The invention relates to wound metallized film capacitors and more particularly to adding an additional auxiliary capacitor onto a first wound capacitor section to produce a dual value capacitor.
One method of making a dual value capacitor is disclosed in U.S. Pat. No. 3,921,041 issued to Stockman on Nov. 18, 1975. In this capacitor, one of the two basic metallized films has the metallized layer burned from its surface for a selected distance and an insulated sheet is inserted to encircle the capacitor at least once and to extend from one end of the roll. The winding is then completed around the insulated sheet and then both ends of the capacitor are shooped with one end providing the base plate and the other end, being divided into two sections by the extending insulated sheet, providing the other two sections of the dual value capacitor.
This method is not practical; however, when a small number of turns is desired to be added to the first section since there will be an insufficient end area on the auxiliary capacitor to attach a contact to. If a metal contact tab is added to the metallized film it will cause the film to burn away around the contact tab because of the small area involved. If the auxiliary section is end sprayed with metal, even if there is sufficient area to provide good electrical contact, the metallized film will burn away because of the small area involved.
Current technology generally makes use of a separate capacitor if an auxiliary capacitor having a small number of turns is required in conjunction with a large capacitor. One such application of a large capacitor with an auxiliary small capacitor is in the fluorescent lighting ballast which requires a large 3.95 microfarad capacitor and a small 0.05 microfarad capacitor for a typical rapid start design. It would be desirable to utilize a wound metallized film construction to take advantage of the self-healing or clearing properties of the metallized film. When a short occurs the metallized film will self-heal and clear itself so that a short is only temporary and does not destroy the whole capacitor as happens in conventional metal foil capacitors. The need has thus developed for a wound metallized film capacitor having first and second sections which may be easily constructed to provide a compact capacitor having both a core capacitor and an auxiliary capacitor having a small number of turns within the same unitary structure.