Electrostatic coating system

Abstract

A system for electrostatic deposition of a coating on an article from a charging device includes a direct current high voltage supply having first and second terminals across which the high voltage is impressed. The article to be coated is held at a potential substantially equal to the potential on one of the high voltage terminals. The article is suspended for relatively free movement toward and away from the device. The system includes a high-speed switch coupled between the high voltage terminals and a control circuit for controlling the switch. A sensing resistor in the high voltage output circuit monitors the current between the high voltage terminals. The control circuit includes a slope-detecting or fixed difference circuit responsive to the sensed current to detect too-rapid changes therein, such changes being indicative of close proximity of the article to the charging device and the possibility of high voltage arcing therebetween. The fixed difference circuit compares a sampled signal, related to the sensed current by a fixed difference, with a signal related to the instantaneous value of the sensed current to predict a condition conducive to high voltage arcing. The control circuit additionally includes an automatic ranging circuit for comparing a sampled signal related to the actual current through the sensing resistor with a signal related to a predetermined multiple of the instantaneous current through the sensing resistor. The automatic ranging circuit therefore is not concerned with the absolute magnitude of the current through the sensing resistor, as is the slope-detecting circuit, but rather, the automatic ranging circuit considers only the difference between a sampled signal related to current through the sensing resistor and a signal related to the instantaneous value of current through the sensing resistor. The high voltage supply further includes a high voltage transformer having primary and secondary windings. The primary winding includes a supply terminal coupled to a relatively low voltage supply and two terminals coupled to a switching circuit which controls current flow through the primary winding. The resulting voltage variations across the secondary winding are rectified and multiplied to produce the high voltage. A resistive voltage divider senses the high voltage. A regulator coupled between the low voltage supply and the supply terminal regulates the voltage provided to the supply terminal, thereby regulating the high voltage.

Dependencies are inferred from the claim text.