When positively charged minute ink droplets 608 from a defective nozzle impact and cling on a negatively charged deflector electrode 320, the positive charge at a side of a condenser 609 opposite to the side connected to the deflector electrode 320 flows to the ground via a FET 618 of a photo-coupler...http://www.google.com.au/patents/US20020171717?utm_source=gb-gplus-sharePatent US20020171717 - Ink jet recording device capable of detecting defective nozzle with high signal-to-noise ratio
