Electrostatic spraying device and method using an alternating polarity high potential

A method of coating a target which is either so arranged, or is of a nature, that it accumulates charge, said method comprising the steps:

• supplying liquid to a nozzle;
• and, liquid feed to the nozzle being induced by one of capillary and gravity action,
• applying an alternating high potential produced by and output from high voltage circuitry so that liquid sprayed in atomized form from said nozzle is electrostatically charged,
• a frequency of said alternating high potential being such that each cloud of atomized particles charged with one polarity issues from said nozzle and deposits on said target substantially without discharging any particles from a preceding cloud of atomized particles carrying an opposite charge while successive clouds are air-borne, whereby deposited spray clouds substantially neutralize each other at said target.

An electrostatic spraying device comprising:

• a nozzle having a nozzle outlet;
• means for supplying liquid to said nozzle;
• and high voltage circuitry arranged so that, in use, liquid emerging at said nozzle outlet is subject to an electrical field sufficiently high to draw said liquid from said nozzle in the form of at least one ligament which breaks up into charged droplets to form a spray,
• said high voltage circuitry comprising a bi-polar voltage generator producing an output voltage which alternates with a frequency such that each cloud of liquid droplets charged with one polarity issues from said nozzle substantially without discharging droplets of a preceding cloud carrying an opposite charge at least while successive clouds are in flight from said nozzle to a target, in which said high voltage circuitry comprises a voltage multiplying circuit having:
• input terminal means for connection to a pulsed voltage source;
• output terminal means for providing an output voltage which is bipolar and is a multiple of a voltage applied to said input terminal means;
• two parallel sets of serially connected capacitors connected between said input and output terminal means;
• a plurality of paired bi-directional switches interconnecting said sets of capacitors in such a way that each junction between successive capacitors in one of said sets is connected to a pair of said switches which couple said junction to opposite sides of a capacitor of another set;
• and
• means for effecting alternating operation of said switches of each pair in phased relation with said pulsed voltage source and for cyclically varying such phased relation to cause said output voltage at said output terminal means to alternate between opposite polarities with a frequency no greater than 10 Hz.