I am unable to find the documentation for this barrel jack and I was wondering why there are three prongs. Is the third a grounding wire? Does it matter which end is power or ground? Thank you.
There’s a pinout in the accompanying datasheet:
Pin 3 is electrically optional and a lot of designers will wind up only using its sturdy solder tab for a mechanical anchoring, if at all. Pin 3 could be used for a plug-detect circuit: pins 2 & 3 disconnect when a barrel is inserted.
I can’t tell from the schematic, is pin 1 power or ground?
We can’t tell, either.
Some are AC.
Most are DC and I’ll hazard a guess that 85% are tip positive. The power supply and the receiving device need to agree on their polarity but the tip pos/neg choice is arbitrary.
Lot’s more about the polarity iconography here: