v, and the other is driving 5.1v, then there will be current flowing. If one pump is driving a tank to 35 PSI, and another is driving it to 33 PSI (and no more), then the eventually some of the gas will flow backwards through the 33PSI pump as the 35PSI pump attempts to attain pressures higher than the other pump can manage.


I'm not familiar with usb specs, so I can't say what kind of protection mechanisms are built in for this or if its a major issue.

 

[pcgeek11]

I agree that it doesn't make any sense to connect them that way. I do not however think any damage would be done even with a tenth of a volt difference. the USB is designed for 0.5 amperes continuous power at 5 vdc.

 

[WHAMPOM]

USB uses a master/slave star architecture. Master supplies power.

Question:
If I plug in a usb wallwart (think cellphone charger) into my pc usb port. And the PC is a default master (supplies power to usb connected slave devices, like your cellphone). What would happen? Both the PC and Charger are trying to supply the power. Would the PC pop? Is there power protection built into the circuitry? :\

Lookup A and B/mini/micro USB connectors, I doubt you can make such a connection without a special adapter. Plus both power circuits have diodes and unless you reversed the positive/negative no current would flow.

 

[serpretetsky]

I agree that it doesn't make any sense to connect them that way. I do not however think any damage would be done even with a tenth of a volt difference. the USB is designed for 0.5 amperes continuous power at 5 vdc.

Sorry I didn't catch this post. USB power connection wires range between 20AWG and 28AWG. I don't know if it's copper or aluminum, i'll assume copper.

Using http://www.cirris.com/testing/resistance/wire.html
and using 0.1 V of difference we find the following:

3 foot cable
20/28 gauge.
3.33 Amps/0.51 Amps

6 foot cable
20/28 gauge
1.67 Amps/0.26 Amps

So, if we assume the cables are copper, then for three feet of usb cable a difference of 0.1 V could make a fairly large current depending on the thickness of cable used.

For 6 feet, 20 gauge wire can still draw up to 1.67 amps.

However, this is all assuming that the usb connections would maintain that 0.1V difference while pumping that current. More than likely, the voltage would difference would lower as the current increases.

Also, once again, I dont know if there is built over current protection.

 

[shabby]

One simply does not cross the streams...

 

[greenhawk]

Generally, bad things happen.

It comes down to the design of the power supplies as some will have over voltage protection, so if you have one at 5v and the other at 5.1V, the 5v will cut out, so doing nothing. If the 5V has a over voltage check that is less than the 5.1V, it will short the power to the ground on the wire to ensure a 5V. This means the 5.1V power supply will work hard to drive the 5V up to 5.1V, generally resulting in it dieing (too much current needed). Of course, if the 5V power supply is cheap, it might not be designed to take a high voltage/current for extended periods of time, so part of it will die, and the whole power supply stops working as a safety feature.

Though if the 5V unit can take the power, the 5.1V unit will over heat trying to get up to 5.1V, which will trip out a good power supply, but kill a cheap one. The 5.1V could also have a current protection circuit which turns off the power supply if too it goes to high to protect itself. So it will turn on/off as fast as it can, or might just turn off until it is powered down (ie: there is a fault on the attached equipment, so not going to work until it forgets there is a fault).

Other things might happen, but generally it is wise not to go looking for trouble as cheap gear never dies nicely.

PS - there is a usb cable with two ends on it which look to allow this sort of setup to work, but it is designed to take this into account so is safe. The cable is a SUB cross over and acts as a null modem cable for data transfer between two devices.

 

[piasabird]

This is an interesting question. Some USB plugs are powered (5w?) and some are not. For instance a lot of USB plugs on smaller devices like tablets do not have power but they can be powered to a specific watt and amp rating. Exceed the amps rating for your device and your device goes into overload. BEWARE of USB Damage. On some motherboards all the USB ports are not individually powered except on some motherboards that exceed the USB requirements. Sometimes you can kill every USB Port all at once if all the USB ports come from a single hub/controller chip. It might also be possible if you plug too many devices in that use power that you might not get a full 5-5.1 watts. USB is capable of multiple hubs with multiple hubs.

Gigabyte has been advertising that they are isolating the power on certain USB ports. For music and microphones this reduces noise in the power circuitry. See specifications for DAC.

USB has 4 wires 2 for power and 2 for data. Putting data and power right next to each other can cause data errors if you have dirty power.

 

[piasabird]

USB OTG on a tablet???

You can use a powered HUB???

This is an interesting Video:

http://www.youtube.com/watch?v=tsRSlXJ2ZUo

This kind of defeats the purpose of using a tablet.

Android Tablet with mouse pointer and keyboard and possibly a game controller. Has some references to some useful android apps.

In sleep mode your tablet might also charge itself.