For linear and switching power supplies, when lowering the AC input voltage from 115vac down under 90Vac will turn the power supply off. Some power supplies can go under 90VAC and the power supply will will output a stable DC voltage. What determines a power supplies cut off voltage or threshold? I can't find anything that tell on the datasheets on power supplies about this. Linear power supplies react differently compared to a switching power supply when lowering the AC input voltage under 90vac. What kind of power supply can the output of a power supply can stay stable with an AC input under 80VAC?