connectordcschematics
I'm sure this information is somewhere around the internet, but I can't find it.
I have a device that takes a DC barrel jack. I don't know the polarity it expects though. There is a marking though. It looks like this:
___
--- 12VDC
I'm 99% sure it's a symbol to indicate the polarity, but which is it? Positive inside or positive outside?
Those are barrel power connectors.
Looking at Digikey, it looks like common inner diameters with a 5.5mm outer diameter are 2mm, 2.1mm, and 2.5mm, but that doesn't mean that your target application doesn't have a custom size which doesn't match any of these.
The one I usually use for my projects is 2.1mm*5.5mm if I can, but as far as I know this is by no means a rule of thumb.
Knowing what the jack is being used for may help in identifying a correct size.
Is there any reason something like this can't be used:
D1 is there just in case; it provides switching from battery to external power even if the jack doesn't break the contact, or if it momentarily fails to do so.
All switching is on the ground side. The J_PWR jack breaks the ground-side connection of the battery when external power is applied. The J_PWR jack and battery share a common ground-side network which goes through J_IN. The ground for both the battery and J_PWR is interrupted if nothing is plugged into J_IN.
How J_IN works is that when a mono phone plug (TS) is plugged into a stereo jack (TRS), it bridges the sleeve and ring because the two are fused in a mono plug.
AUDIO BOX should still have a power switch to prevent draining the battery when something is plugged into J_IN, and nothing into J_PWR.
Note that this is a simplified design. Transformer-based AC adapters usually put out only a half-wave rectified AC: it is DC, but doesn't have a flat voltage. The device has to provide regulation for itself.
Thus, between the positive input terminal of J_PWR, and D1, we would typically have a three-terminal voltage regulator, flanked by some capacitors. The battery drives the internal, regulated voltage line.
Best Answer
Unless there is a figure like below, or some wording like "positive centre" then you can't tell.
or:
A supply can use positive or negative centre, as Olin says there is no standard. This is why you get the polarity switches on many of the universal DC supplies.