Varistors, also known as "voltage sensitive resistors," have the core function of providing overvoltage protection for electronic circuits. It is like an intelligent voltage "safety valve", with a unique and critical working principle:
Nonlinear volt ampere characteristic: This is the most essential characteristic of varistors. Its resistance value is not constant, but strongly depends on the voltage applied across its terminals.
At normal voltage: When the circuit voltage operates within its nominal rated voltage (varistor voltage) range, the varistor exhibits extremely high resistance values (up to megaohms). At this point, it is equivalent to an almost open circuit state, with minimal impact on the normal operation of the circuit, and only a very small leakage current flows.
Under overvoltage: When there is an instantaneous surge voltage in the circuit that is higher than the voltage of the varistor (such as lightning induction, switch operation induced spikes, electrostatic discharge), the characteristics of the varistor undergo drastic changes. Its resistance value drops sharply (can be as low as a few ohms), instantly becoming highly conductive.
Clamping and energy absorption: When the varistor conducts due to overvoltage:
Clamp voltage: It quickly limits the voltage applied at its two ends (clamp) to a relatively safe, pre-set level (although this voltage is still higher than its nominal varistor voltage, it is far below the dangerous voltage that may damage the backend equipment). This prevents excessive voltage from directly impacting the precision electronic components at the back end, such as IC chips, transistors, capacitors, etc.
Discharge current: It provides a low impedance path for dangerous transient high currents (surge currents) to flow through itself to the ground wire, rather than through the protected circuit.
Energy absorption: In this process, the varistor absorbs most of the energy of the surge pulse (dissipated in the form of heat), thereby protecting other components in the circuit from damage caused by energy impact.
Summarize the core logical chain of its function:
>Detected abnormal high voltage (overvoltage/surge) → Sudden decrease in self resistance value → Formation of low impedance path → Bypass/discharge dangerous high current from the protected circuit to ground → Simultaneously clamp the overvoltage at a safe level → Absorb and dissipate surge energy → Protect sensitive electronic devices at the back end from damage.
Key application scenarios:
AC power input protection: for household appliances and industrial equipment power inlets, to prevent grid fluctuations and lightning induced surges.
DC power line protection: The output terminals of switch power supplies and adapters are protected against overvoltage caused by switch noise or sudden load changes.
Signal line protection: communication lines (telephone lines, network lines), data line interfaces to prevent electrostatic discharge or induced lightning strikes.
Semiconductor device protection: protects the input/output ports of transistors, thyristors, integrated circuits, etc.
Relay and contactor contact protection: Absorb the back electromotive force generated when the coil loses power.
Image metaphor:
The varistor can be imagined as an intelligent dam (safety valve) built on a river channel (circuit):
When the water flow (normal current) is gentle, the dam remains motionless (high resistance) and does not affect the river flow.
When a sudden flash flood (surge voltage/current) occurs, the gates of the dam automatically open in an instant (sudden drop in resistance), quickly diverting the surging flood (surge current) to the spillway (ground wire) to prevent the flood from washing away downstream villages (electronic components), while controlling the reservoir water level (circuit voltage) at a safe height (clamp voltage).
Core values:
Varistors, with their fast response, high current capability, and relatively low cost, have become the first and most commonly used line of defense in electronic devices to resist transient overvoltages, absorb surge energy, and protect core circuits, greatly improving the reliability and service life of electronic devices.