Protection analysis of varistor

The varistor is a voltage limiting type protection device. Using the non-linear characteristics of the varistor, when an overvoltage occurs between the two poles of the varistor, the varistor can clamp the voltage to a relatively fixed voltage value, thereby protecting the latter circuit. The response time of the varistor is ns, which is faster than the gas discharge tube and slightly slower than the TVS tube. Under normal circumstances, the response speed of the overvoltage protection for the electronic circuit can meet the requirements.

The Zv of the varistor and the total impedance of the circuit (including the surge source impedance Zs) form a voltage divider, so the control voltage of the varistor is V=VsZv/(Zs+Zv). The resistance of Zv can be reduced from the normal megaohm level to a few ohms, or even less than 1 Ω. It can be seen that Zv flows a large current in an instant, and the overvoltage mostly falls on Zs, and the input voltage of the electric appliance is relatively stable, so that it can play a protective role.

The straight line segment is the total impedance Zs, and the curve is the characteristic curve of the varistor. The two intersect at point Q, that is, the protection working point, and the corresponding control voltage is V. It is the work of using the varistor and then adding it to the consumer. Voltage. Vs is the surge voltage, which has exceeded the withstand voltage value VL of the electrical appliance. After the varistor is applied, the working voltage V of the electrical appliance is less than the withstand voltage value VL, thereby protecting the electrical appliance. Different line impedances have different protection characteristics. From the perspective of protection effect, the larger the Zs is, the better the protection effect is. If Zs=0, that is, the circuit impedance is zero, the varistor cannot protect.