Schematic Reference

Power Supply For USB Devices

More and more equipment is sold that runs off internal rechargeable batteries. Although a matching charger is usually supplied in the package, there are also devices that can only be charged via a USB port. That is not surprising in the case of USB MP3 players, which have to ‘dock’ in the PC anyway for some time for the purpose of file transferring. Still, the same ‘feature’ can be a serious disadvantage, for example, on ‘computer-free’ holidays. Sometimes it makes you wonder how simple the solutions to such problems actually turn out to be. After all, if it’s just a supply voltage we’re after, then a USB port is easily imitated.

The circuit shown here is nothing but a 7805 in a dead standard configuration. The innovation, if any, might be USB connector to which the MP3 player can be connected. The 7805 comes in different flavors most devices can supply 1 A, but there are also more advanced variants that achieve up to 1.5A. Because a USB device is never allowed to draw more than 500 mA from the port it is plugged into, the circuit shown here should be able to supply charging and/or operating current to up to two (or three) USB devices at the same time. The input voltage may be a direct voltage of anything between 7 and 24 volts, so for use at home or abroad a simple wall cube with DC output is sufficient.

Another useful bit to make yourself might be a cable with an inline fuse and a cigarette lighter plug so you can tap into a vehicle supply (note that this may be up to 14.4 V with a running engine). At an output current of 1 A and an input voltage of just 7 V, the 7805 already dissipates 2 watts. Assuming you’re using the most commonly seen version of the 7805, the TO-220 case with its metal tab will have a thermal resistance of about 50 °C/W. Also assuming that the ambient temperature is 20 °C, the 7805’s internal (chip) temperature will be around 120 °C. In most cases, 150 °C is the specified maximum, so ample cooling must be provided especially in a car and with relatively high input voltages.