They use a 1000µfd after the bridge, then into voltage regulators, so I would say the 18v is not critical, they will finsh up with around 24vdc into the regulators, but it depends what max they recommend, BTW with a bridge, you can also feed in DC of the same value if you wish, so if you had a 24VDC supply, you could connect that into it, polarity does not matter, the bridge will take care of it.
Al.
CNC, Mechatronics Integration and Custom Machine Design
“Logic will get you from A to B. Imagination will take you everywhere.”
Albert E.