What Is a Varistor?
A varistor is a semiconductor device with two electrodes whose resistance changes depending on the applied voltage.
The term varistor is derived from the combination of “variable” and “resistor,” meaning “variable resistance.” For this reason, it is sometimes called a non-direct resistance or a voltage-dependent resistance.
It is characterized by the fact that voltage and current are not proportional. When the voltage applied to the Varistor is low, the resistance is high, and when the voltage is high, the resistance is low.
Uses of Varistors
Varistors vary in resistance according to voltage. This characteristic can be used to protect IC elements from static electricity or to protect electronic equipment from lightning surges.
If an abnormal voltage is applied to an element, such as an IC or an electronic device, it may lead to malfunction or destruction. In addition, when a high voltage is applied to a varistor, the resistance of the Varistor becomes low. This allows current to flow through the circuit more easily, and the voltage drop in the line impedance can reduce the load on the electronic equipment. Other uses of varistors include prevention of electrostatic discharge and shattering.
1. Prevention of Static Electricity Discharge
Electronic devices with external interface terminals, such as cell phones, music players, and USB devices, which are used in everyday life, are difficult to shield from static electricity and require the use of components to prevent static electricity. This is because the technological sophistication and difficulty of manufacturing these devices makes them susceptible to electrostatic discharge, which can easily destroy them.
Until now, the method of preventing static electricity has been to use Zener diodes, which provide a stable and constant voltage, but the development of small and inexpensive multilayer chip varistors has led to the use of varistors.
2. Anti-Dispersion
Commutator motor is a generic term for electric motors and power devices that have a mechanical commutator and brushes to switch the flowing current according to the rotation phase and to keep the power of the rotating shaft in a constant direction.
One type of commutator motor is a brush DC motor that has a part called a brush through which a direct current flows. The commutator, which rotates intermittently, generates high voltage and sparks, causing the brush to wear out and noise to be generated. Varistors are used to prevent this.
Principle of Varistors
Varistors consist of a zinc oxide-based ceramic semiconductor sandwiched between two electrodes. Varistor characteristics can be expressed as I=KV^α, where I is the current and V is the voltage. where K is a constant specific to the device and α is the voltage non-linearity coefficient (α coefficient).
The voltage non-linearity coefficient is a coefficient that expresses the curvature after the refraction point, which is the point where the resistance transitions from low to high. The equivalent circuit of a Varistor consists of two upside-down Zener diodes connected in parallel with a capacitor.
From this, we can see that the varistor has a capacitor component, which means that the voltage across the varistor is low, and that the varistor has a small amount of capacitance when it has a high resistance.
Up to a certain voltage, the varistor has a structure that does not allow current to flow due to its high resistance, but when a load exceeding a certain voltage is applied, the voltage becomes higher than the resistance, and a large current flows due to the quantum mechanical tunneling effect. Therefore, when a high-voltage load is applied to an element or electronic device, the varistor serves to dissipate static electricity to ground and so on.
Other Information on Varistors
Varistor Characteristics
Varistors have a limited life span. You should select a varistor whose life can be properly determined based on the voltage applied to the varistor, the varistor’s withstand capability, and a linear graph that shows the results of a surge waveform, which defines both the state in which the output is released and the state in which it is short-circuited. If the stipulations are greatly exceeded, the product may be damaged or shattered, leading to injury.
Another similar structure is the Zener diode, but there is a slight difference because it has symmetrical current-voltage characteristics and has no polarity.