Selection of Varistors
This article outlines the methods for selecting varistors, focusing on parameter selection and important considerations for their application.
1. Selection of Varistor Parameters
(1) Selection of Varistor Voltage (V1mA)
For overvoltage protection applications, the varistor voltage value should exceed the actual circuit voltage. It can generally be selected using the following formula: V1mA = a * U / (b * c)
Where:
a — Power supply voltage fluctuation coefficient (typically 1.2);
U — DC operating voltage or RMS AC voltage of the circuit;
b — Varistor voltage tolerance (typically 0.85);
c — Aging coefficient of the varistor component (typically 0.9).
The V1mA value calculated using the formula above is 1.5 times the DC operating voltage; however, for AC applications, the peak voltage must be considered, so the calculated result should be multiplied by √2.
(2) Selection of Surge Current Capacity
The surge current capacity specified by manufacturers is typically the maximum current a varistor can withstand—under pulse testing conditions defined by product standards (regarding waveform, number of surges, and time intervals)—while maintaining a change in varistor voltage of less than ±10% of the initial value. The number of surges a varistor can withstand depends on the waveform, amplitude, and time interval; if the current waveform amplitude is reduced by 50%, the allowable number of surges can be doubled. Therefore, in practical applications, the surge current absorbed by the varistor should be lower than the product's maximum surge current capacity to ensure a longer service life.
When selecting the surge current capacity, the primary consideration is whether the varistor is intended for lightning protection or for suppressing switching overvoltages within electronic instruments and equipment. Generally, the peak voltage of an induced lightning strike is approximately 3 to 5 times the operating voltage. If the primary purpose is lightning protection, lightning-rated varistors should be selected; these are available in various surge current capacities, such as 3kA, 5kA, and 20kA. Actual measured lightning currents typically range from 200A to 3000A, with the vast majority falling below 10kA. For surge currents generated by internal operations within electronic instruments and equipment—typically less than 500A—general-purpose varistors may be selected.
(3) Selection based on energy handling capability
The energy absorbed by a varistor can be calculated using the following formula:W = K · I · U · T (J)
Where: I = peak current flowing through the varistor;
U = voltage across the varistor when current I flows through it;
T = duration of current I;
K = waveform coefficient (K=1 for a 2ms square wave; K=1.4 for an 8/20μs wave; K≈1.4 for a 10/1000μs wave).
In practical applications, the varistor is required to absorb energy stored in the circuit (such as energy in coils and capacitors, as well as stray energy). When selecting a varistor for such scenarios, the total stored electrical energy in the circuit must be less than the varistor's energy absorption capacity.
Varistors currently on the market vary significantly in capacitance; therefore, selection should be based on the principle of not interfering with normal circuit operation. Standard varistors are suitable for use at frequencies below 300Hz.
2. Precautions for varistor usage
① Protective measures should be implemented for the varistor to prevent damage caused by external, unpredictable factors. If a fuse is installed in the circuit, the installation method should be as shown below.
② Varistors should not be installed near heat-generating or flammable components; a clearance of more than 3mm is recommended to ensure operation within the specified temperature range.
③ The operating voltage (voltage continuously applied across the varistor) must not exceed the maximum continuous operating voltage rating.
④ In applications involving repetitive surge currents, the peak surge current and surge energy passing through the varistor must not exceed the limits defined by its pulse life characteristics.
⑤ When surge pulses are applied repeatedly with short intervals, the average power must remain below the maximum static power rating.
⑥ If a thermistor is connected between the live parts of the equipment and the metal enclosure, necessary measures must be taken to prevent electric shock to personnel.
⑦ Varistors should be protected from direct sunlight during use and should not be operated in open-air or outdoor environments.
⑧ Varistors should not be operated under conditions of high temperature and humidity—including exposure to wind, rain, or water vapor—nor in environments containing dust, salt spray, or harmful gases. Protective housing should be used if necessary.
⑨ Do not clean varistors with solvents such as acetone during use, as this may damage the epoxy resin encapsulation.
⑩ Varistors should be stored in a warehouse where the temperature is below 40°C and the relative humidity is no higher than 75% RH; the storage environment must be free from corrosive gases. Stored varistors must also be protected from direct sunlight.
1. Selection of Varistor Parameters
(1) Selection of Varistor Voltage (V1mA)
For overvoltage protection applications, the varistor voltage value should exceed the actual circuit voltage. It can generally be selected using the following formula: V1mA = a * U / (b * c)
Where:
a — Power supply voltage fluctuation coefficient (typically 1.2);
U — DC operating voltage or RMS AC voltage of the circuit;
b — Varistor voltage tolerance (typically 0.85);
c — Aging coefficient of the varistor component (typically 0.9).
The V1mA value calculated using the formula above is 1.5 times the DC operating voltage; however, for AC applications, the peak voltage must be considered, so the calculated result should be multiplied by √2.
(2) Selection of Surge Current Capacity
The surge current capacity specified by manufacturers is typically the maximum current a varistor can withstand—under pulse testing conditions defined by product standards (regarding waveform, number of surges, and time intervals)—while maintaining a change in varistor voltage of less than ±10% of the initial value. The number of surges a varistor can withstand depends on the waveform, amplitude, and time interval; if the current waveform amplitude is reduced by 50%, the allowable number of surges can be doubled. Therefore, in practical applications, the surge current absorbed by the varistor should be lower than the product's maximum surge current capacity to ensure a longer service life.
When selecting the surge current capacity, the primary consideration is whether the varistor is intended for lightning protection or for suppressing switching overvoltages within electronic instruments and equipment. Generally, the peak voltage of an induced lightning strike is approximately 3 to 5 times the operating voltage. If the primary purpose is lightning protection, lightning-rated varistors should be selected; these are available in various surge current capacities, such as 3kA, 5kA, and 20kA. Actual measured lightning currents typically range from 200A to 3000A, with the vast majority falling below 10kA. For surge currents generated by internal operations within electronic instruments and equipment—typically less than 500A—general-purpose varistors may be selected.
(3) Selection based on energy handling capability
The energy absorbed by a varistor can be calculated using the following formula:W = K · I · U · T (J)
Where: I = peak current flowing through the varistor;
U = voltage across the varistor when current I flows through it;
T = duration of current I;
K = waveform coefficient (K=1 for a 2ms square wave; K=1.4 for an 8/20μs wave; K≈1.4 for a 10/1000μs wave).
In practical applications, the varistor is required to absorb energy stored in the circuit (such as energy in coils and capacitors, as well as stray energy). When selecting a varistor for such scenarios, the total stored electrical energy in the circuit must be less than the varistor's energy absorption capacity.
Varistors currently on the market vary significantly in capacitance; therefore, selection should be based on the principle of not interfering with normal circuit operation. Standard varistors are suitable for use at frequencies below 300Hz.
2. Precautions for varistor usage
① Protective measures should be implemented for the varistor to prevent damage caused by external, unpredictable factors. If a fuse is installed in the circuit, the installation method should be as shown below.
② Varistors should not be installed near heat-generating or flammable components; a clearance of more than 3mm is recommended to ensure operation within the specified temperature range.
③ The operating voltage (voltage continuously applied across the varistor) must not exceed the maximum continuous operating voltage rating.
④ In applications involving repetitive surge currents, the peak surge current and surge energy passing through the varistor must not exceed the limits defined by its pulse life characteristics.
⑤ When surge pulses are applied repeatedly with short intervals, the average power must remain below the maximum static power rating.
⑥ If a thermistor is connected between the live parts of the equipment and the metal enclosure, necessary measures must be taken to prevent electric shock to personnel.
⑦ Varistors should be protected from direct sunlight during use and should not be operated in open-air or outdoor environments.
⑧ Varistors should not be operated under conditions of high temperature and humidity—including exposure to wind, rain, or water vapor—nor in environments containing dust, salt spray, or harmful gases. Protective housing should be used if necessary.
⑨ Do not clean varistors with solvents such as acetone during use, as this may damage the epoxy resin encapsulation.
⑩ Varistors should be stored in a warehouse where the temperature is below 40°C and the relative humidity is no higher than 75% RH; the storage environment must be free from corrosive gases. Stored varistors must also be protected from direct sunlight.
Aug 07,2026