Power Supply for USB Devices

Published:2011/8/17 2:00:00 Author:Amy From:SeekIC

Roman Mittermayr

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 o 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 1.5 A. 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 hove 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.

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