Varistor is generally used 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. .

The varistor is generally used in parallel in the circuit. When the voltage across the resistor changes abruptly, the short circuit of the resistor blows the current fuse to protect it. Varistors are used in circuits and are often used for power supply overvoltage protection and regulation.

Varistor parameters

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 is 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 Il: Current flowing when a maximum DC voltage Udc is applied to the varistor. When measuring leakage current, the voltage of Udc=0.83U1mA is usually applied to the varistor (sometimes 0.75U1mA is also used). 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.

5 impulse current and repeated impact times

Precautions for use

1, the calculation of varistor voltage:

Generally, it can be calculated by the following formula: U1mA=KUac where: K is the coefficient related to power quality, generally taking K=(2~3), the city with better power quality can be smaller, and the power quality is worse in rural areas (especially Mountain area) may be larger. Uac is the rms voltage of the AC power supply. For 220V ~ 240V AC power supply lightning arrester, varistor with varistor voltage of 470V ~ 620V should be selected. Select a varistor with a higher varistor voltage to reduce the failure rate and extend the service life, but the residual voltage is slightly increased.

2. Calculation of nominal discharge current:

The nominal discharge current of the varistor should be greater than the surge current required to withstand or the maximum surge current that may occur each year. The nominal discharge current should be calculated from the value of the impact resistance of the pressure sensitive resistor in the calibration curve for more than 10 times, which is about 30% of the maximum impact flow rate (ie 0.3 IP).

3. Parallel connection of varistor:

When a varistor does not meet the requirements of the nominal discharge current, multiple varistor should be used in parallel. Sometimes in order to lower the limiting voltage, multiple varistors are used in parallel even if the nominal discharge current meets the requirements. It is important to note that when the varistor is used in parallel, it is necessary to strictly select the parameters with the same parameters (for example: ΔU1mA≤3V, Δα≤3) for pairing to ensure uniform distribution of current.

4, matters needing attention

The temperature fuse should have good thermal coupling with the varistor. When the varistor fails (high impedance short circuit), the heat generated by it will blow the temperature fuse, and the failed varistor will be separated from the circuit to ensure the equipment. Safety. When a higher power frequency temporary overvoltage acts on the varistor, the varistor may instantaneously break through the short circuit (low impedance short circuit), and the temperature fuse may not be blown, and may also ignite. In order to avoid this phenomenon, an impact-resistant power frequency fuse can be connected in series with each varistor (single power frequency fuses may not blow when aging fails)

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.