System for energizing a fluorescent tube

A fluorescent tube (which may or may not have its heater coils shorted) is energized by a substantially constant current. However, the direction of the substantially constant current is periodically changed. The magnitude of the substantially constant current may be varied by varying the setting of a control member. For example, the movable arm of a potentiometer may be varied. The potentiometer may be included in an optically coupled circuit to give isolated control to the substantially constant current when the position of the movable tap is changed. The movement of the movable arm of the potentiometer causes the duration of the output pulse from a pulse width modulator to vary accordingly. During the production of this output pulse, a switch (e.g. a field effect transistor) is closed so that current is able to flow from a rectified alternating voltage to an energy storage member such as a coil. The coil may be connected to the center tap of an autotransformer to introduce stored energy to the autotransformer during the time that the switch is open. The autotransformer is also connected, at positions displaced from the center tap, to switches which become alternately closed at progressive periods of time to change the direction of the flow of the current through the fluorescent tube.