Varistor Selection
This article mainly discusses the selection method of varistors, focusing on the parameter selection method and precautions for their use.
1. Varistor Parameter Selection Method
(1) Selection of Varistor Voltage V1mA.
For overvoltage protection applications, the varistor voltage value should be greater than 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, generally taken as 1.2;
U---Effective value of DC operating voltage or AC voltage in the fluctuating circuit;
b---Varistor voltage error, generally taken as 0.85;
c---Aging coefficient of the varistor element, generally taken as 0.9.

The actual value of V1mA calculated by the above formula is 1.5 times the DC operating voltage. In AC conditions, the voltage peak value must be considered; therefore, the calculation result should be multiplied by √2.

(2) Selection of Current Capacity
The current capacity specified in a product is usually the maximum current that the varistor can withstand when the rate of change of its varistor voltage is less than ±10% of the initial value under pulse testing according to the waveform, number of impacts, and interval time specified in the product standard. The number of impacts a varistor can withstand is a function of the waveform, amplitude, and interval time. When the current waveform amplitude decreases by 50%, the number of impacts can double. Therefore, in practical applications, the surge current absorbed by the varistor should be less than the maximum current capacity of the product to ensure a longer service life.

When selecting the current capacity, the main consideration is whether the varistor is used for lightning protection or to prevent operational overvoltages inside electronic instruments and equipment. Generally, the peak value of induced lightning voltage is about 3.5 times the operating voltage. If it is mainly used for lightning protection, lightning-protected varistors can be selected. These are available in different types with current capacities such as 3kA, 5kA, and 20kA. The actual detected lightning current is in the range of 200-3000A, with the vast majority being less than 10kA. The surge current generated during the internal operation of electronic instruments and equipment is generally less than 500A, so a general-purpose varistor can be selected.

(3) Selection of Energy Capacity
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 generated 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 energy stored in the circuit (such as energy in coils and capacitors, and stray energy) must be absorbed by the varistor. In such cases, when selecting a varistor, the total electrical energy stored in the circuit must be less than the energy that the varistor can absorb.

Currently, the capacitance of varistors varies greatly due to different types. Therefore, the principle for selection should be to avoid affecting the normal operation of the circuit. Generally, varistors are suitable for use at frequencies below 300Hz.

2. Precautions for using varistors
① Protective measures should be taken for varistors to prevent damage to the varistor and device caused by uncertain external factors. If a fuse is installed in the circuit, the installation method is as follows.

② Varistors should not be installed near heat-generating or flammable components. A gap of more than 3mm is recommended to ensure that it operates within the specified operating temperature range.

③ The operating voltage of the varistor (the voltage continuously applied across the varistor) should not exceed the maximum continuous operating voltage value.

④ In applications where surge currents are repeatedly generated, the peak surge current and surge energy through the varistor should not exceed the pulse life characteristic specifications.

⑤ When surge pulses are repeatedly applied to the varistor with very short intervals, the average power at this time should be lower than the maximum static power.

⑥ If a thermistor is connected between a live part of an equipment and its metal casing, necessary measures must be taken to prevent electric shock accidents.

⑦ Varistors should not be exposed to direct sunlight during use and should not be operated outdoors.

⑧ Varistors should not be operated under conditions of wind, rain, water vapor, high temperature and humidity, or in environments with sand, salt dew, or harmful gases. If necessary, a protective box should be used for protection.

⑨ Do not clean varistors with solvents such as acetone, as this may damage the epoxy resin encapsulation.

⑩ Varistors should be stored in a warehouse with a temperature below 40℃ and a relative humidity not exceeding 75%RH. The warehouse should be free of corrosive gases. Varistors stored in the warehouse should also not be exposed to direct sunlight.