Knowledge analysis of varistor

What is a varistor? What is its function? The varistor is one of the indispensable electronic components in electronic technology engineering. So do you know these little knowledge about varistor? This article will reveal to you how to measure the quality of varistor and other practical knowledge.

Varistor

"Varistor" is a resistive device with nonlinear volt-ampere characteristics. It is mainly used to clamp the voltage when the circuit is subjected to overvoltage and absorb excess current to protect sensitive devices. The English name is "VoltageDependentResistor", abbreviated as "VDR", or "Varistor". The resistor material of the varistor is a semiconductor, so it is a variety of semiconductor resistors. The main materials of "zinc oxide" (ZnO) varistor, which is now widely used, are composed of the divalent element zinc (Zn) and the hexavalent element oxygen (O). So from a material point of view, zinc oxide varistor is a "Group II-VI oxide semiconductor". In Taiwan, China, varistors are called "surge absorbers" and sometimes "electrical shock (surge) suppressors (absorbers)".

The varistor is a voltage-limiting protection device. Utilizing the nonlinear characteristics of the varistor, when overvoltage occurs between the two poles of the varistor, the varistor can clamp the voltage to a relatively fixed voltage value, thereby protecting the subsequent circuit. The main parameters of the varistor are: varistor voltage, current capacity, junction capacitance, response time, etc.

The response time of the varistor is ns level, which is faster than the gas discharge tube and slightly slower than the TVS tube. Generally, its response speed can meet the requirements for over-voltage protection of electronic circuits. The junction capacitance of a varistor is generally in the order of hundreds to thousands of Pf. In many cases, it is not suitable to be directly used in the protection of high-frequency signal lines. When used in the protection of AC circuits, its large junction capacitance will increase leakage. Current needs to be fully considered when designing protective circuits. The flow capacity of the varistor is larger, but smaller than that of the gas discharge tube. Varistor, referred to as VDR, is a voltage-sensitive nonlinear overvoltage protection semiconductor component.

How does a varistor work?

When the voltage applied to the varistor is lower than its threshold, the current flowing through it is extremely small, and it is equivalent to a resistor with infinite resistance. That is, when the voltage applied to it is lower than its threshold, it is equivalent to an off-state switch. When the voltage applied to the varistor exceeds its threshold, the current flowing through it surges, which is equivalent to an infinitesimal resistor. That is, when the voltage applied to it is higher than its threshold, it is equivalent to a closed switch.

Main parameters of varistor

The main parameters of the varistor include nominal voltage, voltage ratio, maximum control voltage, residual voltage ratio, current capacity, leakage current, voltage temperature coefficient, current temperature coefficient, voltage nonlinear coefficient, insulation resistance, static capacitance, etc.

1. Nominal voltage The nominal voltage refers to the voltage value across the varistor when a 1mA DC current passes through it.

2. Voltage ratio refers to the ratio of the voltage value generated when the current of the varistor is 1mA to the voltage value generated when the current of the varistor is 0.1mA.

3. Maximum limit voltage The maximum limit voltage refers to the highest voltage value that the two ends of the varistor can withstand.

4. Residual voltage ratio: When the current flowing through the varistor reaches a certain value, the voltage generated across it is called this current value, which is the residual voltage. The residual voltage ratio is the ratio of the residual voltage to the nominal voltage.

5. Flow capacity. Flow capacity, also called flow capacity, refers to the maximum pulse (peak) current allowed to pass through the varistor under specified conditions (standard impulse current is applied at specified time intervals and times). value.

6. Leakage current Leakage current and waiting current refer to the current flowing through the varistor under the specified temperature and maximum DC voltage.

7. Voltage temperature coefficient The voltage temperature coefficient refers to the rate of change of the nominal voltage of the varistor within a specified temperature range (temperature is 20~70°C), that is, when the current through the varistor remains constant, the temperature The relative change at both ends of the varistor when changing 1°C.

8. Current temperature coefficient The current temperature coefficient refers to the relative change in the current flowing through the varistor when the temperature changes by 1°C when the voltage across the varistor remains constant.

9. The voltage nonlinear coefficient refers to the ratio of the static resistance value to the dynamic resistance value of the varistor under a given external voltage.

10. Insulation resistance Insulation resistance refers to the resistance value between the lead wire (pin) of the varistor and the insulating surface of the resistor.

11. Static capacitance Static capacitance refers to the inherent capacitance of the varistor itself.

Simple measurement methods and steps for varistor

1) Fill the constant temperature oil pan with salad oil.

2) Place the thermometer horizontally into the oil pan (the thermometer must be completely immersed in the oil).

3) Connect a pair of alligator clips to the test ends of a pair of multimeter pens respectively.

4) Clamp the two lead ends of a varistor on the alligator clip, insert the other end of the multimeter pen into the multimeter, turn on the multimeter and set it to the appropriate gear for resistance measurement. At this time, the multimeter shows that the varistor is completely immersed in oil at room temperature. Within, this multimeter displays the resistance value of the varistor at room temperature.

5) Put the alligator clip into the oil pan filled with salad and oil for this multimeter, so that the alligator clip and the varistor are all immersed in the oil to monitor the temperature change of the oil in the oil pan.

6) When the multimeter that monitors the oil temperature displays a relatively stable value, it means that the oil temperature is relatively stable. At this time, the constant temperature resistance in the oil can be tested, such as the resistance and accuracy of the varistor used for monitoring on the alligator clip and the pressure to be measured. When the resistance accuracy of the sensitive resistors is the same, comparative measurements can be made.

Precautions for measuring varistor

1) The temperature change of the oil in the oil pan should be controlled within the range of 20±0.1℃~~30±0.1℃.

2) When the constant temperature cannot reach the control accuracy, a varistor with the same resistance value and accuracy can be used as the oil temperature monitoring resistor for comparative measurement.

3) The two multimeters should be calibrated accurately before measurement, and at least the consistency of the two meters should be checked.

4) The varistor to be measured should be kept at constant temperature in oil for no less than 2 minutes, and should be completely immersed in oil.

5) The temperature probe, monitoring varistor and resistance to be measured should be placed in the same position in the oil pan to ensure accurate measurement.

I hope you can learn something about varistor through this article. The use of varistor is regular, and following some necessary rules can make the life of varistor longer. The above is the analysis of relevant knowledge about varistor, I hope it can help you.