Fixed frequency, variable duty cycle, square wave dimmer for high intensity gaseous discharge lamp

Claim 1

A dimmer circuit connectable to a dc voltage source for applying a constant frequency, variable duty voltage to a high intensity discharge lamp, a change in the ratio of applying voltage from the dc source compared to the overall period of applying such voltage and a greatly reduced voltage providing a range of dimming currents to the lamp, comprising an inductor in series with the lamp, a dual switching circuit connected to the lamp and inductor including a pair of transistor networks alternatively and complementarily conducting, thereby directing current from the dc voltage source in opposite directions through the lamp at a low frequency, a first electronic on-off switch connected alternatively to provide and to deny the voltage from the source to the lamp during the time the first of said pair of transistor networks is conducting, at a high frequency rate, a second electronic on-off switch connected alternatively to provide and to deny the voltage from the source to the lamp during the time the second of said pair of transistor networks is conducting, at the same high frequency rate, first and second timer means connected respectively to said first and second switches, each said first and second timer means being connected to a constant-frequency, cycle-initiating signal at a high frequency, current sensing means connected for receiving a current which passes through one of said first and second switches and through the lamp when one of said first and second switches is on, and voltage triggering means connected to said current sensing means and to a preset voltage level for producing a cycle terminating signal to each said first and second timer means, a predetermined preset voltage level producing a terminating signal dependent on the time the resulting current flow through the lamp reaches a predetermined level, said inductor maintaining reduced current flow at the greatly reduced voltage through the lamp following the occurrence of the terminating signal and before the occurrence of the succeeding cycle-initiating signal.