Varistor: definition, function, work and test (2023)

Catalog

Ⅰ What is a varistor?

A varistor is a device with a non-linear volt-ampere characteristic. It is mainly used to fix the voltage when the circuit experiences an overvoltage and absorb excess current to protect sensitive equipment. It is also called "voltage dependent resistance"abbreviated as"VDR". The material of the body of the resistor of a varistor is asemiconductor, so it is a question of a large number of semiconductor resistors. The currently widespread "zinc oxide" (ZnO) varistor consists of the divalent element zinc (Zn) and the hexavalent element oxygen (O) as its main material. In terms of material, the zinc oxide varistor is therefore a type of "II-VI oxide semiconductor".

a varistor

A varistor is a voltage-limited protective device. Using the non-linear characteristics of the varistor, when an overvoltage occurs between the two poles of the varistor, the varistor can fix the voltage to a relatively fixed voltage value, thereby achieving the protection of the subsequent circuit. The main parameters of the varistor are the varistor voltage, current capacity, junction capacity, response time, etc.

ⅡHow do varistors work?

The response time of varistor is ns, which is faster than gas discharge tube and slightly slower than TVS tube. In general, the response speed of the electronic circuit surge protector can meet the requirements. The junction capacitance of a varistor is typically on the order of hundreds to thousands of pf. In many cases it should not be applied directly to protect high frequency signal lines. When applied to AC circuit protection, the large junction capacitance increases the leakage current. The current must be fully considered when designing the protection circuit. The varistor has a higher flow capacity but is smaller than a gas arrestor.

When the voltage applied to the varistor is below its threshold, the current flowing through it is extremely small, equivalent to a resistor of infinite resistance. That is, when the voltage applied to it is below its threshold, it corresponds to an off switch.

When the voltage applied to the varistor exceeds its threshold, the current flowing through it increases dramatically, which is equivalent to an infinitely small resistance. In other words, if the voltage applied across it is greater than its threshold, it corresponds to a switch in the closed state.

ⅢHauptparameter des Varistors

The main parameters of the varistor are rated voltage, voltage ratio, maximum control voltage, residual voltage ratio, current capacity, leakage current, voltage temperature coefficient, current temperature coefficient, non-linear voltage coefficient, insulation resistance, static capacity, etc.

1. Onominal voltagerefers to the voltage reading across the varistor when a 1mA DC current is passed through it.

2. Otensionrefers to the relationship between the voltage value generated when the varistor current is 1 mA and the voltage value generated when the varistor current is 0.1 mA.

3. Omaximum limit voltagerefers to the highest voltage value that the two ends of the varistor can withstand.

4.residual stress ratio: When the current flowing through the varistor has a certain value, the voltage generated by it is called the current value as residual voltage. The residual voltage ratio is the ratio between the residual voltage and the nominal voltage.

5. Othroughput capacityalso called flow capacity, which refers to the maximum pulse current (peak current) that is allowed to flow through the varistor under certain conditions (with a certain time interval and a certain number of times the standard inrush current is applied).

6. Thwleakage current and quiescent currentthey refer to the current that flows through the varistor at the specified temperature and maximum DC voltage.

7. Ovoltage temperature coefficientdenotes the rate of change of the varistor nominal voltage within a certain temperature range (temperature 20 ~ 70 °C), i.e. H. with constant current through the varistor, the relative change of both ends of the varistor with temperature change 1℃.

8. Ocurrent temperature coefficientrefers to the relative change in current flowing through the varistor when the temperature across the varistor remains constant and the temperature changes by 1°C.

9. Ononlinear stress coefficientrefers to the static partresistance valueon the dynamic resistance of a varistor at a given applied voltage.

10. OIsolationswiderstandrefers to the resistance value between the wire (pin) of the varistor and the insulating surface of the resistor body.

11. Ostatic capacityrefers to the intrinsic capacitance of the varistor itself.

ⅣThe function of varistors

The main function of the varistor is to protect the transient voltage in the circuit. Due to the functional principle described above, the varistor corresponds to a switch. Only when the voltage is greater than its threshold and the switch is closed does the current flowing through it increase and the effects on other circuits do not change significantly, reducing the impact of the overvoltage on sensitive downstream circuits. This varistor protection function can be used multiple times and can also be made into a one-off protection device similar to a current fuse.

The varistor protection function is widely used. For example, the circuit of a home color TV uses a varistor to complete the surge protection function. When the voltage exceeds a threshold, the varistor returns its clamping characteristics. The overvoltage is reduced in such a way that the subsequent circuit works in the safe voltage range.

The varistor is primarily used to protect against transient overvoltages in a circuit, but due to its volt-ampere characteristics similar to a zener semiconductor, it also has a variety of circuit element functions. For example, the varistor is a kind of small, high-voltage DC voltage stabilizing element with a stable voltage of thousands of volts or more, which cannot be achieved with the silicon zener. The varistor can be used as a voltage fluctuation detection element, DC level shift bit element, starting fluorescent element, voltage compensation element, etc.

ⅤMetaloxid-Varistor

The most common varistor is a metal oxide varistor (MOV), which contains a ceramic block made up of zinc oxide particles and a small amount of other metal oxides or polymers, sandwiched between two sheets of metal. A diode effect occurs at the transition between neighboring particles and oxides. Due to the large number of jumbled particles, this corresponds to a large number of backlinked particles.diodes. At low voltage there is only a small reverse leakage current. When a high voltage occurs, reverse collapse of the diode occurs due to hot electrons and tunneling, and a large current flows. Therefore, the current-voltage characteristic of a varistor is highly non-linear: high-impedance at low voltage and low-impedance at high voltage.

Metal oxide varistors are currently the most common voltage clamping devices and can be used for different voltages and currents. The use of metal oxides in their structure means that MOVs absorb short-term voltage transients very effectively and have higher power handling capabilities.

Like ordinary varistors, metal oxide varistors start conducting at a certain voltage and stop conducting when the voltage is less than the threshold voltage. The main difference between the standard silicon carbide (SiC) varistor and the MOV type varistor is that the leakage current of the zinc oxide material through the MOV is very small under normal operating conditions, and its operating speed is much faster when momentarily stuck.

MOVs typically have radial connectors and a hard blue or black epoxy outer shell that closely resembles the disc.ceramic capacitorsand can be physically mounted on printed circuit boards and PCBs in a similar manner. The structure of a typical metal oxide varistor is as follows:

Metal Oxide Varistor Structure

In order to choose the right MOV for a specific application, it is necessary to understand the source impedance and the possible power of the transient pulse. For input line or phase transients, choosing the right MOV is a bit more difficult as the power supply characteristics are often unknown. In general, the electrical protection against transients and overvoltages of the MOV selection circuit is usually just an assumption.

However, metal oxide varistors can be used for a wide range of varistor voltages, from around 10 volts to over 1000 volts AC or DC, so knowing your supply voltage can help you make a choice. For example, choose MOV or silicon varistor. For voltage, the maximum continuous square root means that the voltage rating squared should be slightly higher than the highest expected voltage of the power supply. For example, a 120 volt power supply is 130 volts rms and a 230 volt power supply is 260 volts rms.

The maximum value of the peak current that the varistor uses depends on the width of the transient pulse and the number of pulse repetitions. An assumption can be made about the width of the transient pulse, which typically has a duration of 20 to 50 microseconds (μs). If the maximum pulse current is insufficient, the varistor may overheat and become damaged. Therefore, if the varistor is to function without failure or degradation, it must be able to quickly dissipate the energy absorbed by the transient pulse and safely return to its pre-pulse state.

ⅥCharacteristics of a damaged varistor

A resistor is the most common component in electrical equipment, but it is not the component with the highest damage rate. An open circuit is the most common type of resistive damage. It is rare that resistance becomes large and it is very rare that resistance becomes small. Common types arecarbon film resistors,metal film resistors, wirewound resistors and fuse resistors. Firsttwo types of resistanceare most commonly used. Its damage characteristics are low resistance (below 100 Ω;) and high resistance (above 100 Ω;). The second is that when the low-value resistor is damaged, it usually burns and darkens, which is easy to find, and when the high-value resistor is damaged, only a few traces are left. Wire wound resistors are generally used for high current limiting, and the resistance is not large. When the cylindrical wire resistor burns out, part of it turns black or the surface bursts, cracks. The cement resistor is a type of wire-wound resistor that can be destroyed if burned, otherwise no visible traces will remain. If the fuse blows, some surfaces will explode and others will leave no trace, but they will never burn or turn black.

Ⅶ How are varistors tested?

1. Preparation before the varistor measurement

Connect the two test leads (either positive or negative) to the two ends of the resistor to measure the actual value of the resistor. To improve the measurement accuracy, the range is selected according to the nominal value of the measured resistance. Because of the non-linear relationship of the ohm scale, the center portion of the scale is correct. Therefore, the pointer value should be as far as possible from half the scale, i.e. H. within the range of 20% to 80% of full scale radians. Depending on the error level of the resistance, an error of ±5%, ±10% or ±20% is allowed between the reading and the nominal resistance. If the error range is exceeded, the resistance has changed from the standard value.

2. How do you measure the quality of a varistor?

Evaluation of the varistor generally requires a power supply with a wide voltage regulator, and has good current-limiting effect. A voltmeter with good accuracy is connected parallel to the varistor during the measurement. Connect the adjustable power cord to both ends of the varistor.

The voltmeter shows the voltage of the power supply. You need to adjust the tension slowly, and you will find that after reaching a certain tension, the tension suddenly drops. The voltage at the last moment before dropping is the protective value of the varistor.

When DC voltage is applied to the varistor, its resistance can change from MΩ (megohms) to mΩ (milliohms). When the voltage is low, the varistor works in the region of leakage current with large resistance, and the leakage current is small; when the voltage rises in the non-linear region, the current changes within a relatively large range and the voltage does not change much, showing good voltage-limiting characteristics; When the voltage is increased again, the varistor enters saturation and has a very low linear resistance. Due to the large current, the varistor will overheat over time and burn out or even explode.

a multimeter

3. Selection of the varistor

When selecting a varistor, the specific conditions of the circuit should be considered, and in general the following principles should be followed:

(1) Selection of varistor voltage V1mA

Depending on the choice of supply voltage, the supply voltage permanently applied to the varistor must not exceed the "maximum continuous operating voltage" value in the specification. That is, the maximum DC operating voltage of the varistor must be greater than the DC operating voltage VIN of the power line (signal line), i.e. H. VDC ≥ VIN; When selecting a 220V AC power supply, the range of mains operating voltage must be fully considered. The general range of fluctuation in the house electricity network is 25%. A varistor with a varistor voltage of 470V to 620V should be selected. Selecting a varistor with a higher varistor voltage can reduce the failure rate and increase the service life, but the residual voltage will increase slightly.

(2) Traffic selection

The rated leakage current of the varistor must be greater than the required overcurrent or the maximum overcurrent that can occur during operation of the device. The rated discharge current should be calculated by pushing the value of more than 10 surges on the rated life curve, which is approximately 30% of the maximum surge flux (ie 0.3IP).

(3) Clamp voltage selection

The varistor reverse voltage must be less than the maximum voltage (i.e. safe voltage) that the protected component or equipment can withstand.

(4) The choice of the capacitor Cp

For high-frequency transmission signals, the capacitance Cp must be smaller and vice versa.

(5) Matching the internal resistance (Resistance matching)

The relationship between the internal resistance R (R≥2Ω) of the protected component (line) and the internal contact resistance Rv of the varistor: R≥5Rv; For the protected component with small internal resistance, you should try to use a large capacitor varistor without affecting the signal transmission speed.

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