What is the function of a varistor?

Varistor (varistor) is a voltage limiting protective device based on nonlinear volt ampere characteristics. Its core function is to achieve transient voltage clamping and energy absorption when the circuit encounters abnormal overvoltage, thereby protecting sensitive electronic components from damage. The following is a detailed analysis of its core function and working principle:

1、 Core function: Transient overvoltage protection
 
 
 
Voltage Clamping
 
When the voltage of the circuit is lower than the threshold (varistor voltage), the varistor is in a high resistance state (with a resistance value of megaohms), and the leakage current is extremely small (usually<100 μ A), which is equivalent to a "disconnected" state and has no effect on the circuit.
 
When the voltage exceeds the threshold, its resistance drops sharply within nanoseconds (can be as low as milliohms), which is equivalent to "closing" a switch, forcing the voltage to clamp within a safe range (i.e. residual voltage), to avoid overvoltage on subsequent equipment.
 
Typical scenario: Household appliances (such as color TV power supplies) use 470V varistors. When the peak surge voltage is greater than 470V, they are pulled down to a safe value.
 

Energy absorption and surge suppression
 
Absorb transient high-energy pulses such as lightning strikes and switch surges (such as 8/20 μ s waveform), with a current carrying capacity of over 70kA, and convert electrical energy into thermal energy for release.
Suppress the back electromotive force generated by the power-off of inductive loads (such as motor coils) to prevent component breakdown.
2、 Extended functions and application scenarios
Typical applications of functional types and mechanisms of action

Lightning surge protection response speed<25ns, fast discharge of lightning induced current communication base station, power input stage, security system
Electromagnetic interference suppression absorbs high-frequency transient noise (such as EFT pulses), reduces circuit EMI signal line and data line protection
Start control utilizes voltage sensitive characteristics to control circuit on/off (such as stable motor speed operation), industrial motor drive, and fluorescent lamp start
Multiple parallel connections of voltage equalization and level shifting are used to achieve voltage balance, or for high-voltage DC voltage stabilization (thousands of volts level) in power system lightning arresters and high-voltage insulators
3、 Key features and usage limitations
Advantages:
Extremely fast response (nanosecond level), with current carrying capacity far exceeding TVS diodes.
Good temperature stability, wide operating temperature range (-40 ℃~+85 ℃).

limitations:
The junction capacitance is large (several hundred to several thousand pF), and low capacitance devices (such as TVS tubes) need to be used in high-frequency circuits.
Weak ability to withstand sustained current: only suitable for transient protection, long-term overvoltage can cause thermal failure.
Aging characteristics: The pressure-sensitive voltage may drift after multiple impacts, and regular testing is required.
IV. Typical Application Fields
Power system:
AC/DC input level lightning protection (such as switch mode power supply, UPS).
Communication equipment:
Surge protection for telephone lines and Ethernet ports.
Consumer Electronics:
Mobile phone charger, TV power board overvoltage protection.

Automotive Electronics: