Overvoltage components are divided into clamping type and switching type according to their functions. Switching overvoltage elements are well-known lightning protection devices, including ceramic gas discharge tubes, semiconductor discharge tubes and glass discharge tubes. The clamping type is that the overvoltage elements include transient suppression diodes, plug-in varistors, patch varistors and ESD discharge diodes. Then this article mainly focuses on the plug-in varistor (MOV) of clamping overvoltage protection device.
A brief introduction of MOV varistor
Varistor is the most commonly used voltage limiting device. The varistors include silicon carbide varistors and zinc oxide varistors. Zinc oxide (ZnO) varistor is commonly used, which is an overvoltage protection device made of zinc oxide as raw material, adding various trace metal oxides, mixing, forming and sintering, and it is coated with epoxy resin (pigment can be added). Using the nonlinear characteristics of piezoresistors, when the voltage appears between the two poles of piezoresistors, piezoresistors can clamp the voltage to a relatively fixed value, thus realizing the protection of the subsequent circuit.
Second, the plug-in (MOV) varistor working principle
MOV varistor is equivalent to a variable resistor, which is connected in parallel with the circuit. When the circuit is in normal use, the varistor has high impedance and small leakage current, which can be regarded as an open circuit and has little influence on the circuit. However, when the high surge voltage comes, the resistance value of the varistor decreases instantly (its resistance value can be changed from Ω (megaohm) to Ω (milliohm)), so that it can flow too much current and clamp the overvoltage at a certain value. Because the surge carrying capacity of varistor depends on its physical size, it is possible to obtain different surge current values.
The biggest characteristic of varistor is that when the voltage applied to it is lower than the threshold value "UN", the current flowing through it is extremely small, which is equivalent to a closed valve; when the voltage exceeds UN, the resistance value becomes smaller. The resistance value becomes smaller, the current flowing through it increases sharply, the influence on other circuits changes less, and the influence of overvoltage on subsequent sensitive circuits decreases. With this function, the abnormal overvoltage in the circuit can be suppressed and the circuit can be protected from overvoltage damage.
III. Characteristic parameters of MOV varistor
(1) Nominal varistor voltage (V): refers to the voltage value at both ends of varistor when pulse current of specified duration (generally 1mA, duration is generally below 400mS);
(2) Voltage ratio: refers to the ratio between the voltage value generated when the current of the varistor is 1mA and the voltage value generated when the current of the varistor is 0.1mA;
(3) Maximum limiting voltage (V): the maximum pulse peak current Ip that the varistor can bear and the voltage peak at both ends of the varistor under the specified waveform;
(4) Residual voltage ratio: When the current of the varistor is a certain value, the voltage generated at both ends of the varistor is called the residual voltage of the current value. Residual voltage ratio is the ratio of residual voltage to nominal voltage;
(5) Current capacity (kA): The current capacity, also known as flow capacity, refers to the maximum pulse (peak) current value allowed to pass through the varistor under specified conditions (specified time interval and times, and standard impulse current applied);
(6) Leakage current (mA): Leakage current, also called waiting current, refers to the current flowing through the varistor at the specified temperature and the maximum DC voltage;
(7) Voltage temperature coefficient: refers to the rate of change of transformer called voltage within the specified temperature range (temperature is 20℃~70℃), that is, when the current of transformer is kept constant and the temperature changes to 1℃, the relative change of voltage at both ends of transformer;
(8) Current temperature coefficient: when the voltage at both ends of the varistor remains constant, when the temperature changes to 1℃, the relative change of the varistor current;
(9) Voltage nonlinear coefficient: refers to the ratio of static resistance value to dynamic resistance value of pressure sensitive resistor under a given extra voltage;
(10) Insulation resistance: refers to the resistance value between the lead (pin) of the varistor and the insulation surface of the resistor;
(11) electrostatic capacity (PF): refers to the inherent capacitance of the varistor itself;
(12) rated power: work at a specific ambient temperature of 85℃ for 1000 hours, and the maximum power change is less than 10%;
(13) maximum impulse current (8/20us): a specific pulse current (8/20us waveform) impacts the rheostat once or at an interval of 5 minutes), and the change of the pressure sensitive voltage is still within 10;
Four, plug-in (MOV) varistor selection steps
(1) the voltage of the varistor should be higher than the normal working voltage of the user, and a certain margin should be reserved;
(2) The flow rate of varistor should be determined according to the design index of lightning protection circuit, and the flow rate of varistor should be greater than the design flow capacity of lightning protection circuit;
V. Characteristics of MOV varistor
The response time of varistor is ns grade, which is faster than air discharge tube and slightly slower than TVS tube. In general, the response speed of varistor used for overvoltage protection of electronic circuit can meet the requirements. The junction capacitance of varistor is generally in the range of hundreds to thousands of Pf, which should not be directly applied to the protection of high-frequency signal lines in most cases. When applied to the protection of AC circuits, the junction capacitance will increase leakage current, so it needs to be fully considered when designing protective circuits. The current capacity of varistor is large, but smaller than that of gas discharge tube.
VI. Application of MOV varistor
It is mainly used for surge protection of AC/DC power circuit and various low frequency signals, control circuit board, power system, surge suppressor, safety system, motor protection, automotive electronic system, household appliances and related lightning protection modules.
Seven, our company shunhai technology agent SOCAY shuokai plug-in (MOV) varistor product series are as follows
Specification series: 05D series, 07D series, 10D series, 14D series, 20D series, 25D series, 32D series, 34S series, 40D series and 53D series;
Product shape and specification: there are two main packaging forms: wafer shape and square piece shape, as well as patch shape, high current module and high voltage module;
Product voltage: 18 v-1800 v;
Product characteristics: large flux (100A~70kA), larger volume, larger surge current, complete variety and wide application range;
The above content mainly introduces the detailed explanation of MOV varistor. If you need MOV varistor or patch varistor, please leave a message or call 0755-28100016; Massive varistors are in stock with complete specifications. Welcome to inquire about the price or take samples for testing.
Shenzhen Shunhai Technology Co., Ltd. is an integrated supplier of electronic components, semiconductor devices and protection devices, Mainly engaged in TVS transient suppression diode, ESD electrostatic protector, GDT ceramic gas discharge tube, TSS semiconductor discharge tube, NTC thermistor, varistor, fuse, alloy resistor, chip resistor, precision resistor, wafer resistor, chip fuse, self-recovery fuse, chip capacitor, tantalum capacitor, electrolytic capacitor, aluminum electrolytic capacitor, supercapacitor, connector and other related product lines. Brands can be elected from Huade, Shuokai, Saruiwei, Yineng, Tesscara, KEMET, Tianer, Wangquan, Dayi, Murata, Taiyi, TDK, AVX, Weishi, Fenghua, Huaxin Branch, Sanying, Guoju, Lilong, Housheng, Guangluo, Shunluo, etc.
