Charging a sat nav from a CD USB port

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Technical numbskull has an electronics question...

I got a sat nav for Christmas (Tom Tom One, if it matters for the following) and it came with 2 types of chargers: one goes in the cigarette lighter, the other uses the PC USB port. My car doesn't have a cigarette lighter, but my CD player has a USB port on the front for playing music off USB sticks and such. So I was wondering: a) is it possible and b) is it safe to use the CD USB port to charge the sat nav whilst on the move?

Thanks in advance!
 
I would guess that the CD's USB port will not give out any power, or not enough to charge anything.
 
A USB port just provides 5VDC relative to ground power besides the 2 data pins, so I think it would be okay.

Does the ciggy lighter plug say the current supply it provides? A USB port can deliver 100 mA if it meets the USB standards.
 
It would have to have some power, but it will not be more than 2.5W - that's a USB standard. But since the device does come with a USB charger, I'm assuming that isn't going to be an issue.

I'd start by checking the manual for the CD player and see if it gives any specs for the USB port. If not... get out a voltmeter and try checking the voltage and current of the port

usb_test_cable.gif


Hopefully you can get accurate measurements without having to move anything on the data buses... I don't really know a ton about how USB works.
 
If you measure current across the pins without putting a load in there you run a good risk of ruining your head unit.
 

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