Laser cathode voltage controller

Abstract

A voltage controller for converting AC line voltage (which may fluctuate, or vary from site to site) to AC power (having substantially constant average power) for a laser tube cathode heater. The voltage controller of the invention receives standard AC line voltage and a feedback signal proportional to the voltage applied to the cathode of a laser gas tube by the cathode heater. In a preferred embodiment, the feedback signal is processed to generate a DC signal representing time-averaged power applied to the cathode. Preferably, the DC signal is then compared with a preset reference voltage in a comparator, and the comparator output is fed to the data input of a flip flop circuit. A clock signal derived from the AC line voltage is supplied to the clock input of the flip flop. The flip-flop will thus respond to the comparator output once per line cycle (approximately at a zero crossing). The flip-flop output is employed to control a solid state switch, which turns power to the cathode heater on or off for full line cycle intervals. In response to variations in the line voltage, the ratio of 'on' to 'off' cycles will change to maintain substantially constant average power to the cathode heater. The control technique employed in the inventive apparatus avoids switching transients and DC current in the cathode heater's transformer primary.