Printed circuit resistor network with ultrasonically welded fuses - has resistance value trimmed by laser cutting for use in electric motor speed controls

摘要

The printed circuit consists of a sinuous metal tracks (2), made for example of aluminium, deposited on an insulating substrate (1). Typically, the form of the tracks allow two or more individual resistive paths to be accessible through contacts (3,4,5,) to external circuits. Track resistances are typically of the order of 3 ohm and are laser trimmed by cuts (6,7) of selectable length in wide sections (2a,2b) of the tracks. At certain points (8a,8b) along the tracks, the metal is cut (10) by the laser beam. The gaps so formed are bridged by ribbons or wires (9) made of the same conducting material as the track. The bridging sections are of calibrated cross-sectional area and form fuses in the circuit. The electrical properties of the fuses so formed are more uniform and stable than fuses formed from narrowed sections of the tracks themselves. The bridging section is welded to the track surface on either side of the cut by an ultra-sonic welder. The fuse section is embedded in a protective coating of a silicone compound after formation.