# Does the input polarity matter for a 12V DC adapter?

I bought a car cigarette DC adapter yesterday with the following specs:

Input:  DC 12V
Output: DC 17V


I'm planning to connect it to a 12V battery I have at home, but I was wondering if the adapter input polarity matter or could damage the adapter if reversed?

• If the adapter connects to the battery through a full-wave bridge, and the device it's powering is galvanically isolated from the input, then the input polarity won't matter. Do you have any details (manufacturer, part number, etc.) you could post about the adapter? – EM Fields Jun 6 '14 at 12:17
• @EMFields Why do you mention a full-wave bridge if we are talking about DC input? – Nazar Jun 6 '14 at 13:39

The adapter might have some sort of protection inside, but it will not work (and possibly it will fry) if you reverse the polarity. I bet you have this kind of plug:

The tip is the positive contact while the side plates should be connected to the negative pole of the battery.

For extra safety, and since you are working with a somewhat big battery, add a fuse in series with your adapter: if you mess something up the fuse blows and the wires don't catch fire. The fuse should be rated a bit above the maximum input current that should be written somewhere on your adapter. Please note that it's well possible that there already is a fuse and that it's inside the above plug.

• The one in your photo most likely has a fuse already in it- the knurled ring at the end can be unscrewed, releasing an internal glass fuse. – Spehro Pefhany Jun 6 '14 at 12:42
• That's exactly what I thought. Some has it and some has not, and the unscrewable ring can be easily missed... – Vladimir Cravero Jun 6 '14 at 13:00
• Yeah I found the fuse inside it :) that was the greatest answer ever :) thank you – Bassel Alkhateeb Jun 9 '14 at 11:19

Yes, it matters. Check it carefully or you probably will damage the device.

• By "device" you mean the adapter? Or the othe connected device i.e. my laptop?? Please notice that Im talking about the Adapter input not its output – Bassel Alkhateeb Jun 6 '14 at 10:21

The polarity of the adapter matters and it should be mentioned on the device itself. If not, you may first need to check whether it works with your car cigarette socket. Then you can find the polarity of your car socket using a multimeter to be certain. Please have a look at this answer too.
And as Vladimir has mentioned, add a fuse of proper rating when you are powering from an external battery. The DC socket in your car is also 'fuse'd to limit the current drawn by the device attached.