Printhead assembly with ink supply shut off

A printhead assembly with a printhead and a shut off valve that has a valve body defining an ink inlet for connection to an ink supply, an ink outlet connected to the printhead IC. A diaphragm is biased into sealing engagement with the valve seat to provide a fluid seal between the ink inlet and the ink outlet. An actuator unseals the diaphragm from the valve seat when energized. The valve protects the ink in the ink supply from contaminants that can migrate up the ink line during shut down periods. The diaphragm is biased to a closed position and so seals the ink supply from the printhead as a default condition even in the event of a power failure. The bias is strong enough to provide the fluid seal so that the seal is not compromised when the pressure difference between the inlet and the outlet is small.