USB Charging

[JoeWawaw]

Hi, i have a PSU that can deliver 5A at +5V. I would like to use it to charge my phone. I was wondering though, whether its high current capability would affect my phone (would the 5A be flowing through my phone, or is it somehow limited inside the phone?). I'm not sure whether to add a current limiting circuit (transistor, or even a simple series resistor). Could someone tell me whether i should add a current limiting circuit in series with the phone to limit it at a safe current (1A is what im looking for) thanks.

 

[Ian Rogers]

The phone takes what it takes... The PSU can deliver up to 5A.. If the phone only takes 2A then that's all it will use..

The problem is when it's the other way round... Trying to get 5A from a 2A source... I believe there is another issue when charging Iphones... They use resistors in their chargers, Some potential divider to D+... I haven't made one but there is documentation on the web...

 

[JoeWawaw]

ok sure thanks. But lets say the phone takes 2A, but i wanted it to only pull 1A from the source? would i add a series resistor on the V+ pin to limit the current, or would i use some sort of transistor circuit to limit it?

 

[Ian Rogers]

Why would you want to limit the current the phone needs? If your PSU is a fixed 5V then adding a resistor will drop the voltage as well as the current!... There are transistor circuits that could control the current, but you'll still have a voltage drop!

 

[audioguru]

Why do you want to limit the current? The phone uses only as much current as it needs. The charger circuit in the phone limits the current when the battery is charging. If it has a modern Lithium battery then the charger circuit in the phone shuts off and draws NO CURRENT when it detects that the battery is fully charged.

A car battery can supply hundreds of amps to the starter motor. But when you plug in your phone it draws only as much current as it needs without limiting the current with a huge resistor or something.

 

[JoeWawaw]

ok sure, i understand you guys. thanks

 

[WTP Pepper]

Some phones (I think iPhone chargers do) have resistors across the D+ & D- pins to determine the current the phone will draw from a charger. I think it's for fast charging capability some chargers can support.

 

[TaDa]

For an iPhone I believe you need to hold the data lines at 2V to tell the phone it can draw more than 500mA
You can achieve this by having a 75k/50k resistor divider between +5v and 0v for each data line - feed the data line from the join
I am unsure if you need to keep the datalines separate or if you can just join the two data lines together and use one divider
If you are charging other devices, this voltage differ - for my Galaxy Tab I used 33k/10k which is about 1.2 volts - and I definitely bound both datalines
At the time I was tempted to create some sort of switchable dongle - but I didn't;-)

 

[Menticol]

Yes, Indeed Tada.

The girl in this video explains how to get different charging speeds for Apple products. But I strongly recommend against using the fast charging mode, as it dramatically shortens the battery useful life.

 

[RCinFLA]

Smartphones and tablets generally will not charge at their maximum rate without company/model specific knowledge about the charger they are connected to.

USB charging from a generic Vbus +5vdc supply from a computer USB port or generic USB charger is often limited to less then 100 mA. This is due to the fact that most smartphones rely on the charger to perform the current limiting to the phone's required maximum charge rate based on size of cellphone's battery. The phone must be sure it is connected to its proper (usually company unique) charger because the phone is relying on charger to limit the charge current to about 0.8 times the Li-Ion capacity, mAH rate, in mA's.

If the phone does not get the proper detection that it is its proper matching charger then it reduces its charge rate to avoid overloading the computer/laptop USB port, and more important to the phone, to avoid the linear regulator in the phone from overheating. By letting the charger do the maximum charge rate current limiting, the phone's linear regulator can go into full saturation bypass mode thereby reducing the IR heating loss in the phone.

Although USB 1 and USB 2 specs allow for up to 500 mA after negotiation from initial 100 mA, many phone manufacturers find this too complicated and just limit the charge current to less then 100 mA from an unidentified charger.

When the smartphone sees Vbus of 5vdc but does not get the proper detection of its unique charger then the phone's charge regulator goes into linear mode, regulating charge current to a lower level to avoid overheating in phone. It can take much longer to recharge under this condition.

USB 1 and USB 2 standard allow up to 500 mA draw from USB Vbus after negotiation from the initial 100 mA limit. There is total crazy variations on how particular manufactures adhere to official USB specs on Vbus loading. USB 3 standard has tried to bring some consistency to this and allow for higher then 500 mA loading but it will take some time, if ever, for manufactures to comply. Some laptops, for example will totally shut down their Vbus sourcing if a connected device tries to draw more then 100 mA without proper USB negotiation. Some laptops will only allow a maximum of 100 mA draw to conserve their own batteries.

 

[TaDa]

Yikes!
I had previously thought the gadget would just draw what it needed - but I guess batteries are non-linear and could draw serious current if they are very flat.
That, further coupled with inbuilt regulators not being capable of dissipating much heat should they try to regulate the supply to the battery in some way, is not good
Now, given that the battery is always being fast charged by the supplied charger, I would presume that fast charging should be ok subject to the above regulation limitations
So, all that said and with the power wart for my android tablet not being intelligent in any way (just a plain old PSU with a resistor ladder similar to the one I described earlier) I remain confident I'm alright.....
...but depending on how clever your top of the range ipod charger is your mileage may vary.

 

[RCinFLA]

scenario may go like this:

You search the web to find out how to make your phone think it is connected to its correct charger (like above You Tube video for iPhone). You make the proper resistor divider network connected the proper way to to USB data lines. You attach this you your 5 vdc 3 amp capable regulated power supply.

The phone will momentarily draw high charge rate then drop back down to 100 mA charge rate. Reason for this is the power supply does not current limit to the phones expected maximum battery charge rate. This is because the phone does monitor the current as a safety feature. In its initial expected constant current bypass mode, the battery (something like a 1500 mAH battery) draws 1.8 to 2.5 amps which is detected by phone to be excessive. The phone reverts to linear regulation mode at 100 mA current to avoid damaging battery (or worse).

Some phones will stay lock in the low charge rate as long as the 5v from USB is applied. Some may attempt to recycle after some time but bounced back to low current when excessive charge current is detected. Some phone do cycle base on temperature sensing of the linear series pass regulator, going to high current then dropping back or shutting down charging until things cool down. It is still hard on battery with greater then 80% C charge rate.