Limit current without affecting voltage

Thread Starter


Joined Nov 27, 2009
Hey haven't posted here in a while,

Anyway I'm interested in making a phone charger via my laptops USB port.
The reason why a USB is good is because the output on my charger is 5V at 800mA.
However a USB can only provide 500mA, less than that if that.

So i decided 300mA would be a good safe option to choose... (a little more than half the max rating)

So how do i limit the current to 300mA without reducing the 5V going to the phone?

You cant particularly use resistors because that would drop the voltage, wouldn't it?


Joined Jan 11, 2009
Q: "So how do i limit the current to 300mA without reducing the 5V going to the phone?"

A: You may not be able to ( or need to ) it depends upon the V vs I characteristic of the phone in changing.


Joined May 9, 2009
First, the 500mA limit on USB ports is rarely implemented. You can usually draw 1A from them without tripping them. They have short circuit and overload protection built in anyway. In my experience this is shared between ports, so a failure in one device will shut down all other devices but it does not affect the computer.

It is next to impossible to drop current without changing voltage, however, many chargers short the data lines on the USB cable together to notify the phone that they can provide a lot of current, whereas the computer doesn't, so this may be enough to tell the phone to go to a slow charge mode.

John P

Joined Oct 14, 2008
I suppose you could have a low-resistance logic-level MOSFET that's turned on most of the time, and an amplifier checking the voltage drop across a small resistance and limiting the current flow through the FET if it gets too high. Sounds reasonably straightforward, but there would be a small amount of voltage loss.


Joined Apr 2, 2009
If one can charge a phone via USB by using a data cable than U do not need a separate charge.

If u are planning to build a charger cause ur phone does not charge when connected using USB data cable, then it will never charge thru any or modified USB charger that has a current limitation at 500mA.

Point is if the phone has the capability to charge from 5V at 500mA, then u do not need to make anything. Just connect the appropriate USB cable and use a proper USB port


Joined May 9, 2009
A USB port will not give more than about 100mA. Unless the device are able to negotiate more current
This is true but it is rarely implemented, which is why you can get devices like mini hotplates working on USB with no enumeration chip.


Joined Jul 26, 2010
There are plenty of bench power supplies that are capable of limiting current while keeping the voltage at the same level instead of just shutting down so obviously it can be done. I have one at work that operates in that manner.

Might not be so hard to do if I thought about it for a while but it's late.