Thyratron Control of Discharge Lamps

The general principles of thyratron control are discussed, the various points of difference between circuits being mentioned, together with some recently introduced improvements. Two valves are connected in reverse parallel, in series with the load. The brightness is controlled by varying the period of conduction of the valves, the ignition angle being varied by grid phase control. Hotcathode lamp filaments must be continuously heated from a separate current supply. Each lamp requires a separate ballast. Cold-cathode lamps require normal leakage-flux transformers only. Reduction of light to 1% or 2% of its maximum value is obtained with simple circuits, but improved stability can be obtained with special ballast arrangements which give a further reduction of light by an approximate factor of 10. Power factor can be corrected on the load side of the dimmer, with improvement in dimmer load rating and increased flexibility of use. Waveforms illustrating proper circuit conditions and various modes of malfunctioning are included.