Variable inductance ballast apparatus for HID lamp

Claim 1

A variable inductance apparatus for ballasting a high-intensity discharge lamp, said apparatus comprising: a. a laminated iron core member having an E-I configuration wherein the E-conformed part thereof comprises a main leg member having three transverse leg members connected thereto proximate the ends and mid-portion thereof and projecting at right angles therefrom; the I-conformed part of said core member having dimensions similar to said main leg member and positioned as a yoke proximate the projecting ends of said transverse leg members; and non-magnetic gaps having predetermined spacings provided intermediate said yoke member and at least two of the proximate projecting ends of said transverse leg members; b. a main winding having a predetermined number of turns wrapped about one of said transverse leg members; means for connecting said main winding in series circuit relationship with an AC energizing source and said high-intensity discharge lamp to be ballasted; and said main winding and said core member coacting to provide two closed magnetic paths; c. a control winding wrapped about a leg member of said core member other than the said leg member about which said main winding is wrapped and said control winding also encircling the flux path of only one of said closed magnetic paths which are formed by said core member and said main winding; and d. bilateral switch means having output terminals and a control terminal, said switch means having a closed state in which conduction therethrough is provided at said output terminals, and said switch means having an open state in which an open circuit is provided at said output terminals, and said control terminal is adapted to receive an actuating signal to control the conducting state of said switch means; said control winding connected to the control terminal of said switch means to actuate and switch means to a closed state at a predetermined time in each half cycle of energizing potential for said HID lamp as normally operated; whereby on closing of said switch means the resulting counter mmf generated in one of said closed magnetic paths of said core member decreases by a predetermined amount the inductance of said apparatus to control in a predetermined fashion the average power delivered to said HID lamp.