I have 2 brand new humidifiers. One is a tabletop and the other is freestanding; made by different companies. The humidity in the house is good now but they will continually run if I don't manually turn them off. I have had the same thing happen to other humidifiers after I have had them a while but these are brand new.
Most cold type portable humidifiers have 2 controls. One works like a limit switch based on the humidity setting you desire. Above that point the unit will be 'on' and below it, 'off'. The other setting determines how much vapor is sent into room as in high setting will give largest cloud of mist and use up water the quickest.
If you set humidity regulator control to a level very low, as a test, unit should shut off. If this is not possible, the humidity level in the room must be extremely low and you still get static electricity shocks from touching metal screws on light switches in a carpeted room. Read instruction books carefully and maybe get a thermometer/hygrometer that gives you indoor humidity level.