In the fluorescent lamp art both preheat type and rapid start type fluorescent lamps are commonly encountered. In the preheat type of fluorescent lamp, heater current flows through the electrode only during lamp ignition whereupon an external voltage sensitive starter opens the heater current circuit and discontinues heater current flow. In contrast the rapid start fluorescent lamp normally has a constant heater current flow through each electrode both during ignition and operation of the lamp. Unfortunately, heater current flow during operation of the rapid start fluorescent lamp is lost power which obviously reduces the operational efficiency of rapid start fluorescent lamps.
Numerous suggestions have been made for enhancing efficiency of rapid start fluorescent lamps. For example, U.S. Pat. Nos. 4,052,687; 4,097,779; 4,114,968; 4,156,831 and 4,171,519, all assigned to the Assignee of the present application, suggest numerous configurations for enhanced operation of rapid start fluorescent lamps. Generally, each provides a thermally responsive circuit breaker suitable for use in discontinuing heater current upon operation of the fluorescent lamp.
However, it has been found that problems still exist. For example, it has been found that a problem occurs whenever the thermally responsive circuit breaker extends in a direction parallel to the electrode of a lamp but for a distance greater than the length of the electrode. Thereupon, positioning the lamp envelope properly is encumbered because the envelope tends to undesirably contact the circuit breaker when the circuit breaker and lamp electrodes are being inserted into the envelope. Such an encounter is obviously undesirable, and especially so in a high production manufacturing process.
In another aspect, it has been found that positioning of the circuit breaker too close to the electrode has a deleterious effect upon the life span of the fluorescent lamp. More specifically, closely spacing the circuit breaker to an electrode generating heat tends to significantly reduce the life span of the fluorescent lamp wherein the electrode is located.
In still another aspect of the invention, it has been found that the consistency and repeatability of operation of a circuit breaker disposed within a discharge lamp can be adversely affected when left unprotected from the environment within the discharge lamp. In other words, a bimetal switch means placed nearby an activated electrode within a discharge lamp may be deleteriously affected when protection is not provided such that the bimetal switch surfaces are not isolated from the environment surrounding the lamp electrode.