How to use varistor in parallel in the circuit?
The varistor (MOV) is a non-linear resistance element with zinc oxide (ZnO) as its main component. The surge current tolerance and nonlinear coefficient of this component are very large. When the threshold voltage is below the threshold voltage, the resistance is very high and there is almost no current flow. If the threshold voltage is exceeded, the resistance is drastically reduced, and the amplified current can be discharged. Due to this characteristic, as a protective element for electronic and electrical equipment, the absorption of abnormal voltage and the absorption of lightning surges play a large role. .
1 varistor voltage UN (U1mA): A flag voltage that is normally turned on by a voltage at a mA current of 1 mA across a varistor. This voltage is called a varistor voltage UN. The varistor voltage is also commonly referred to by the symbol U1mA. The error range of the varistor voltage is generally ±10%. In test and practical use, the varistor voltage is usually reduced by 10% from the normal value as the criterion for varistor failure.
2 Maximum continuous working voltage UC: refers to the maximum AC voltage (effective value) Uac or maximum DC voltage Udc that the varistor can withstand for a long time. General Uac≈0.64U1mA, Udc≈0.83U1mA
3 Maximum clamp voltage (limit voltage) VC: The maximum clamp voltage value refers to the voltage appearing on the varistor when a specified 8/20 μs wave impulse current IX(A) is applied to the varistor.
4 Leakage current: The current flowing when the maximum DC voltage Udc is applied to the varistor. When measuring the leakage current, the voltage of Udc=0.83U1mA is usually applied to the varistor (sometimes also 0.75U1mA). Static leakage current is generally required to be ≤ 20μA (also required to be ≤ 10μA). In actual use, it is not the magnitude of the static leakage current value itself, but its stability, that is, the rate of change after the impact test or under high temperature conditions. The rate of change is not more than double after the impact test or under high temperature conditions, which is considered to be stable.
Since the varistor (MOV) has a large parasitic capacitance, it is used in an AC power system, and a considerable leakage current is generated. After using the varistor with poor performance for a period of time, the leakage current may become hot and may self-explosion. To solve this problem, a gas discharge tube is inserted between the varistor. In the above figure, the varistor is connected in series with the gas discharge tube. Since the parasitic capacitance of the gas discharge tube is small, the total capacitance of the series branch can be reduced to several pF. In this branch, the gas discharge tube will act as a switch. When there is no transient voltage, it can separate the varistor from the system, leaving the varistor with almost no leakage current. However, this has the disadvantage that the reaction time is the sum of the reaction times of the devices. For example, the reaction time of the varistor is 25 ns, the reaction time of the gas discharge tube is 100 ns, and the reaction time of R2, G, and R3 is 150 ns. To improve the reaction time, the R1 varistor is added, so that the reaction time is 25 ns.
Commonly used devices for preventing surge or transient immunity include gas discharge tubes, metal oxide varistors, silicon transient voltage absorbing diodes, and solid discharge tubes, and combinations thereof. The AC power lightning protection circuit and its device are generally a combination of a gas discharge tube and a varistor.