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What is common negative?
#1
Question 
hello i'm new to solar setup. while searching for solar charge controller i read about common negative Solar charge controller and wanted to ask what is this common negative and common positive solar charge controller? is this depends on your application?
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#2
I believe the terminology refers to which side is grounded. It is most commonly done with Common Negative which would also be Grounded. Common Positive isn't used as much.

The big thing here is, never ever mix the two together in a setup without proper isolation. Otherwise your Pos and Neg can, and most likely, will be shorted out, considering they are each bonded to ground at some point.

Older vehicles, especially those that ran on 6V, usually used Common Positive and was later phased out with Common Negative when 12V batteries were used.

As far as charge controllers, I haven't seen any Common Positive (not saying there aren't any, just I have seen them) and are probably far fewer than Common Negative.
On the circuitry side, it probably means they used P-Mosfets instead of N-Mosfets for the gate controllers (I'm guessing here).
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#3
"Grounding" implies a functional earth, used for protective devices,  this isn't the same as common negative or positive.

Unless the panels are installed within a vehicle/boat/caravan the panels most negative terminal should not be grounded (chassis ground), however any conductive framework or mounting hardware can/could be, assuming by doing so the electrical installation guidelines (for your location) are adhered to.
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#4
Agreed with Sean, Grounding means putting it to zero, it does not mean a + or a -. In many setups you can actually have a situation like -12v (blue) / 0 (black) / +12V (red) with a 24V difference between the 2 (blue-red in this example) wires.

Given how electrons work, it's generally a bad idea to connect any negative terminals to casings of anything, because you'd actually polarize the casing/hull of the object if you do so. In the situation of many flashlights and lasers, it's the + connected to the case and the '-' is the one disconnected and this is physically better.

Commons are another matter, but generally yes, negatives are the most used commons. As a designer, I'd choose the common depending on the situation.
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