Correct use of varistors(II)

Three: Parallel connection of varistors:

When a varistor does not 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 connected in parallel even if the nominal discharge current meets the requirements. It is important to note that when varistors are used in parallel, they must be strictly selected with the same parameters (for example, ΔU1mA≤3V, Δα≤3) to match to ensure even distribution of current.

The failure mode of the varistor is usually a short circuit. In order to prevent the varistor from failing to cause a short circuit of the power supply and cause a fire, a temperature fuse or a thermal detachment mechanism can be connected in series with each varistor. The temperature fuse should have good thermal coupling with the varistor. When the varistor fails (short circuit with high impedance), the heat it generates will blow the temperature fuse to break the failed varistor and the circuit, ensuring the equipment Security. When the higher power frequency temporarily overvoltage acts on the varistor, it may cause the varistor to instantaneously short-circuit (short-impedance short-circuit), and the temperature fuse is too late to blow, and may also cause fire. To prevent this from happening, an impact-resistant power line fuse can be connected in series with each varistor (single-phase power fuses may not be blown in the event of an aging failure).

The varistor can also be used in series with the ceramic gas discharge tube. The ceramic gas discharge tube does not conduct during normal operation. The varistor has no leakage current and can greatly extend the service life. When exposed to a surge, the ceramic gas discharge tube first strikes. Wear, and then by the varistor limit the surge voltage, the total residual pressure is the sum of the two, a slight increase (tens of volts); after the impact of the past, because the varistor limits the current, the discharge tube can not maintain conduction The arc extinguishes, resumes the normal working condition; After the varistor short-circuit failure, due to the ceramic gas discharge tube's large power frequency current will quickly fail, but the vast majority of its failure mode is open, so not easy Causes fire.