Air-conditioning varistor application case
July 30, 2021
Varistors are widely used in the power supply circuits of air conditioner circuit boards as overvoltage protection and high surge absorption. Varistors are a voltage-limiting protection device. Utilizing the non-linear characteristics of the varistor, when an overvoltage occurs between the two poles of the varistor, the varistor can clamp the voltage to a relatively fixed voltage value, thereby realizing the protection of the subsequent circuit. The main parameters of varistor are: varistor voltage, current capacity, junction capacitance, response time, etc.
Zinc oxide varistor is generally used in the power supply circuit of air conditioner circuit board, where the varistor varistor voltage is 470V. When the transient surge voltage exceeds 470V (non-effective value), the varistor reflects him The clamping characteristics of the air conditioner can pull the excessively high voltage down. In addition, the peak value of the lightning or transient, random, non-periodic current and voltage is also effectively absorbed, so that the back-end circuit of the air conditioner circuit board power supply can work within a range .
1. Principles of varistor protection for air-conditioning circuit boards
The varistor JEC1 is connected in parallel at both ends of the primary coil of the power transformer, and forms a series circuit with the fuse to suppress the surge voltage. Under normal voltage conditions, the resistance of the varistor is very large, up to megohm level, and the current flowing through it is only microampere level, which can be ignored. It is in an open circuit state and has no effect on the operation of the circuit. But when the power supply voltage exceeds its design breakdown voltage, its resistance suddenly decreases to a few ohms or even a few ten ohms, and the instantaneous current can reach thousands of amperes. The varistor immediately changes from cut-off to conduction. It is connected in parallel with the power supply, so the fuse of the power supply will be blown out soon to prevent the main circuit board from burning out. In addition, when encountering a slight transient surge of the power grid, the varistor will absorb and buffer this surge clutter; in addition, when encountering lightning strikes, transformers and other inductive circuits for switching operations, the transient overvoltage generated The resistance value of the varistor will suddenly decrease, and the passing current is very large, which plays a role of draining and protecting the entire circuit. This effect is called overvoltage protection. The code name of the varistor in the circuit diagram is often represented by ZNR.
2. Common failure phenomena and repair methods of varistor of air conditioner circuit board
The common failure phenomenon of varistor is bursting or burning, which causes a short circuit in the circuit. This type of fault is mostly caused by improper selection of varistor, long power supply over-voltage time, wrong connection of power supply with high voltage (due to thunder, wind, lightning, etc.), poor component quality, etc. The varistor is a one-time component, and it should be replaced with the fuse in time if it is damaged. If only the fuse is replaced, and the varistor is not replaced, other components on the circuit board will be burnt out when the overvoltage occurs again. The above information is provided by the R&D department of Dongguan Ampfort Electronics Co., Ltd. For more information, please visit the website Ampfort Information.