Electrical Engineering ⇒ Topic : Varistor (Thyrite)

Varistor (Thyrite).

A varistor is a nonlinear resistor whose resistance decreases as the voltage increases. Therefore, a varistor is a voltage-dependent resistor .It is made of silicon -carbide powder and is built in the shape of a disc. The V/I characteristic of a typical varistor is shown in Fig. (A). The curve shows that the current increases dramatically with increasing voltage. Thus when the voltage increases from 1.5 kV to 10 kV, the current rises from 1 mA to 100 A. Varistors are placed in parallel with critical components which might be damaged by high transient voltages. Under normal conditions, the varistor remains in high-resistance state and draws very little current. On the application of surge, the varistor is driven to its low-resistance state. The varistor then conducts a relatively large amount of current and dissipates much of the surge as heat. Thus the component is saved from damage.

                                                              figure (a)