I have a device that plugs into the wall or runs off batteries. I would like to use this in my car but it did NOT come with a car adapter. It runs @ 9V and the polarity is center pin - My question is will it be cheaper to make something I can get the plug for dirt cheap but getting my car from 12 down to 9 I dont know how to do it and what to buy. OR would it just be easier to buy one that had switching voltage on it.
Like this http://cgi.ebay.com/CAR-AUTO-TO-3V-4-5V-6V...1QQcmdZViewItem

I say get the ebay thing. Boom, done! Make sure the unit is can supply enough current for your device (it most like can). But read the back of your device and see if it requires any more than 300mA (the maximum rating for the ebay thing), if it does then this isnt going to work.

Yeah when I get home I will see, I know you can run it off 4 AA batteries so it shouldnt be to much. I totally understand the amp thing. Thanks for the advice.