Varistors are specialized electronic components designed to protect electrical circuits from voltage spikes and transient surges. Often referred to as Transient Voltage Spikes Suppressors (TVSS) or Transient Voltage Surge Suppressors (TVSS), they are semiconductor devices that act as non-linear circuit elements and exhibit a characteristic where their resistance decreases exponentially with increasing voltage. This property allows them to conduct significant current only when the applied voltage exceeds a certain threshold, making them highly effective in suppressing voltage spikes during transient events.
How Do Varistors Work?
The function of a varistor is based on its metal-oxide semiconductor structure. When a high voltage is applied, electrons are forced through the oxide layer, creating a conductive channel. This sudden conduction results in a rapid release of stored electrical energy, effectively clamping the voltage spike to a manageable level.
Types of Varistors
- Metal-Oxide Varistors (MOV): MOVs are the most common type of varistor due to their cost-effectiveness and ability to handle high energy pulses. They are widely used in surge protection devices for applications such as protecting computers, networking equipment, and industrial machinery.
- Multi-Layer Varistors (MLV): MLVs consist of multiple layers of metal-oxide semiconductor material, providing higher capacity and better voltage handling compared to standard MOVs. They are ideal for protecting sensitive electronics in low to medium voltage applications.
Limitations
Varistors have certain limitations:
- They cannot protect against sustained over-voltages or long-duration power surges.
- They do not address issues like inrush currents during equipment startup or over-current due to short circuits.
Comparison with Other Suppressors
- Transient-Voltage-Suppression (TVS) Diodes: These devices are similar to varistors but rely on Zener diode technology, which allows them to operate at lower clamping voltages. However, they lack the capacity to handle high-energy surges and degrade more easily from repeated exposure to voltage spikes.
- Gas-Tube Suppressors: Gas-tube suppressors use a spark gap and often radioactive material to provide consistent breakdown voltages. They have longer response times and higher clamping voltages but can handle significant fault currents and multiple high-voltage events without degradation.
Safe Usage
- Always incorporate a series-connected thermal fuse in surge suppressors to safely disconnect the varistor in case of catastrophic failure.
- Use devices with internal thermal protection to limit the risk of fire.
- Follow manufacturer recommendations for proper installation and application.
Their limitations highlight the need for careful installation and consideration of alternative solutions when appropriate.