12 volt question for electrical gurus

[Aaron Coffee]

I have a docking station for an xm radio that I would like to run off of a rechargeable 12 volt power pack/jump starter. The docking station didn't come with a 12 volt cord, the 120 power cord has the following information "input: 120vac 60hz 19w Output: 12vdc 1a" My question is do I need a 12 volt cord that will only draw on amp or will the docking station only draw the amps it needs? Or in other words do I just need a straight 12 volt dc lighter plug cord with the correct tip and polarity or do I need something else?
Thanks
Aaron

[ssrjim]

I have a docking station for an xm radio that I would like to run off of a rechargeable 12 volt power pack/jump starter. The docking station didn't come with a 12 volt cord, the 120 power cord has the following information "input: 120vac 60hz 19w Output: 12vdc 1a" My question is do I need a 12 volt cord that will only draw on amp or will the docking station only draw the amps it needs? Or in other words do I just need a straight 12 volt dc lighter plug cord with the correct tip and polarity or do I need something else?
Thanks
Aaron

My guess is it will draw what it needs. Maybe put a 1 amp fuse in your line. Good luck

[kludge]

12V from the lighter outlet or a "12V" lead-acid battery can be as high as 14.4V or so.

I can't gurantee that your circuit won't be damaged by 14.4V.

The best thing to do is fully charge the power pack and then measure the voltage. If the voltage is right around 12V, then I would use a 2A fuse and call it a day. If it's up around 14.4 volts, then I would use as many series 2A diodes as it took to get the voltage down to ~12V. And a fuse.

Each diode will have a ~0.6V to ~0.7V drop. If you need to drop 2 volts, use 3 diodes, for example.

I built myself a battery pack for my iPod dock using 8xAA NiMH 2500mAh batteries. 8x1.5V=12V Inside a tear I imagine the batteries would last 6-hours or so. (To avoid damage don't let the batteries get below 0.9V per cell)

I got my 8xAA battery holder at Radio Shack. I could have used 10xAA since my dock uses a 15V/3A wall wart, but 8 works fine.

And just so you know, I designed the electronics inside this (shameless plug): Klipsch iGroove SXT

[wannabefree]

I wouldn't worry too much about the voltage. Consumer devices have to be pretty tolerant of input voltage variations, particularly those designed for cars or wall warts (those little black plug in the wall power converters). If you try to put in a regulator to drop the potential 14v to 12v you can end up with a regulator that isn't regulating. Linear regulators, like the 7800 series mentioned above, need several volts above their output voltage to overcome the internal losses. Bottom line, don't sweat it.

Now, as far as current. The 1A listed is what the maker of your power pack, since you indicated it was the "power cord," is the charger for the pack, right? This is DC input to the battery, not to your radio. Your radio will draw what it needs, and if it draws too much it may blow a fuse in the pack, or it may just draw down the battery sooner than later. Time will tell...