Fluorescent lamps need a ballast to generate high voltage and large current to energize. The ballast controls the electrical characteristics of the fluorescent lamp.
When the lamp is turned on, the electronic ballast is at both ends of the lamp, which generates a high-voltage shock to make the lamp light up and energize. A self-oscillation circuit is formed in the electronic ballast, and the oscillation circuit is controlled by a transistor.
Many electronic ballasts fail due to the lamp. When the lamp is short-circuited, the service life is reached, or the lamp is taken out, an overcurrent condition will occur, which will cause the cathode of the lamp to open.
Due to the power factor, the load resistance becomes lower. In the start-up phase, the ballast works more than three times under abnormal working current and high oscillation frequency; The switching circuit generates overcurrent and causes the ballast to malfunction.

Resettable polymer fuses can provide protection for the lamp when it reaches the end of its life and protection against transistor failure.

Since the ballast often fails due to the simultaneous opening of the upper and lower voltage switches of the transistor, the fault protection of the transistor is of great significance.

First of all, the polymer fuse has the performance of automatic recovery, which can reduce the number of repairs and services of the product, thereby reducing the cost.
Secondly, because the resettable fuse can operate in a very short time to protect some of the more sensitive resistors in the circuit. The reliability and service life of the ballast can be improved.
Third, the power consumption of the resettable fuse is very low, and it will not consume energy due to extreme heat generation under normal current working conditions. Under normal operating current, the resistance is very small (usually only a few tenths of ohms), so it will not form an oscillating circuit.
Fourth, the self-resetting fuse is small in size and occupies a small space on the circuit board, which is convenient for design.