Detailed introductions of varistors (MOV) and negative temperature coefficient thermistors (NTC), including working principles, characteristics, applications, and comparisons with gas discharge tubes (GDT) :
l Sintered from zinc oxide (ZnO) particles, with nonlinear volt-ampere characteristics.
l When the voltage exceeds the threshold (varistor voltage), the resistance drops sharply to discharge the inrush current; When the voltage returns to normal, the resistance returns to the high resistance state.
l Varistor voltage (Vn) : A threshold voltage measured at 1mA DC (e.g. 470V, 680V).
l Current-carrying capacity: typically several hundred A to tens of kA (used to absorb lightning strikes or switch surges).
l Response time: nanosecond level (faster than GDT, slower than TVS).
l Capacitance: Higher (tens to hundreds of pF), not suitable for high frequency circuits.
l Advantages: Low cost, strong current-carrying capacity, widely used for AC power protection.
l Disadvantages:
¢ Aging problem: Varistor voltage drift after multiple surges, possible short circuit and fire (fuse required).
¢ Leakage current: A small leakage current at high voltage over a long period of time may heat up.
l Ac power protection: such as the lightning protection module at the power input end (MOV + GDT + fuse).
l Dc circuit surge absorption: such as motor, relay contact protection.
l Sintered from oxides of metals such as manganese, cobalt, and nickel, the resistance decreases exponentially as the temperature rises.
l There are two types:
¢ Power-type NTC: used to suppress inrush current (such as when the power starts up).
¢ Temperature-sensing NTC: For temperature sensing (such as battery temperature monitoring).
l Zero power resistance: Resistance at room temperature (e.g. 25 ° C) (e.g. 5Ω, 10Ω).
l Maximum steady-state current: The current allowed for long-term operation.
l Dissipation coefficient: The ability to balance its own heat generation and heat dissipation.
l Advantages:
¢ Effectively suppress startup inrush current (such as the momentary high current when charging a capacitor).
¢ Low cost, simple structure.
l Disadvantage:
¢ Heat problem: Reduced resistance during operation, may continue to heat up (with relay or bypass circuit required).
¢ Recovery time: After power-off, cooling is required to return to high resistance (not suitable for frequent switching scenarios).
l Power surge suppression: Connected in series to the AC/DC power input, limiting the startup current.
l Circuit protection: Prevents capacitors and rectifier Bridges from being damaged by surge current.
Characteristics | Gas discharge tube (GDT) | Varistor (MOV) | NTC thermistor |
Core Functions | Overvoltage protection | Overvoltage protection | Surge current limiting |
Response speed | Nanosecond level | Nanoseconds | Millisecond (temperature change) |
Flow capacity | Extremely high (kA class) | High (kA) | Low (dependent on resistance value) |
Failure mode | Open circuit/Leak | Short circuit (may catch fire) | Resistance drift after aging |
Typical Applications | Communication lightning protection, high voltage isolation | Power surge protection | Power start current suppression |
1 The AC input - [fuse] - [NTC] - [GDT] - [MOV] - [was] - level after circuit
l NTC: Limit the inrush current at startup.
l GDT: Absorbs high-energy surges such as lightning strikes (Level 1).
l MOV: Clamped medium-pressure surge (second stage).
l TVS: Fine protection of sensitive devices (Level 3).
l MOV:
¢ The varistor voltage needs to be higher than the maximum operating voltage of the circuit (470V for 220VAC).
¢ The current-carrying capacity is selected according to the surge level (e.g. 10kA, 20kA).
l NTC:
¢ Select based on the maximum steady-state current and initial resistance value.
¢ A "relay bypass" scheme is required for frequent switching scenarios.
If you need a specific model recommendation or circuit design details, you can further describe the application scenario (such as power supply voltage, protection level, etc.)!
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