Today, lighting systems based on gas discharge phenomenon take up a fundamental place in artificial lighting field. Efficient design of supply systems and studies about its importance in electrical power distribution systems make necessary achievement of previous models for this kind of loads. Main objective consists in reproducing lamp behaviour in the most extensive domain and integrating it in circuit analysis software. Understanding of the way in which lamp impedance responds as a component of the circuit can be a useful tool to reduce costs and improve performance. Difficulty in obtaining models is caused by complexity of physical phenomena that occur inside discharge tube. This difficulty is related to its negative impedance. Although physical modelling to reproduce lamp behaviour with accuracy can be developed, these models are so complex that become useless to study circuits. In fact, highly sophisticated models are not always necessary to get valuable information about lamp discharge process. An approach, less frequently used, consists in the description of lamp conductance G(t) by means of a single differential equation. Knowledge of mathematical models for G(t) allows deriving the current and voltage waveforms of discharge lamps from the circuit equation. These kinds of models can be applied at low and at high frequencies successfully. In this paper physic assumptions about plasma properties have been used in order to find a mathematical expression of discharge conductance.
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