Methods of using varistors

Varistors are generally used in parallel in circuits. When the voltage across the resistor changes dramatically, the resistor short-circuits and blows the current fuse, which plays a protective role. Varistors are often used in circuits for power supply overvoltage protection and voltage stabilization. Today, a netizen from the Electronic Components Technology Network Forum will share with you some of his experiences and methods of using varistors at work. Parameters that need to be understood in circuit design: 1. Varistor voltage UN (U1mA): The voltage when a 1mA DC current passes through the varistor is usually used to indicate whether it is conducting. This voltage is called the varistor voltage UN. The varistor voltage is also often represented by the symbol U1mA. The error range of the varistor voltage is generally ±10%. In experiments and actual use, a 10% drop in the varistor voltage from the normal value is usually used as a criterion for varistor failure.

2. Maximum continuous working voltage UC: refers to the maximum AC voltage (effective value) Uac or maximum DC voltage Udc that the varistor can withstand for a long time. Generally, Uac≈0.64U1mA, Udc≈0.83U1mA

3. Maximum clamping voltage (limiting voltage) VC: The maximum clamping voltage value refers to the voltage appearing on the varistor when the specified 8/20μs wave impulse current IX (A) is applied to the varistor.

4. Leakage current Il: The current that flows when the maximum DC voltage Udc is applied to the varistor. When measuring the leakage current, a voltage of Udc = 0.83U1mA is usually applied to the varistor (sometimes 0.75U1mA is also used). The static leakage current Il is generally required to be ≤20μA (sometimes ≤10μA). In actual use, what is more concerned is not the size of the static leakage current value itself, but its stability, that is, the rate of change after the impact test or under high temperature conditions. If the rate of change does not exceed one time after the impact test or under high temperature conditions, it is considered stable.

Methods and steps

1. Calculation of varistor voltage:

Generally, it can be calculated by the following formula:

U1mA=KUac Where: K is a coefficient related to power quality, generally K=(2~3), a smaller value can be used in cities with better power quality, and a larger value can be used in rural areas with poor power quality (especially mountainous areas). Uac is the effective value of the AC power supply voltage. For 220V~240V AC power supply lightning arresters, it is more appropriate to use a varistor with a varistor voltage of 470V~620V. Using a varistor with a higher varistor voltage can reduce the failure rate and extend the service life, but the residual voltage will increase slightly.

2. Calculation of nominal discharge current:

The nominal discharge current of the varistor should be greater than the required surge current or the maximum surge current that may occur each year. The nominal discharge current should be calculated based on the value of more than 10 surges in the surge life rating curve of the varistor, which is about 30% of the maximum surge current (i.e. 0.3 IP).

3. Parallel connection of varistors:

When one varistor cannot meet the requirements of the nominal discharge current, multiple varistors should be used in parallel. Sometimes, in order to reduce the limiting voltage, multiple varistors are used in parallel even if the nominal discharge current meets the requirements. It should be noted that when varistors are used in parallel, they must be strictly selected with the same parameters (for example: ΔU1mA≤3V, Δα≤3) for pairing to ensure uniform current distribution.

Precautions

The temperature fuse should have good thermal coupling with the varistor. When the varistor fails (high impedance short circuit), the heat it generates will melt the temperature fuse, separating the failed varistor from the circuit to ensure the safety of the equipment. When a high power frequency temporary overvoltage acts on the varistor, it may cause the varistor to break down and short circuit (low impedance short circuit) instantly, and the temperature fuse will not have time to melt, and may even catch fire. To avoid this phenomenon, an impact-resistant power frequency fuse can be connected in series with each varistor (a single power frequency fuse may not melt when it ages and fails).