Varistors are widely used in household appliances and other electronic products, serving functions such as overvoltage protection, lightning protection, surge current suppression, pulse spike absorption, amplitude limiting, high-voltage arc extinguishing, noise reduction, and protection of semiconductor components. This article introduces varistors in three parts: principle, parameters, and selection methods.
I. Working principle of varistors
A varistor, as the name suggests, is a resistor whose resistance changes with voltage within a certain current and voltage range; it is also called a "voltage-sensitive resistor," abbreviated as "VDR." The characteristic of this type of resistor is that its resistance can rapidly decrease from the megaohm level to the milliohm level as the applied voltage gradually increases. At low voltages, the varistor operates in the leakage current region, exhibiting a high resistance state and maintaining a low leakage current. When the voltage exceeds a certain threshold and enters the nonlinear region, the voltage remains relatively stable despite a wide range of current variations, demonstrating good voltage limiting characteristics. Once the voltage rises further into the saturation region, the varistor will exhibit a low resistance state. Due to the large current at this point, prolonged operation can lead to overheating or even burnout. Under normal operating conditions, the varistor is usually in the leakage current region, only entering the nonlinear region for current discharge when subjected to surge impacts, and generally does not enter the saturation region.
From a materials perspective, the main material of varistors is semiconductor. Among them, the widely used zinc oxide varistor is composed of divalent zinc and hexavalent oxygen, belonging to the group II-VI oxide semiconductors. This material characteristic endows zinc oxide varistors with unique voltage sensitivity and nonlinear characteristics.
II. Applications and Functions of Varistors
Varistors, a unique electronic component, play a crucial role in circuit protection. Their core characteristic lies in their ability to control current flow when the applied voltage is below a certain threshold, acting like a tightly closed gate; however, once the voltage exceeds this threshold, the current increases dramatically, as if the gate were to open instantly. This characteristic makes varistors an ideal choice for suppressing abnormal overvoltages in circuits, effectively protecting them from damage.
Furthermore, varistors also possess transient voltage suppression capabilities, allowing them to replace combinations of transient suppression diodes, Zener diodes, and capacitors, providing comprehensive protection for ICs and other electronic devices. Whether it's electrostatic discharge, surges, or other transient currents (such as lightning strikes), varistors react rapidly, reducing their resistance to conduct large currents, thus ensuring the integrity of ICs or electrical equipment. When the voltage is below their operating range, the varistor exhibits high resistance, nearly operating as an open circuit, having no impact on the normal operation of the device or equipment.
III. Nominal Parameters of Varistors
Varistors are marked with "MY". Adding specific letters indicates their application area; for example, "J" represents household use. The letters W, G, P, L, H, Z, B, C, N, and K are associated with functions such as voltage regulation, overvoltage protection, high-frequency circuits, lightning protection, arc suppression, noise reduction, compensation, demagnetization, high energy efficiency, or high reliability, respectively. It is important to note that although varistors can absorb a large amount of surge energy, they cannot withstand continuous currents exceeding milliamperes. Therefore, this must be fully considered when designing overvoltage protection circuits.
IV. Characteristic Parameters of Varistors
① Varistor voltage UN (U1mA): This is an important parameter of the varistor, representing the voltage value when a 1mA DC current flows through it. This voltage indicates whether the varistor has started to conduct, and is usually represented by the symbol U1mA. Its error range is usually controlled within ±10%. In practical applications, a drop in varistor voltage to 90% of its normal value is generally considered the criterion for judging varistor failure.
② Maximum continuous operating voltage UC: This parameter defines the maximum AC or DC voltage that the varistor can withstand over a long period of time. Generally, the maximum AC voltage Uac is about 64% of the varistor voltage, while the maximum DC voltage Udc is about 83%.
③ Maximum Inrush Current (IP): This reflects the maximum 8/20μs wave inrush current peak value that the varistor can withstand. "Can withstand" here means that the rate of change of the varistor voltage after the inrush will not exceed 10%. The IP value for a single inrush is usually given in the technical specifications.
④ Maximum clamping voltage (limiting voltage) VC: When a specified 8/20μs surge current is applied to the varistor, a voltage value will appear across the varistor. This is the maximum clamping voltage or limiting voltage. This parameter is very important in practical applications because it directly affects the protective effect of the varistor.
⑤ Rated Energy E: Rated energy measures the maximum energy a varistor can withstand under a specified waveform impact. After this energy impact, the rate of change of the varistor voltage remains within 10%. The calculation formula is: E = KIPVC * T, where IP and VC are defined parameters, T represents the pulse width, and K is a waveform-related constant. For common 8/20μs and 10/1000μs waves, the K value is 1.4; for a 2ms square wave, the K value is 1.
⑥ Rated power (maximum average power) Pm: The maximum average power that a varistor can withstand when subjected to multiple consecutive impacts with short intervals at room temperature, resulting in a heat accumulation effect. It should be noted that although a varistor can withstand significant pulse power, its average power handling capability is relatively low.
⑦ Capacitance C0: The capacitance present between the two electrodes of the varistor, typically ranging from a few picofarads to several hundred nanofarads. It is worth noting that smaller varistors often have higher varistor voltages and smaller capacitances.
⑧ Leakage current Il: The current flowing through the varistor when the maximum DC voltage Udc is applied. When measuring leakage current, the voltage typically used is Udc = 0.83U1mA (sometimes 0.75U1mA). The static leakage current Il is usually required to not exceed 20 microamps (or 10 microamps). However, in practical applications, we are more concerned with the stability of the leakage current, especially whether its rate of change is within one time limit under impact testing or high-temperature environments, to ensure its stability.
⑨ Nonlinearity index α: This index measures the degree of influence of voltage changes on current. Its formula is I=KUα or α=log log. According to the formula, the larger the α value, the more significant the influence of voltage changes on current, and the better the nonlinear characteristics of the varistor. Simultaneously, α can also be understood as the reciprocal of the slope at each point on the volt-ampere characteristic; the flatter the characteristic region, the larger the α value (for example, in the leakage current region and saturation region, α=1, known as the low α region). In instrument measurement, I2=1mA and I1=0.1mA are usually set to calculate αT, whose value is 1/log(U1mA/U0.1mA).
Varistor parameters
1. Varistor voltage UN: also known as breakdown voltage or threshold voltage. It refers to the voltage value under a specified current, and in most cases, it is the voltage value measured when a 1mA DC current is applied to the varistor.
2. Leakage current: also known as standby current, refers to the current flowing through the varistor when it is at a specified temperature and maximum continuous DC voltage.
3. Voltage ratio: refers to the ratio of the voltage value of the varistor when the current is 1mA to the voltage value of the varistor when the current is 0.1mA.
4. The nonlinear coefficient 'a' is the reciprocal of the logarithm of the pressure ratio.
5. Limiting Voltage Up: In a broad sense, the limiting voltage refers to the peak value across the varistor when an inrush current flows into it. As a performance indicator for varistors, the limiting voltage Up refers to the peak value of the voltage across the varistor when an inrush current flows in at waveform 8/20 and peak value of the specified value. The basic function of a varistor is to suppress transient abnormal voltages, so the limiting voltage is one of its most important performance parameters.
6. Voltage Limiting Ratio Rp: The ratio of the limiting voltage Up to the varistor voltage is called the voltage limiting ratio Rp. The actual limiting voltage of the device should be lower than the specified value. The smaller Rp is, the closer it is to 1, the better the voltage limiting performance of the device. Therefore, the technical specifications specify the maximum allowable value of Up.
7. Maximum continuous operating voltage Uac, Udc: This refers to the maximum effective value of AC voltage Uac, or the maximum DC voltage Udc, that the varistor can withstand over a long period. The principle for determining Uac is: the peak value of the AC voltage should not exceed the lower limit of the varistor voltage tolerance; the principle for determining Udc is: the power consumption of the varistor is roughly the same under Uac and Udc, thus Uac = 0.64UN, Udc = 0.83UN, and Udc = 1.3Uac.
8. Current Capacity: Current capacity, also known as flow rate, refers to the maximum pulse (peak) current value allowed to pass through a varistor under specified conditions (specified time intervals and number of times, applying a standard inrush current). The peak value of the maximum pulse current is the maximum pulse current value at an ambient temperature of 25℃, where the variation of the varistor voltage does not exceed ±10% for a specified inrush current waveform and a specified number of inrush currents.
9. Maximum energy: refers to the maximum surge energy allowed to pass through the varistor under specified conditions (specified time intervals and number of times, application of standard surge current).
10. Rated power: refers to the maximum average power that the varistor can withstand under a certain current pulse group. Being able to withstand means that the varistor voltage after the impact is no more than ±10% compared with the voltage before the impact, and there is no visible mechanical damage.
11. Static capacitance: refers to the inherent capacitance of the varistor itself.
12. Voltage temperature coefficient: refers to the rate of change of the nominal voltage of a varistor within a specified temperature range. Specifically, it refers to the relative change in voltage across the varistor when the temperature changes by 1°C, provided the current through the varistor remains constant. The corresponding current temperature coefficient refers to the relative change in current flowing through the varistor when the temperature changes by 1°C, provided the voltage across the varistor remains constant.
13. Insulation resistance: refers to the resistance value between the lead (pin) of a varistor and the insulating surface of the resistive body.
14. Insulation withstand voltage: refers to the voltage that the electrode leads of the varistor can withstand between its encapsulation layer surface. In addition to the electrical characteristics mentioned above, the environmental characteristics of the varistor include: climate, steady-state damp heat, rapid temperature change, upper limit temperature durability, damp heat environment durability, flame retardancy, moisture resistance, etc. The mechanical characteristics include: vibration, shock, solderability, terminal strength, etc. In addition, there are operating temperature range and storage temperature range, etc.
Methods for selecting varistors
3.1 Selection Principles
The selection principles for varistors are: to consider all aspects, meet standards, make compromises, and rely on experiments.
Specifically, forward-looking considerations should include: the upper limit of the normal fluctuation range of the system voltage; the highest transient voltage and its duration under fault conditions; the peak voltage and source impedance (or inrush current) of the impulse source; and the duration and frequency of the impulse.
The following factors should be considered: the withstand voltage level of the protected object; the inherent capacitance and resistive leakage current of the varistor allowed by the protected object.
The basic requirements for considering both the preceding and following conditions are as follows: Under the maximum impulse voltage of the expected impulse source, the limiting voltage of the varistor should be lower than the impulse withstand voltage of the protected object; Under the upper limit of the normal fluctuation range of the system voltage, and under fault and highest ambient temperature conditions, the expected service life of the varistor should be greater than the design requirement; The current carrying capacity, rated energy, and power consumption of the varistor should be greater than the predetermined maximum impulse current, impulse energy, and average power consumption of the impulse source; Under specified conditions, the impulse life of the varistor should be greater than the number of impulses from the impulse source during its service life; When abnormal conditions occur in the system voltage and impulse source exceeding the expected values, the varistor will not catch fire, will not explode and endanger adjacent components, and will not cause electric shock; The influence of the capacitance and nonlinear current of the varistor on the protected object or system should be within the allowable range.
A varistor is a resistive device with nonlinear current-voltage characteristics, mainly used for voltage clamping when a circuit is subjected to overvoltage, absorbing excess current to protect sensitive components. The resistive material of a varistor is a semiconductor, making it a type of semiconductor resistor. The widely used zinc oxide (ZnO) varistor is composed primarily of divalent zinc (Zn) and hexavalent oxygen (O). Therefore, from a materials science perspective, a zinc oxide varistor is a group II-VI oxide semiconductor.
Function and working principle of varistors
The function of varistor
A varistor is a special type of resistor whose main function is to protect circuits from overvoltage damage. When the voltage in a circuit exceeds a predetermined range, the varistor quickly changes its resistance, thereby absorbing and dissipating the excessive electrical energy. This protects other components from damage.
Working principle of varistor
Varistors are made of semiconductor materials and have non-linear resistance characteristics. When the voltage in the circuit is below a certain threshold, the resistance of the varistor is very high, and almost no current flows through it. However, once the voltage exceeds the threshold, the resistance of the varistor drops rapidly, forming a low-resistance path that allows excess electrical energy to pass through and be dispersed.
Varistor Models and Parameters
Varistor model
There are many common types of varistors, the most common of which are "MOV" (Metal Oxide Varistor) and "VDR" (Voltage Dependent Resistor).
Parameters of a varistor
The main parameters of a varistor include:
Resistance range: This refers to the range of resistance values of the varistor under operating conditions. Common resistance ranges range from a few ohms to several thousand ohms.
Rated voltage: This refers to the maximum voltage that a varistor can withstand. Exceeding this voltage will cause the resistance to drop sharply. Rated voltage is usually measured in volts (V).
Rated power: This indicates the maximum power that a varistor can absorb and dissipate at its rated voltage. Rated power is usually measured in watts (W).
In addition, there are other parameters such as response time and tolerance, which may vary depending on the specific varistor model.
In summary, varistors play a protective role in circuits, absorbing and dissipating excessive electrical energy to prevent damage to other components. Their working principle is achieved through the non-linear resistive characteristics of semiconductor materials. Common varistor models include "MOV" and "VDR," and their main parameters include resistance range, rated voltage, and rated power. Understanding these parameters can help in selecting a suitable varistor to meet specific circuit protection needs.
