Improper use of thermistor

We know that thermistors are used in a wide range of applications, such as high sensitivity to temperature, low thermal inertia, long life, small size, simple structure, and various shapes. The magnitude of the current affects the change in the body temperature of the thermistor. Specifically, when the thermistor exhibits a high-resistance state, the current decreases, the thermistor body temperature decreases, the resistance decreases, and the current flowing through the thermistor increases, so that the cycle causes the thermistor Always in a state of change. This state has the following hazards:

(1) The thermistor always has power consumption in the preheating start circuit, which is generally 4% of the total power. Reduce the lumen factor of electronic ballasts or electronic energy-saving lamps. After testing, the power consumption of the 40W fluorescent electronic ballast thermistor is greater than 1.6W, and the power consumption of the 18W electronic energy-saving lamp thermistor is about 0.8W. At a luminous flux of 50 lumens per watt of power, the 40W and 18W electronic ballasts thus lose 70 and 40 lumens, respectively.

(2) The heat generated by the power consumption of the thermistor causes the temperature inside the compact fluorescent lamp and the electronic ballast case to rise, which may cause damage to other electronic components, particularly transistors and electrolytic capacitors, and increase the failure rate.

(3) After the fluorescent lamp is turned on, the filament circuit always has a current passing through the filament due to the existence of the thermistor, thereby forming an emission current and shortening the service life of the cathode.

(4) The thermistor in the preheating circuit is still in a high temperature environment above 80 °C after the lamp is lit, which tends to cause the deterioration of the thermistor grain boundary resistance performance, so that the temperature resistance coefficient changes and the preheating time becomes longer. When the smashing is severe, the inrush current generated at the moment of starting will burn the power tube. If the cathode is in a warm-up state for a long time, it will eventually damage the lamp and the electronic ballast.

In addition, our tests prove that the thermistor exhibits considerable capacitance. In a higher frequency line, using a thermistor in parallel with the starting capacitor C1 will directly destroy the output characteristics of the ballast.