Wednesday, September 02, 2009

Getting 9 Volts from USB

USB Adaptor 5 Volt to 9 VoltThe "USB Mobile Phone Adaptor Kit" I ordered from Swamp Industries turned up in the mail the next day. The kit is impressive, with both car and mains USB adaptors to supply power and assorted plugs for differnet phones . However, the USB Adaptor, which boosts 5 Volts from a USB socket to 9 Volts, does not supply sufficient current to run my Huawei D100 3G Router and HUAWEI E169 3G USB modem.

The modem with the US wireless 3G modem plugged in uses about 270 mA, which should be just within what the USB adaptor supplies (300 mA +-5% at 9 Volts). However, the router uses up to 500 mA for a few seconds when it first starts up. When I turn the router on it starts to boot, then the green light on the USB adaptor turns red (presumably to indicate it is overloaded) and the modem goes off.

One good point is that the adaptor doesn't seem to be harmed by the overoad and is able to protect itself. So I did a quick calculation and decided that a 80 uF capacitor would store sufficient power while the router boots (the calculation involves Amps, Volts, Watts, Jules, and Farads). So, in theory, if I connect the capacitor (which will cost about $2) across the power terminals of the DC supply from the USB adaptor and wait a few seconds before turning on the router, the capacitor will be charged up with enough power to supply the router when it starts and it should then run fine.

If there are no more postings for a few days, that will be because my netbook has blown up, while trying this. ;-)

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