Hi guys..

with USB devices - say for example my HTC One X or the Samsung Galaxy S2 etc.. they just look for a micro USB to charge from etc..

what's I'd like to understand it - the smarts, for charging the battery inside - are they done by the phone, or by the USB charger external.. ie if I could just hookup my simple old 0-30Vdc up to 5Amp bench power supply.. set it to 5Volts (accurately) and then connect that to the right pins on the USB cable.. is that at good as anything ..? ie from the perspective of the phone, looking back at the power supply - is it just looking for a dumb 5Vdc (clean, no ripple etc) ..??

this then I guess leads on to the greater question - why can't I just take a 12V input, put it accross a 7805 (to-220 package) and then put the output into my phone.. what is wrong with that etc..?

I have a 12V charger.. which just gives you a USB output - there is quite a bit of circuitry in there.. just wondering what else it would do on top of the 7805 etc..?

I hope my question is not too confusing.. just trying to get my head around it a little.

Thanks!

 

There are three USB specifications — USB 1.0, 2.0, and 3.0 — but we’ll be focusing on USB 2.0, as it’s by far the most common variant. We’ll point out where 1.0 and 3.0 are significantly different. The other important fact is that in any USB network, there is one host and one device. In almost every case, your PC is the host, and your smartphone/tablet/camera is the device. Power always flows from the host to the device, but data can flow in both directions.

A USB socket has four pins and and a USB cable has four wires. The inside pins carry data (D+ and D-), and the outside pins provide a 5-volt power supply. In terms of actual current (milliamps or mA), there are three kinds of USB port dictated by the current specs: a standard downstream port, a charging downstream port, and a dedicated charging port. The first two can be found on your computer (and should be labeled as such), and the third kind applies to “dumb” wall chargers. In the USB 1.0 and 2.0 specs, a standard downstream port is capable of delivering up to 500mA (0.5A); in USB 3.0, it moves up to 900mA (0.9A). The charging downstream and dedicated charging ports provide up to 1500mA (1.5A).

The USB spec also allows for a “sleep-and-charge” port, which is where the USB ports on a powered-down computer remain active. You might’ve noticed this on your desktop PC, where there’s always some power flowing through the motherboard, but some laptops are also capable of sleep-and-charge.

Now, this is what the spec dictates, but in actual fact there are plenty of USB chargers that break these specs — mostly the wall-wart variety. Apple’s iPad charger, for example, provides 2.1A at 5V; Amazon’s Kindle Fire charger outputs 1.8; and car chargers can output anything from 1A to 2.1A.

So basically some chargers would have some other circuitry to support some of these specs.....

There is a huge variance, then, between normal USB 2.0 ports rated at 500mA and dedicated charging ports which range all the way up to 2100mA. This leads to a rather important question: If you take a smartphone which came with a 900mA wall charger, and plug it into a 2100mA iPad charger, will it blow up?

In short, no: You can plug any USB device into any USB cable and into any USB port, and nothing will blow up — and in fact, using a more powerful charger should speed up battery charging.

The longer answer is that the age of your device plays an important role, dictating both how fast it can be charged, and whether it can be charged using a wall charger at all. In 2007, the USB Implementers Forum released the Battery Charging Specification, which standardized faster ways of charging USB devices, either by pumping more amps through your PC’s USB ports, or by using a wall charger. Shortly thereafter, USB devices that implemented this spec started to arrive.

SO as long as you are supplying 5 volts on the proper connections on the USB port of the phone, it should be just fine, since the internal circuitry of your phone takes care of the charging of the battery and monitoring this charge.

 

I've heard some Apple products use some sort of "protection scheme" to keep you from doing exactly what you are (quite reasonably) attempting to do. So this is not always 100% effective, but should not cause any damage.

The 7805 will still need the input and output caps to stay stable. And it has a nasty self-destruct mode if you unplug the 12V but the connected device is back-supplying the 5V output: it will die immediately and permanently. The fix for that is to add a reverse diode across the 7805. Anything like a 1N400x would do fine there, as would anything Radio Shack sells as a "rectifier" diode.