What Is a Varistor and How Does It Protect Circuits from Voltage Spikes?

A varistor (specifically a Metal Oxide Varistor, or MOV) is a voltage-dependent, nonlinear resistor that acts as an open circuit under normal voltage but instantly drops its resistance to near-zero to shunt dangerous voltage spikes away from sensitive components. In a real circuit, an MOV changes a potentially destructive, microsecond-long 2000V transient spike into a harmless, thermally dissipated heat burst, clamping the voltage to a safe level before it can puncture the gate oxide of downstream silicon. Think of it like a mechanical pressure relief valve on a steam boiler: under normal operating pressure, the valve remains sealed and invisible to the system, but the moment pressure exceeds a critical threshold, it blows open to vent the excess energy and prevent a catastrophic explosion.

Where You Meet Varistors in Practice

If you have ever opened a piece of mains-powered electronics, you have likely seen an MOV. They are the thick, epoxy-coated discs (usually blue, yellow, or black) with two wire leads, typically sitting right behind the input fuse and before the bridge rectifier. Here is where they are doing the heavy lifting in modern electrical and electronic systems:

• Switch-Mode Power Supplies (SMPS): Placed across the Line and Neutral (or Line and Ground) inputs of everything from laptop chargers to ATX PC power supplies to absorb grid switching surges and nearby lightning transients.
• Surge Protector Power Strips: The "joule rating" on a $30 power strip is almost entirely dictated by the physical size and parallel array of the MOVs inside the plastic housing.
• HVAC Control Boards: Used to suppress the massive inductive kickback generated when compressor contactors and blower motors switch off, preventing the microcontroller from brown-out resetting.
• Smart Home Relays and IoT Nodes: ESP32 and Arduino-based AC-switching modules use small MOVs across relay contacts to extinguish arcs and protect the low-voltage DC isolation barrier.

Worked Numeric Example: Sizing an MOV for a 120V AC Line

Selecting the wrong MOV is a fast track to either a blown component on day one or a fire hazard on day 100. You cannot simply slap a "120V" rated MOV on a 120V AC line. Here is the exact bench math for sizing an MOV for a standard North American 120V AC mains input, referencing Littelfuse application guidelines.

1. Calculate the Peak Line Voltage: AC voltage is measured in RMS. The peak voltage is $V_{RMS} \times \sqrt{2}$. For a 120V line, $120 \times 1.414 = 169.7V$ peak.
2. Account for Grid Tolerance: Mains voltage can legally fluctuate by +10% to +15% depending on your local utility. Let us assume a 15% high-line tolerance: $169.7V \times 1.15 = 195.1V$ maximum continuous peak voltage.
3. Select the MCOV (Maximum Continuous Operating Voltage): The MOV's MCOV must be strictly greater than this worst-case peak voltage to prevent the MOV from conducting during normal operation and overheating. We need an MCOV $\ge 200V$ DC.
4. Choose the Standard Part: A standard 130V RMS-rated MOV (such as the Littelfuse V130LA20CP) has an MCOV of 175V RMS / 225V DC. This safely clears our 195.1V worst-case peak requirement.

With this part selected, if a 2000V surge hits the line, the MOV will clamp the voltage seen by your downstream bridge rectifier and bulk capacitors to a maximum of 340V, which is well within the 400V or 450V rating of standard SMPS electrolytic capacitors.

Real-World Scenario Walkthrough: The Fried ESP32 Smart Relay

To understand what happens when you omit this component, let us look at a real-world failure from a DIY smart thermostat project built in early 2026.

The Setup: A maker designed a custom Wi-Fi thermostat using an ESP32-WROOM-32 module and a mechanical relay to switch a 24V AC HVAC contactor coil. The 24V AC was stepped down via a small transformer, rectified, and regulated down to 3.3V using an AMS1117 LDO to power the ESP32.

The Numbers: The 24V AC coil is highly inductive. When the relay contacts open to turn off the AC compressor, the collapsing magnetic field generates an inductive kickback spike. Without suppression, $V = L(di/dt)$ dictates that a fast current interruption in a high-inductance coil can easily generate a 400V to 600V transient spike across the relay contacts.

The Outcome: During bench testing, the system worked perfectly. But after three days of intermittent compressor cycling on the wall, the ESP32 began randomly resetting. A week later, the AMS1117 LDO failed short-circuit, sending raw unregulated voltage into the 3.3V rail and instantly bricking the ESP32 module.

What Went Wrong: The maker relied on the transformer to provide galvanic isolation, forgetting about parasitic inter-winding capacitance. The 500V inductive kickback from the 24V AC coil coupled through the transformer's parasitic capacitance directly into the secondary DC side. Because there was no MOV across the relay contacts or the AC coil to absorb the $di/dt$ energy, the spike punched through the LDO's internal junctions. The fix required adding a 30V RMS rated MOV (like a Littelfuse V30ZA3P) directly across the AC coil terminals, clamping the kickback to a safe ~65V before it could couple to the DC side.

Varistor vs. TVS Diode vs. Fuse: Clearing Up the Confusion

A common mistake on the workbench is confusing MOVs with TVS (Transient Voltage Suppression) diodes, or assuming an MOV acts like a fuse. According to the semiconductor protection tutorials on Electronics Tutorials, while all three protect circuits, their physics and failure modes are entirely different.

FAQ: Varistor Degradation and Bench Testing