A varistor is a device with a non-linear volt-ampere characteristic. When the voltage applied to the varistor is lower than its threshold value, the current flowing through it is extremely small, which is equivalent to a resistor with infinite resistance, vice versa. The most common varistor is a metal oxide varistor (MOV).

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Ⅰ What is a varistor?

A varistor is a device with a non-linear volt-ampere characteristic. It is mainly used to clamp the voltage when the circuit is subjected to overvoltage and absorb excess current to protect sensitive devices. It is also called "Voltage-Dependent Resistor" abbreviated as "VDR". The material of the resistor body of a varistor is a semiconductor, so it is a variety of semiconductor resistors. The "zinc oxide" (ZnO) varistor, which is now widely used, has the main material composed of the divalent element zinc (Zn) and the hexavalent element oxygen (O). So from the perspective of materials, the zinc oxide varistor is a kind of "II-VI oxide semiconductor".

A varistor

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

Ⅱ How do varistors work?

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, the response speed of overvoltage protection for electronic circuits can meet the requirements. The junction capacitance of a varistor is generally in the order of hundreds to thousands of Pf. In many cases, it should not be directly applied to the protection of high-frequency signal lines. When applied to the protection of AC circuits, the large junction capacitance will increase the leakage. The current needs to be fully considered when designing the protection circuit. The varistor has a larger flow capacity but is smaller than a gas discharge tube.

When the voltage applied to the varistor is lower than its threshold, the current flowing through it is extremely small, which is equivalent to a resistor with infinite resistance. That is when the voltage applied to it is below 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 increases sharply, which is equivalent to an infinitely small resistance. In other words, when the voltage applied to it is higher than its threshold, it is equivalent to a closed state switch.

Ⅲ Main parameters of the varistor

The main parameters of the varistor are 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. The nominal voltage refers to the voltage value across the varistor when a 1mA DC is passed.

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

3. The maximum limiting 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 is a certain value, the voltage generated across it is called this current value as the residual voltage. The residual voltage ratio is the ratio of the residual voltage to the nominal voltage.

5. The through-current capacity is also called the through-flow capacity, which refers to the maximum pulse (peak) current allowed to pass through the varistor under specified conditions (with a specified time interval and number of times, standard inrush current is applied).

6. Thw leakage current and waiting current refer to the current flowing through the varistor at the specified temperature and maximum DC voltage.

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

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

9. The voltage non-linear coefficient refers to the ratio of static resistance value to the dynamic resistance value of a varistor under a given applied voltage.

10. The insulation resistance refers to the resistance value between the lead (pin) of the varistor and the insulating surface of the resistor body.

11. The static capacitance refers to the inherent capacitance of the varistor itself.

Ⅳ The function of varistors

The main function of the varistor is to protect the transient voltage in the circuit. Due to its working principle as described above, the varistor is equivalent to a switch. Only when the voltage is higher than its threshold, and the switch is closed, the current flowing through it surges and the impact on other circuits does not change much, thereby reducing the impact of overvoltage on subsequent sensitive circuits. This protection function of the varistor can be used repeatedly and can also be made into a one-time protection device similar to a current fuse.

The protection function of the varistor has been widely used. For example, the power circuit of a household color TV uses a varistor to complete the overvoltage protection function. When the voltage exceeds a threshold value, the varistor reflects its clamping characteristics. The excessive voltage is pulled low so that the subsequent circuit works within the safe voltage range.

The varistor is mainly used for transient overvoltage protection in a circuit, but because of its volt-ampere characteristics similar to a semiconductor Zener, it also has a variety of circuit element functions. For example, the varistor is a kind of DC high voltage small current-voltage stabilizing element with a stable voltage of thousands of volts or more, which cannot be reached by silicon Zener. Varistor can be used as a voltage fluctuation detection element, a DC level shifter Bit element, a fluorescent starting element, a voltage equalizing element, and so on.

Ⅴ Metal oxide varistor

The most common varistor is a metal oxide varistor (MOV), which contains a ceramic block composed of zinc oxide particles and a small number of other metal oxides or polymers, sandwiched between two metal sheets. A diode effect is formed at the junction of particles and adjacent oxides. Due to a large number of messy particles, it is equivalent to a large number of back-connected diodes. There is only a small reverse leakage current at low voltage. When high voltage is encountered, the diode reverse collapse occurs due to hot electrons and tunneling effect, and a large current flows. Therefore, the current-voltage characteristic curve of a varistor is highly non-linear: high resistance at low voltage and low resistance at high voltage.

Metal oxide varistors are currently the most common voltage clamping devices and can be used for various voltages and currents. The use of metal oxides in its structure means that MOVs are very effective in absorbing short-term voltage transients and have higher energy handling capabilities.

Like ordinary varistors, metal oxide varistors begin to conduct at a certain voltage and stop conducting when the voltage is lower 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 in the clamping transient.

MOVs usually have radial leads and a hard outer blue or black epoxy coating, which is very similar to disc ceramic capacitors and can be physically mounted on circuit boards and PCBs in a similar manner. The structure of a typical metal oxide varistor is as follows:

Metal oxide varistor structure

To select the correct MOV for a specific application, it is necessary to understand the source impedance and the possible pulse power of the transient. For input line or phase transients, the selection of the correct MOV is slightly more difficult, because the characteristics of the power supply are generally unknown. Generally speaking, the electrical protection of MOV selection circuit power transients and spikes is usually just an educated guess.

However, metal oxide varistors can be used for a variety of varistor voltages, from about 10 volts to more than 1000 volts AC or DC, so it can help you choose by knowing the supply voltage. For example, choose MOV or silicon varistor. For voltage, its maximum continuous root means square voltage rating should be slightly higher than the highest expected power supply voltage. For example, a 120-volt power supply is 130 volts rms, and 230 volts is a 260 volts rms supply.

The maximum surge current value that the varistor will use depends on the transient pulse width and the number of pulse repetitions. An assumption can be made about the width of the transient pulse, which is usually 20 to 50 microseconds (μs) long. If the peak pulse current rating is insufficient, the varistor may overheat and be damaged. Therefore, if the varistor operates without any failure or degradation, it must be able to quickly dissipate the absorbed energy of the transient pulse and safely return to its pre-pulse state.

Ⅵ Characteristics of a damaged varistor

A resistor is the most numerous component in electrical equipment, but it is not the component with the highest damage rate. An open circuit is the most common type of resistance damage. It is rare for resistance to become large, and it is very rare for resistance to become small. Common types are carbon film resistors, metal film resistors, wire wound resistors, and fuse resistors. The first two types of resistors are the most widely used. Their damage characteristics are low resistance (below 100Ω;) and high resistance (above 100Ω;). The second is that when the low resistance resistor is damaged, it is often burnt and blackened, which is easy to find, and when the high resistance resistor is damaged, there are few traces. Wire-wound resistors are generally used for high current limiting, and the resistance is not large. When the cylindrical wire-wound resistor is burned out, some of it will become black or the surface will explode, crack. Cement resistance is a kind of wire wound resistance, which may break when burned out, otherwise, there will be no visible traces. When the fuse is burnt out, some surfaces will blast off, and some will have no trace, but they will never burn and become black.

Ⅶ How to test varistors?  

1. Preparation before varistor measurement

Connect the two test leads (regardless of positive and negative) to the two ends of the resistor to measure the actual resistance value. To improve the measurement accuracy, the range is selected according to the nominal value of the measured resistance. Due to the non-linear relationship of the ohm scale, the middle section of the scale is fine. Therefore, the pointer value should fall as far as possible to the middle of the scale, that is, within the range of 20% to 80% of the radian of the full scale. According to the resistance error level, an error of ± 5%, ± 10%, or ± 20% is allowed between the reading and the nominal resistance, respectively. If the error range is exceeded, the resistor has changed the standard value.

2. How to measure the quality of a varistor?

Judging the varistor usually requires a power supply with a wide regulating voltage range, and it has a good current limiting effect. A voltmeter with good precision is connected in parallel across the varistor when measuring. Connect the adjustable power lead to both ends of the varistor.

The voltmeter indicates the power supply voltage. You should slowly adjust the voltage and will observe that the voltage suddenly drops after reaching a certain voltage. The voltage at the last moment before the decrease is the protection value of the varistor.

With the continuous voltage applied to the varistor, its resistance value can change from MΩ (Megohm) to mΩ (Milliohm). When the voltage is low, the varistor works in the leakage current region, showing a large resistance, and the leakage current is small; when the voltage rises into the non-linear region, the current changes within a relatively large range, and the voltage does not change much showing a good voltage limiting characteristic; when the voltage is raised again, the varistor enters the saturation region and presents a very small linear resistance. Due to the large current, the varistor will overheat and burn or even burst over time.

A multimeter

3. Selection of varistor

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

(1) Selection of varistor voltage V1mA

According to the selection of the power supply voltage, the power supply voltage continuously applied across the varistor must not exceed the "maximum continuous operating voltage" value listed 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), that is, VDC ≥ VIN; For the selection of the 220V AC power source, the fluctuation range of the operating voltage of the power grid must be fully considered. The general fluctuation range of the domestic power grid 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 prolong the service life, but the residual voltage is slightly increased.

(2) Selection of traffic

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 during equipment operation. The nominal discharge current should be calculated by pressing the value of more than 10 shocks in the rating curve of times of the surging life, which is about 30% of the maximum shock flux (ie, 0.3IP).

(3) Selection of clamping voltage

The clamping voltage of the varistor must be less than the maximum voltage (ie, safe voltage) that the component or equipment being protected can withstand.

(4) The choice of capacitor Cp

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

(5) Internal resistance matching (Resistance Match)

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

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