Battery powered USB...What's the easiest?

[ronvegas]

Hi,

I'm new to this stuff, so will apologise in advance, if needed.

I am wanting to power my USB modem (voyager 105) which I use for work as a service engineer to check for a DSL connection. Normally, I would connect this to my laptop to power the modem, but what I am looking for is the ability to do away with the laptop at times, and gain the power needed from a more convenient source.

I saw the homebrew USB chargers that you can make, so bought some of these parts from my local electronics store to use with a 9v battery.

(1) I got a 5v voltage regulator (the guy in the shop recommended the 2amp one, instead of getting the 0.5A one I planned on getting) He said that the bigger amp will allow the reg to run cooler and not thermal cut-out, but to ensure the device doesn't want to draw more than the 0.5A rating of USB or it could fry the modem.....is this right?

(2) He suggested using AA batteries to provide the 9v, instead of an actual
9v (PP3) battery, for longer life, and that a 9v PP3 battery will not last very long at all - how long might both examples last?

(3) Will the device, BT voyager USB modem have a voltage tollerance, ie. could I just strap the 9v battery to a butchered USB lead, or will 9v kill the modem?

(4) Or if I used, say, 4 AA batteries giving 6v, and strapped that to the modem would just 1 volt over be OK, or would this still fry the modem?

What the important bit, the volts or the amps, is it the amps that do the frying of devices?

I wanted this to be as easy and as straight forward as possible, I don't want to get to involved or build the best homebrew electronic, semi-conducted device known to man.


I want the easiest way to power my USB modem from batteries, that will last the longest, without frying the modem instantly (life shortening is acceptable!)

Thank you in advance for any help

Ron.

 

[audioguru]

The modem draws only as much current as it needs if its supply voltage is correct.
You don't say how much current the modem uses so we can't calculate battery run times.

a 9V battery supplies a low current for a short time. A 5V regulator needs a minimum input voltage of 7V to 7.5V and the voltage from a 9V battery quickly drops to less then the regulator and the modem won't work.

Four AA alkaline battery cells are 6V when new and 4V when not. the modem might stop working when you need it.

 

[ronvegas]

Thanks for your reply Audioguru.

Unfortunately, There is no voltage/Amp spec on the modem, so unsure what amps it needs. Considering it is purely USB powered, could we assume it will draw no more that what the USB provides (5v / 500mA)?

From your answers, I think I can rule out the 9v battery option with the Reg.

Would 9v from the battery directly (no reg) damage the modem, or would the 6v AA damage it less?

Thanks.

 

[MrAl]

Hi there,

It's not a good idea to try to power something made for +5v with +9v, as that
is almost TWICE the voltage and most likely will do damage. Instead, get a
9v dc wall wart and a 7805 regulator and connect that to the modem. The
modem takes 5v and the regulator puts out 5v, and it doesnt matter that the
7805 can put out up to 1 amp as this wont hurt the modem, only a voltage
too high will hurt it. You probably will need a small heat sink also for the 7805
device.

If you absolutely have to run from battery you will have to make up a battery
pack that puts out about 9v nominal. The run time will be approximately the
modem supply current divided into the ampere hour capacity of the battery
pack, in hours. Thus, using 2000mAh cells (rechargable AA NiMH's) with a
modem current of 500mA (most likely it is actually less than this) the run time
will be roughly 4 hours. If that is not enough time you will have to use
bigger cells.