Two or more panels of a power-operated folding door are supported by a novel articulated arm assembly that ensures that the door closes tightly against a wall-mounted seal and easily opens. The plate-like powder remover is disposed in the powder conveyance path with both lateral edges thereof in a width direction perpendicular to a powder conveyance direction in the powder conveyance path contacting or nearly contacting the inner wall of the powder conveyance path. One of the transistors is switched onoff by a clock signal that is not subject to history-effect timing variations, and the other is switched onoff by a signal that is subject to such variations.