A varistor is a voltage-dependent, non-linear resistor that dramatically drops its electrical resistance when the voltage across it exceeds a specific threshold, effectively shunting destructive transient spikes away from sensitive downstream components. Unlike a standard fixed resistor that obeys Ohm’s Law linearly, a varistor acts as an insulator during normal operation and instantly becomes a near-short circuit during a high-voltage transient event. This dynamic switching is what saves your microcontrollers, power supplies, and home appliances from grid-level surges and inductive kickback.
The Core Varistor Definition and Operating Principle
The most common type of varistor you will encounter on a workbench or inside a commercial power strip is the Metal Oxide Varistor (MOV). Inside the component's epoxy coating is a ceramic mass of zinc oxide grains sandwiched between two metal plates (the electrodes).
At a microscopic level, the boundaries between these zinc oxide grains form semiconductor junctions. Under normal voltage, these junctions block current flow, presenting a massive resistance (often in the megaohm range). But when the voltage exceeds the component's rated threshold, the junctions undergo avalanche breakdown. The resistance plummets to fractions of an ohm, allowing the surge current to bypass your sensitive load and flow safely through the varistor.
What a Varistor Actually Changes in Your Circuit
A common misconception is that a varistor 'blocks' excess voltage like a series fuse blocks excess current. It does not. A varistor is placed in parallel with the load it is protecting.
What it actually changes in a real circuit is the let-through voltage and the impedance of the parallel path. When a surge hits, the varistor clamps the voltage to a specific maximum level (the clamping voltage) by creating a low-impedance path to ground or the return line. The energy from the spike is dissipated as heat within the varistor's zinc oxide mass. Because it absorbs energy as heat, every time a varistor clamps a spike, it degrades slightly. It is a sacrificial component by design.
Numeric Breakdown: Sizing a MOV for a 120V AC Line
You cannot simply slap a '120V' varistor across a 120V AC mains line. If you do, it will likely overheat and fail. Here is the exact math and component selection process for protecting a standard US 120V AC circuit.
The Math: A 120V AC line is an RMS (Root Mean Square) value. The actual peak voltage of the sine wave is $120V \times \sqrt{2}$, which equals roughly 170V peak. Furthermore, utility tolerances allow the nominal 120V line to drift up to 125V or 130V continuously, pushing the peak voltage to over 183V.
The Component: We select the Littelfuse V150LA20AP, a standard 20mm radial leaded MOV.
| Parameter | Datasheet Value (V150LA20AP) | Why It Matters |
|---|---|---|
| Maximum Continuous RMS Voltage | 150V AC | Safely above the 120V nominal (and 125V max) line voltage. |
| Maximum Continuous DC Voltage | 200V DC | Useful if rectifying the AC to DC before the varistor. |
| Varistor Voltage (at 1mA DC test) | 198V to 242V | The voltage at which it begins to conduct slightly. Well above the 170V AC peak. |
| Clamping Voltage (at 50A peak) | 340V | The maximum voltage your downstream load will see during a 50A surge. |
| Peak Surge Current (8/20μs) | 6,500A | Maximum single-pulse survival rating. |
If you had chosen a 130V RMS rated MOV for a 120V AC line, the normal 170V peaks of the AC cycle would exceed the MOV's conduction threshold. The MOV would conduct slightly on every single AC peak, overheat, and enter thermal runaway within minutes of being plugged in.
Where You Meet Varistors in Practice (and What Goes Wrong)
You will find MOVs across the Line and Neutral inputs of switching power supplies, inside surge protector power strips, and across the coils of AC contactors to suppress inductive kickback. However, their implementation dictates whether they save your gear or start a fire.
Real-World Scenario Walkthrough: The Cheap Power Strip Failure
The Setup: You plug a $300 Wi-Fi 7 mesh router into a cheap, unbranded $15 'surge protector' power strip. Inside the strip is a single 14mm MOV connected between Line and Neutral, wired directly across the mains with no thermal cutoff switch.
The Numbers: A nearby lightning strike induces a 2,000V, 500A combination wave surge on the grid (IEEE C62.41 Category B). The MOV reacts in nanoseconds, clamping the voltage at roughly 340V. In that fraction of a millisecond, the MOV is forced to absorb roughly 85 Joules of energy.
The Outcome: The router's internal power supply easily survives the 340V let-through voltage. However, the 14mm MOV inside the strip is only rated for 50 Joules of single-pulse energy.
What Went Wrong: Because the absorbed energy exceeded its rating, the MOV's internal zinc oxide structure fractures and fails in a 'short-circuit' mode. It now acts as a dead short across the 120V AC line. Without a thermal disconnect switch to physically break the connection, the MOV draws massive continuous current from the wall, glowing red hot and eventually catching fire. The room's 15A AFCI breaker eventually trips due to the arc fault, but the power strip is melted to the desk. Modern UL 1449 4th Edition standards mandate thermal fuses to prevent exactly this scenario.
Clearing the Confusion: MOVs vs. TVS Diodes vs. Thermistors
People frequently confuse varistors with other protection or sensing components. Here is how to tell them apart on a PCB and in a schematic.
| Component | Trigger Mechanism | Response Time | Energy Handling | Best Use Case |
|---|---|---|---|---|
| MOV (Varistor) | Voltage threshold | ~25 nanoseconds | High (Joules) | AC mains inputs, power supply line protection. |
| TVS Diode | Voltage threshold | <1 picosecond | Low (Millijoules) | Data lines (Ethernet, USB), sensitive IC GPIO pins. |
| NTC Thermistor | Temperature increase | Milliseconds to seconds | N/A (Limits current) | Inrush current limiting on power supply startup. |
The Golden Rule: Use MOVs for heavy, slow-moving power surges (like AC mains lightning strikes). Use TVS diodes for fast, low-energy electrostatic discharges (ESD) on data lines. Never use an NTC thermistor for voltage clamping.
FAQ: Testing and Replacing Varistors
How do I test a varistor with a multimeter?
Set your multimeter to the highest resistance range (or continuity). Place the probes across the varistor's legs. A healthy varistor will read 'OL' (Open Loop) or infinite resistance. If it reads near zero ohms or a low fixed resistance, it has failed short and must be desoldered and replaced.
Can I just leave a blown MOV out of the circuit?
Yes, from a purely functional standpoint. If you desolder a shorted MOV and leave the pads empty, your device will turn on and operate normally. However, you have completely removed its surge protection. The next voltage spike will go straight into the device's power supply or microcontroller.
Do varistors wear out over time?
Yes. Unlike fuses that blow once, MOVs suffer from cumulative degradation. Every time they clamp a transient, their internal structure degrades slightly, and their leakage current increases. In industrial environments with frequent switching transients, MOVs should be inspected or replaced on a scheduled maintenance interval before they enter thermal runaway.
Why are some varistors connected to Ground instead of Neutral?
Connecting Line-to-Neutral protects against differential surges (spikes between the hot and return wires). Connecting Line-to-Ground and Neutral-to-Ground protects against common-mode surges (where the entire line rises in potential relative to the earth). High-quality surge protective devices (SPDs) use a combination of all three topologies for complete protection.
