Overvoltage protection devices: How to choose TVS tube and varistor?

All electrical equipment has a voltage range, and overvoltage is a problem that electrical equipment may encounter. For example, surges caused by the power supply line being struck by lightning, and plugging and unplugging electrical equipment can cause the voltage to suddenly increase.

The voltage exceeds the operating voltage of the electrical equipment, and the electrical equipment is burned and damaged due to overvoltage, causing serious accidents. Therefore, overvoltage protection devices that limit excessive voltage and protect circuits have emerged.

Commonly used overvoltage protection devices include electrostatic suppressors, TVS tubes, varistors, and discharge tubes. Let's first look at the two overvoltage protection devices, TVS tubes and varistors.

1. Overvoltage protection device: TVS tube

The full name of TVS tube is transient voltage suppressor or transient voltage suppression diode, which has extremely fast response time (in nanoseconds) and high surge absorption capacity. When its two ends are subjected to instantaneous high-energy impact, the TVS tube can change the impedance value between the two ends from high impedance to low impedance at a very high speed to absorb the instantaneous large current, thereby clamping the excessive voltage at both ends of the circuit to a fixed value, protecting the subsequent circuits and sensitive components from transient high-voltage spike pulse impact.

Because of this, TVS tubes are used to protect equipment or circuits from static electricity, transient voltages generated when inductive loads are switched, and overvoltages generated by inductive lightning.

Zhixu Electronic Varistor

2. Overvoltage protection device: varistor

A varistor is an overvoltage protection device with nonlinear volt-ampere characteristics. Its material is semiconductor, and it is a special semiconductor resistor. The varistor limits the voltage in the circuit and plays a role in protecting the circuit and other components behind the circuit. The resistance of the varistor is related to the voltage. When the circuit voltage is normal, the resistance of the varistor is infinite. When the circuit voltage is higher than the voltage of the varistor, the resistance becomes smaller, the varistor switch is in a closed state, and the current in the circuit becomes larger. When passing through the varistor, the varistor absorbs excess current and clamps the voltage to a fixed value to protect the circuit and other electronic components.

Zhixu Electronic Varistor

It can be seen that the working principles of TVS tubes and varistors are actually the same, both of which clamp the voltage to a fixed value by absorbing excess current. How should TVS tubes and varistors, both of which are overvoltage protection devices, be selected? Varistors have stronger absorption capacity than TVS tubes. Therefore, varistors are usually recommended for ESD protection, overvoltage protection of power supply parts and parts with large transient energy; TVS tubes have lower limiting voltages and small transient internal resistance. TVS diodes are recommended for parts with poor ESD voltage resistance and small impedance of the protected parts (such as receivers, MICs, audio, etc.).