Power Supply For USB Devices

  
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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
Power Supply For USB Devices - schematic

. 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 flavours 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 have a thermal resistance of about 50 °C/W. Also assuming that...



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