22. MOV for 24VDC Industrial Bus Surge Protection

A Metal Oxide Varistor (MOV) is a surge-protection component 

• At normal voltages → it behaves like an insulator (very high resistance).
• At surge voltages (spikes, lightning, ESD) → it suddenly becomes a conductor, clamping the voltage to a safe level.

• Think of it as a “voltage safety valve” in your circuit.

  

Schematic Symbols:

Simple MOV circuit diagram:

• At low voltages, very little current flows (high resistance).
• When the voltage exceeds a threshold (varistor voltage), the barrier breaks down → MOV conducts heavily → clamps the surge.
• After the surge, MOV returns to a high resistance state.

V-I Characteristics of a TVS Diode:

• Surge protectors and power strips (AC mains).
• Switching power supplies (SMPS).
• Automotive (handling load dumps, jump starts, spikes).
• EV chargers and inverters (protecting power electronics).
• Telecom/Ethernet lines (low-capacitance MOVs).

MOV used in Power Line

1. MCOV (Max Continuous Operating Voltage)
   • Highest AC/DC voltage the MOV can handle continuously.
   • Choose ~20–30% above normal operating voltage.
2. Varistor Voltage (V1mA)
   • Voltage where the MOV starts conducting (at 1 mA).
3. Clamping Voltage (Vc)
   • Maximum voltage during a surge.
   • Must be lower than the circuit’s safe limit.
4. Maximum Surge Current (Imax)
   • Peak surge current (8/20 µs waveform) the MOV can withstand.
5. Energy Rating (Joules)
   • Energy the MOV can absorb in a single surge event.
6. Response Time
   • Very fast (nanoseconds), but slower than TVS diodes.
7. Capacitance
   • Acts like a capacitor → high values can affect high-speed signals.
8. Leakage Current
   • Small current flows even in normal operation; it increases with aging.
9. Endurance / Lifetime
   • Degrades with each surge → voltage drops, leakage rises over time.

Example Littelfuse TMOV Series MOV Specifications:

• Across AC mains: Line-to-neutral, Line-to-ground.
• Across DC power rails: 12V, 24V, 48V systems.
• With fuses: MOV + fuse = safe failure mode.
• With TVS diode: TVS for precision spikes, MOV for bulk energy.

•  AC Power Line Protection: Used across line–neutral in power supplies, adapters, and SMPS to protect against voltage spikes and surges.
• Surge Protection Devices (SPD): Widely used in power strips and surge protectors to clamp lightning and switching transients.
• Power Distribution Systems: Installed in substations and electrical panels to safeguard equipment from lightning surges and grid faults.
• Consumer Electronics: Found in TVs, computers, chargers, and home appliances as a first line of surge protection.

• Gradual Degradation: Every surge hit causes microscopic internal damage, meaning it has a limited lifetime and is not an infinite protection device.
• Low Precision Clamping: It cannot clamp voltage very precisely, making it ideal for rugged power circuits but unsuitable for precise IC-level protection.
• Short-Circuit Failure: Since an MOV can fail in a short-circuit state, you must always use a fuse in series to prevent fire or board damage.
• High Capacitance: The inherent internal capacitance is high ($200$–$1000$ pF), which can distort or "swallow" high-speed digital signals.
• Thermal Runaway Risk: Sustained overvoltage can cause the device to overheat and potentially catch fire without a thermal disconnect.

When designing with a Metal Oxide Varistor (MOV), you must evaluate specific electrical parameters to ensure the device protects the circuit without failing prematurely.

• Operating Voltage Selection: Choose a Varistor Voltage (VRMS) that is at least 20% to 30% higher than your standard line voltage to avoid accidental triggering during minor fluctuations.
• Joule Rating (Energy Absorption): Ensure the MOV is rated to handle the maximum expected transient energy (measured in Joules) to prevent the component from exploding during a major surge.
• Clamping Voltage Level: Select a device with a clamping voltage low enough to protect your sensitive downstream components, but high enough to prevent constant conduction.
• Placement for Safety: Always place a fuse in series before the MOV to prevent a fire if the device fails in a short-circuit state.
• Capacitance Awareness: Avoid using standard MOVs on high-speed data lines, as their high parasitic capacitance (– pF) will distort or "swallow" the digital signal.
• Lead Length Minimization: Keep PCB traces and component leads as short as possible to reduce parasitic inductance, which can slow down the MOV's response time during a fast spike.