What is a varistor
"Varistor", the International Electrotechnical Commission (IEC) is called "voltage dependent resistor" in its standard, and the more widely used name in the industry and academia is "varistor", which is the English word for variable and resistor. The combined combination of words, as the name implies, is a resistor whose resistance value changes sensitively with the applied voltage. Its main purpose is the sensing of abnormal overvoltage, the resistance and the absorption of surge energy.
From the material composition point of view, the varistor is made up of electronic grade powder materials - zinc oxide, cerium oxide, cerium oxide, cobalt oxide, manganese oxide, nickel oxide, chromium oxide and other oxides. The highest content, accounting for about 90%, is the main base.
The figure above shows a typical varistor 20D201K with the applied voltage rising from 180V to 420V, and its resistance value decreases from 18 to 0.42. During this change, the external voltage changes only a little more than 2 times, while the resistance value decreases. With nearly 43 million times, this curve helps us intuitively feel the "pressure-sensitive" characteristics of piezoresistors.
Varistor parameters
The most important parameters of the varistor include: varistor voltage, current capacity, junction capacitance, response time, and so on.
The varistor voltage refers to a corner voltage at which the resistance value of the varistor rapidly decreases after the applied voltage is higher than a certain voltage. However, this voltage measurement is inconvenient. For the purpose of parameter standardization, the International Electrotechnical Commission has artificially specified two DC reference currents for measuring varistor voltages of 1 mA and 0.1 mA, and the voltage measured at this current is artificially defined as varistor voltage. . So strictly speaking, the varistor voltage marked on the varistor manual is not the “pressure-sensitive†voltage of the varistor.
The flow capacity, that is, the maximum peak current value, that is, the maximum surge current peak that the varistor can withstand is 8/20 uS (industry surge test standard waveform). The current varistor manual usually gives two parameters of energy capacity, one is an 8/20uS surge impact indicator, and the other is two 8/20uS surge impact indicators.
Junction capacitance, the varistor has a very large resistance value before conduction, which can be regarded as a dielectric. There is a nano-level capacitance between the two electrodes, which requires special attention. It is also for this reason that varistor is less used in high frequency circuits and digital circuits.
Response time, in general, refers to the time difference between a device triggering from the outside to the beginning of the device, but for varistors, the response time is not defined as such, which requires special attention. In the IEEE standard, the response time of the varistor is defined as shown in the figure above. In the figure, Vc refers to the residual voltage after the varistor has passed the 8/20uS impact, and the varistor response time refers to the time difference from the voltage peak point t1 to the 50% residual pressure point.
The figure above shows the datasheet of the varistor produced by a certain manufacturer. From the figure, we can find the main parameters of the varistor that we are more concerned about.
The difference between varistor and gas discharge tube and TVS tube
Varistors, varistor, gas discharge tube, TVS tube (instantaneous suppression diode) are voltage-limiting surge protection devices, which are used as surge protection in the circuit, so what is the difference between them? What?
In the reaction time, the varistor is between the TVS and the gas discharge tube, the TVS tube is picosecond, the varistor is slightly slower, which is nanosecond; and the gas discharge tube is the slowest, usually tens of nanoseconds. even more.
In the flow capacity, the pressure sensitive electricity is also between the TVS and the gas discharge tube, and the TVS tube is usually only a few hundred A; and the varistor can pass a single 8/20uS of several KA to several tens of KA according to different specifications. Inrush current; for gas discharge tubes, usually 10 k 8/20 uS surge current can be turned on hundreds of times.
In principle, the TVS tube is based on the diode avalanche effect; the varistor is based on the barrier between the zinc oxide grains; and the gas discharge tube is based on the gas breakdown discharge.
In terms of voltage range, TVS tubes are usually 5.5V to 550V; varistor ranges from 10V to 9000V; and gas discharge tubes can range from 75V to 3500V.
The role of varistor in lightning protection circuit
The varistor is often used for lightning protection and surge protection at the power inlet of the instrument. In such applications, the varistor is usually used together with a gas discharge tube, a fuse, a thermistor, etc. Combine to make a difference.
The picture above shows a typical lightning protection and surge protection circuit structure. RV1~RV3 are varistor, GAS is gas discharge tube, F1 and F2 are fuses. RV1 is used to protect the differential mode surge voltage, and RV2, RV3, and GAS are combined to protect the common mode surge voltage. The fuse is used for short-circuit protection to prevent short-circuiting of the varistor breakdown.
The common mode surge protection circuit composed of RV2, RV3 and GAS combines the advantages of varistor and gas discharge tube to compensate for their respective shortcomings. It is a commonly used surge protection circuit. The gas discharge tube has a large flow capacity, but the operation time is long, the leakage current is large, and the varistor has a fast response speed and a small leakage current.
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