If you are researching what is varistors in the context of circuit protection, the most practical answer is this: A varistor (specifically the Metal Oxide Varistor, or MOV) is a voltage-dependent, nonlinear resistor that drops its resistance dramatically when the voltage across it exceeds a specific threshold, acting as a sacrificial shock absorber for transient voltage spikes. They are the bouncers of your power supply—ignoring normal operating voltages but physically shunting destructive, high-voltage spikes away from sensitive silicon before it can melt.
The Core Principle: How the V-I Curve Works
Unlike a standard carbon-film or metal-film resistor that obeys Ohm’s Law linearly (where resistance stays constant regardless of voltage), a varistor has a highly nonlinear Voltage-Current (V-I) curve.
Under normal operating conditions, the MOV exhibits an extremely high impedance, typically in the megaohm range. It draws only a tiny leakage current—usually less than 50 microamps—making it virtually invisible to the circuit. However, when a transient voltage spike pushes the potential across its leads past its rated threshold, the zinc oxide grains inside the component undergo an avalanche breakdown. The resistance plummets to a fraction of an ohm in nanoseconds, creating a low-impedance path that diverts the surge current away from downstream components.
What a Varistor Actually Changes in Your Circuit
A common misconception is that varistors regulate or smooth out normal voltage variations. They do not. An MOV will not fix a 110V sag or clean up 60Hz harmonic distortion.
What it changes is the transient survival rate of your installation. Varistors are always wired in parallel with the load they are protecting (e.g., Line-to-Neutral, Line-to-Ground, or Neutral-to-Ground). When a spike hits, the MOV clamps the voltage to a safe "let-through" level. It changes a potentially lethal 2,000V inductive kickback into a manageable 400V pulse that downstream bridge rectifiers and filter capacitors can absorb without catastrophic dielectric failure.
Numeric Breakdown: Sizing an MOV for a 120V AC Mains Input
Selecting the wrong MOV is a frequent cause of premature board failure. If the continuous RMS voltage rating is too low, the MOV will degrade and catch fire during normal grid fluctuations. If it is too high, it won't clamp low enough to protect your silicon. Here is the exact bench procedure for sizing an MOV for a standard US 120V AC mains input.
- Calculate the Peak Line Voltage: A 120V AC RMS sine wave has a peak voltage of $120 \times \sqrt{2}$, which equals 169.7V.
- Factor in Grid Tolerance: Utilities are typically allowed a +10% variance. $169.7V \times 1.10 = 186.6V$. Your MOV must not trigger at this voltage.
- Select the $V_{M(AC)}$ Rating: Choose an MOV with a Maximum Continuous RMS Voltage ($V_{M(AC)}$) greater than 186.6V. A standard 130V RMS rated MOV (like the Littelfuse TMOV14RP130E) actually has a nominal DC breakdown voltage ($V_N$ at 1mA) of about 205V, keeping it safely above the 186.6V peak.
- Verify the Clamping Voltage ($V_C$): Check the datasheet for the clamping voltage at the expected surge current (e.g., 100A at an 8/20µs waveform). For the TMOV14RP130E, the clamping voltage is 340V. This means downstream components (like a 600V rated bridge rectifier) will only see a maximum of 340V during a surge.
- Check the Energy Rating (Joules): For a 14mm disc MOV, expect an energy absorption rating of roughly 15 to 25 Joules. For harsher industrial environments, step up to a 20mm disc (e.g., Bourns MOV-20D201K) which handles up to 100 Joules.
Where You Meet Varistors in Practice
You will rarely see an MOV in a low-voltage DC logic circuit; they are predominantly found in AC mains interfaces and high-voltage DC motor drives.
- Switched-Mode Power Supplies (SMPS): Look right after the input fuse and bridge rectifier on a laptop charger or LED driver PCB. You will see a blue or yellow disc component bridging the Live and Neutral traces.
- Surge Protective Devices (SPDs): Inside power strips and whole-home surge protectors, arrays of 20mm or 34mm MOVs are wired in parallel to handle massive multi-kiloamp lightning surges.
- HVAC Control Boards: Across the 24VAC control lines and the 120VAC/240VAC blower motor relays to suppress the inductive kickback generated when contactors open.
Real-World Scenario Walkthrough: The Melted MOV in an LED Driver
The Setup: A 12V 5A LED driver (SMPS) is installed in a workshop to power bench lighting. The driver is plugged into a 120V outlet sharing a circuit with a 5HP air compressor. The SMPS contains a standard 14mm MOV rated for 130V RMS.
The Numbers: Every time the compressor's unloader valve kicks off, the collapsing magnetic field in the motor generates an inductive voltage spike of roughly 900V lasting for 40 microseconds. The MOV's 340V clamping threshold is breached. The MOV drops its resistance and shunts the 900V spike down to 340V, absorbing roughly 8 Joules of energy per event.
The Outcome (Short Term): The LED driver survives hundreds of these spikes. The downstream 600V electrolytic capacitors and PWM controller never see the 900V spike.
What Went Wrong (Long Term): MOVs degrade with every surge. The zinc oxide grain boundaries physically break down, causing the leakage current at normal operating voltages to creep up from 20µA to 2mA, then to 20mA. One afternoon, a "lost neutral" event on the street transformer sends a sustained 240V RMS into the 120V workshop outlet. The degraded MOV attempts to clamp this sustained overvoltage. It absorbs hundreds of watts of continuous power, enters thermal runaway, and its epoxy coating cracks, venting green plasma and smoke. Because the SMPS lacked an inline thermal fuse, the PCB traces melted before the branch circuit breaker finally tripped.
Common Confusions: Varistors vs. TVS Diodes vs. Fuses
Makers frequently confuse MOVs with other protection components. Here is how they differ on the bench.
| Component | Primary Function | Response Time | Energy Handling | Best Application |
|---|---|---|---|---|
| MOV (Varistor) | Clamps high-energy AC/DC transients | ~25 nanoseconds | High (10J - 1000J+) | Mains inputs, motor snubbers, SPDs |
| TVS Diode | Clamps fast, low-energy ESD/EFT spikes | ~1 picosecond | Low (1J - 50J) | Data lines, I2C/SPI buses, sensitive ICs |
| Fuse | Opens circuit during overcurrent | Milliseconds to Seconds | N/A (Current-based) | Preventing wire fires and continuous faults |
| X2 Capacitor | Filters high-frequency EMI/RFI noise | N/A (Continuous filtering) | N/A | Across L/N for EMI compliance |
For a deeper dive into the physics of metal oxide grain boundaries and avalanche breakdown, All About Circuits provides an excellent semiconductor-level breakdown of MOV construction.
Frequently Asked Questions
Do varistors wear out over time?
Yes. Unlike fuses that blow once, MOVs degrade incrementally. Every time they clamp a spike, the internal zinc oxide structure suffers microscopic damage. This causes the leakage current to increase and the nominal clamping voltage to drop. Eventually, a degraded MOV will fail short-circuit during a normal voltage swell.
Can I replace a burnt MOV with one that has a higher voltage rating?
No. If you replace a 130V RMS MOV with a 275V RMS MOV to stop it from blowing, you are raising the clamping voltage. A 275V MOV might not clamp until 700V+, which will easily destroy your downstream bridge rectifier and switching MOSFETs. You must replace it with the exact same RMS rating, or investigate why the grid voltage is exceeding normal tolerances.
Why is my MOV placed after the fuse instead of before it?
If an MOV fails short-circuit (its most common catastrophic failure mode), it creates a dead short across the AC mains. The inline fuse is required to blow and clear this short. If the MOV were placed before the fuse, a shorted MOV would draw unlimited current from the grid, resulting in a fire or exploded component.
