Building a simple USB Charger

USB charging is currently the most universal method of charging an electronic device. This article details the process of construction of a cheap and simple USB charger which can be used to charge cell phones, iPods, mp3 players, cameras, etc and almost any device charges through an USB port of your computer.
USB normally contains 4 lines – Vcc(+5V), Data-(D-), Data+(D+) and Gnd(Ground) . The Vcc and the Gnd line are used to power a device and the D- and D+ line are used to transfer data between the host and the device. A USB standard compliant host is expected to maintain 5V within a range of ± 0.5V and provide current of 1 unit load (I unit load = 100mA of current), unless requested for more current. To build a USB charger all you need to do is build a circuit that outputs current within the USB specification.

The Power Source

Choosing a power source will be the first step in building the charger. There are quite a few options available here. One option is to use 2 AA or AAA size batteries that output 2.4V and boost up the voltage to 5V. These batteries are quite popular and easy to find and they occupy less space and contribute a lot in making the charger small and portable. But they have drawbacks – I have one such charger that really heats up within minutes of connecting it to any device and the worse still, the batteries don’t last longer than than 4-5 minutes.
Another option is to use a pack of 4 Ni-Mh batteries. 4 Ni-Mh batteries output something around 5V depending on their charge. Hence, they can be connected directly to the power pins of the USB port. But the drawback is that 4 Ni-Mh batteries occupy a lot of space and they output voltage based on the amount of charge remaining in them.

Note: If you are using Ni-Mh batteries I would suggest you to use batteries of capacity greater than 2000 mAh. Ni-Mh batteries, though rated at 1.2V, output something around 1.4V when fully charged and 1V when fully drained. This variation is most for Ni-Mh batteries of low capacity(less than 1000 mAh ) and least for batteries of high capacity(greater than 1500mAh). A 2000mAh battery will output 1.3V when fully charged and 1.1V when fully drained.

Another popular power source is a 9V battery. The advantage is it is easily available and cheap. The drawback though is - you will have to regulate the 9V to 5V and only then supply it to the device.
For my charger, I will be making use of a 9V battery and also provide an option for connecting 4 Ni-Mh battery pack to be used as the source.

Voltage Regulation

For voltage regulation, the popular linear LM7805 regulator can be used because – it outputs a constant of 5V, is widely available and comes cheap. The only problem is that there will be a lot of energy wastage if the 7805 is used. The 7805 regulates voltage by converting the input voltage to 5V and the rest to heat. In the process of charging a USB device at 5V by providing it a current of 100 mA, the 7805 will be wasting 0.1*(9 - 5)W =0.4W and have efficiency of( 5V/9V) 55% !(In reality the efficiency is much more because the 9V battery will provide 9V only for the first few minutes and then slowly over time drop down its output voltage to nearly 6.8V before draining completely).
But still I went ahead with using the 7805 because I wanted my charger to be a simple and cheap (efficiency was quite low in my list of priorities). Actually charging any battery with another battery is itself not efficient because you will have wasted energy in charging your source battery and again will be wasting some more energy when charging your USB device’s battery through the source battery!

What about the D- and the D+ lines?

Just providing a 5V across the Vcc and Gnd lines will charge almost most USB device but it is not the best way of charging the device. By just providing a 5V across Vcc and Gnd lines and leaving the D- and D+ lines unconnected (technically called floating) will charge the device at only 100 mA. Charging a battery at 100 mA is a slow process. Hence, we have to tell the device that it has been connected to a charger and that the charger can provide more current. Unfortunately, there is no standard way of doing this and what works for one device does not work for another. For most devices, connecting the D- and D+ lines to 5V through a 100K resistor works. For, charging iPods, the D- line has to connected to 5V through a 10K resistor and the D+ line has to be connected to Gnd through a 10K resistor.

The Circuit

Here is the circuit for charging any USB device –

Here is the circuit for charging an iPod –

Construction

The pictures of the charger I built is shown below (I added a 8mm High Brightness White LED to the board as I had a little space left on the board and didn’t want to waste the space).



You will have to attach a heat sink to the 7805 if you want to charge your device with more than 500 mA of current. Optionally, you can place a 0.1uF capacitor across the power pins of the USB connector as close to it as possible to reduce any noise in the power lines.