The defrost heater as a heating element, the power during operation will be affected by voltage fluctuations. According to the reliability test content of the defrost heater, the electric fluctuation range of the defrost heater is set at 187 ~ 242V, this voltage Fluctuations can meet the requirements of most domestic refrigerators on the market.
Voltage fluctuations in the design range of 187 ~ 242V will cause the fluctuation rate of the defrost heater power of the refrigerator to be as high as 48%. When the defrosting power is low, incomplete defrosting will occur, and the residual frost layer of the evaporator will gradually thicken, which will eventually affect the refrigeration performance of the refrigerator; and when the defrosting power is high, the surface temperature of the heater is too high and the heating is too fast , Will lead to safety risks, such as the fuse burned in the actual use, scorching of the liner and other conditions, and even fire. The purpose of the constant power defrost heater program is to stabilize the input voltage of the defrost heater, thereby stabilizing the output power of the defrost heater, so that it is not affected by voltage fluctuations, and fundamentally improve the safety of the refrigerator during defrosting.
In the protection circuit, after the temperature of the Shaw defrost heater rises to a certain temperature value, the change in the resistance value will cause a change in sweat. The change in voltage causes the comparator to switch the output level and turn off the defrost heater. The heating is stopped and hardware protection occurs. The zero-crossing interruption and the voltage level are detected by the photocouple. When the voltage is high, the thyristor is turned on and off for controlling the output power of the defrost heater.