# TVS Diode vs MOV Varistor: Which Surge Protector Belongs Where

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Technical comparison between **TVS Diode** and **MOV (Metal Oxide Varistor)** by [Novapart](https://novapart.co).

## Overview

TVS diodes and MOVs both clamp overvoltage, and the usual question of which one to fit is the wrong question. On anything connected to the mains they are not alternatives; they are two stages of the same protection chain, and the interesting decision is where each one sits.

A MOV is a block of zinc oxide grains. Below its rated voltage the grain boundaries behave as insulators, and above it they conduct hard. That structure is what makes it cheap per joule: surge energy is absorbed across the whole volume of the disc, so a 14 mm part can swallow the kind of energy a lightning-induced transient delivers on a mains line. What the structure cannot do is clamp tightly. The clamping voltage of a MOV is often twice its rated voltage or more, so a 275 V unit may still let 700 V through to whatever sits behind it.

A TVS diode is an avalanche junction with a large die. It clamps tightly, typically only 30 to 40 per cent above its standoff voltage, and it does so fast enough that package inductance, not the silicon, sets the practical response time. What it cannot do is absorb much energy, because everything happens in a thin junction region rather than through a bulk ceramic.

Put those two facts together and the standard architecture follows. The MOV goes at the inlet, after the fuse, and takes the bulk of the energy. Series impedance, often just the inductance of the wiring or a deliberate inductor, follows. Then the TVS sits close to the circuit being protected and cleans up the several hundred volts the MOV let past. Neither device does the other's job well.

The difference that catches people out is wear. A TVS within its ratings does not degrade; it either survives the surge or fails short, and failing short is a well-behaved outcome because the upstream fuse then clears. A MOV degrades every time it conducts. Each surge damages some grain boundaries, the clamping voltage drifts downward and the leakage current creeps up. A MOV that has absorbed years of small transients can end up conducting enough at normal line voltage to heat itself, which is why mains-rated designs pair the varistor with a thermal disconnect and why safety standards care about it. A varistor is a consumable component with a finite life, and it should be treated as one.

On data lines the comparison does not even start. A MOV's capacitance runs to hundreds of picofarads or beyond, which destroys a USB or Ethernet signal outright. Low-capacitance TVS arrays exist precisely for this, down below a picofarad, and they are the only sensible choice for anything carrying edges.

Where it gets genuinely arguable is low-voltage DC input protection, a 24 V industrial supply rail for instance. Here a single TVS is often enough on its own: the available surge energy is far lower than on a mains line, the tight clamping protects downstream silicon properly, and you avoid designing around a component that ages. Reach for a MOV at that voltage only when the environment is genuinely harsh, long cable runs in a plant with inductive switching, and then still put a TVS behind it.

## Comparison Parameters

| Parameter | TVS Diode | MOV (Metal Oxide Varistor) | Recommendation |
|---|---|---|---|
| Clamping tightness | Tight. Clamping factor typically 1.3–1.4x the standoff voltage. | Loose. Clamping voltage often 2x or more the rated voltage. | left |
| Surge energy absorption | Modest. Limited by the silicon die area. | High. A 14 mm disc absorbs orders of magnitude more joules. | right |
| Response time | Picoseconds at the junction; package inductance dominates in practice. | Nanoseconds, slower than a TVS but fast enough for mains transients. | left |
| Wear-out mechanism | No degradation within ratings. Fails short when exceeded. | Degrades cumulatively; clamping voltage drifts and leakage rises with every surge. | left |
| Leakage current when idle | Nanoamps to microamps at standoff voltage. | Microamps rising over life; ageing units can self-heat. | left |
| Capacitance | From under 1 pF for low-capacitance data-line parts. | Hundreds of pF to nanofarads. Unusable on high-speed lines. | left |
| Failure mode | Short circuit, which blows the upstream fuse predictably. | Usually short, but can fail open or vent; needs a thermal disconnect on mains. | left |
| Typical position in the chain | Secondary and board-level protection, close to the victim circuit. | Primary protection at the mains inlet, taking the bulk energy. | None |
| Cost per unit of energy handled | Expensive per joule. | Cheap per joule, which is exactly why it is used first. | right |
| Suitability for data lines | Designed for it. Low capacitance parts protect USB, Ethernet, CAN. | Unsuitable. Capacitance and loose clamping corrupt the signal. | left |
| Catalogue depth at Novapart | Part of 18,661 active TVS references, 64% in stock. | 5,803 active varistors. | left |

## Verdict & Recommendation

Mains input, and you can only fit one thing: a MOV, sized for the expected surge energy and paired with a thermal disconnect. It is the only one of the two that can absorb what a mains transient carries.

Mains input done properly: both. MOV at the inlet after the fuse, some series impedance, TVS near the load. That is the standard architecture because each device covers the other's weakness.

Any data line, USB, Ethernet, CAN, RS-485, HDMI: a low-capacitance TVS array, and nothing else. A varistor's capacitance would corrupt the signal before any surge arrived.

Low-voltage DC rails, 12 V or 24 V: a TVS alone is usually sufficient and preferable, because the surge energy is modest and you avoid a component that degrades with every event.

And whichever you choose, remember the asymmetry in ageing. A TVS is fit-and-forget within its ratings. A MOV is a wear item whose clamping voltage drifts down and whose leakage rises over its life, so in equipment meant to last it needs either monitoring or scheduled replacement.

## Relevant In-Stock Components

| SKU | Name | Manufacturer | Price | Stock |
|---|---|---|---|---|
| [SMAJ6.5CA-TR](https://novapart.co/products/SMAJ6.5CA-TR/tvs-diode-transzorb-smaj-bidirectional-65-v-145-do) | TVS Diode, TRANSZORB SMAJ, Bidirectional, 6.5 V, 14.5 V, DO-214AC (SMA), 2 Pins | STMICROELECTRONICS | €0.0910 | 1000+ |
| [PTVS3V3S1UR,115](https://novapart.co/products/PTVS3V3S1UR,115/tvs-diode-ptvs3-unidirectional-33-v-8-sod-123-2) | TVS Diode, PTVS3, Unidirectional, 3.3 V, 8 V, SOD-123, 2 Pins | NEXPERIA | €0.1550 | 1000+ |
| [SM6T39CAY](https://novapart.co/products/SM6T39CAY/tvs-diode-transil-sm6t-bidirectional-333-v-697-aec) | TVS Diode, Transil SM6T, Bidirectional, 33.3 V, 69.7 V, AEC-Q101, DO-214AA (SMB), 2 Pins | STMICROELECTRONICS | €0.1710 | 1000+ |
| [MCVZ0603M050AGT](https://novapart.co/products/MCVZ0603M050AGT/tvs-varistor-4-v-55-mcvz0603-20-0603-1608-metric) | TVS Varistor, 4 V, 5.5 V, MCVZ0603, 20 V, 0603 [1608 Metric], Multilayer Varistor (MLV) | MULTICOMP PRO | €0.0510 | 1000+ |
| [MCVZ0603M260AGT](https://novapart.co/products/MCVZ0603M260AGT/tvs-varistor-20-v-26-mcvz0603-54-0603-1608-metric) | TVS Varistor, 20 V, 26 V, MCVZ0603, 54 V, 0603 [1608 Metric], Multilayer Varistor (MLV) | MULTICOMP PRO | €0.0230 | 1000+ |
| [1SMA24AT3G](https://novapart.co/products/1SMA24AT3G/tvs-diode-sma-series-unidirectional-24-v-389-do) | TVS Diode, SMA Series, Unidirectional, 24 V, 38.9 V, DO-214AC (SMA), 2 Pins | ONSEMI | €0.1400 | 1000+ |
| [VC0805K121R025](https://novapart.co/products/VC0805K121R025/tvs-varistor-vc-series) | TVS Varistor, VC Series | KEMET / PARTNER STOCK | €0.0830 | 1000+ |
| [MLVB04V18C0R5](https://novapart.co/products/MLVB04V18C0R5/varistors) | VARISTORS | EATON / PARTNER STOCK | €0.0850 | 1000+ |
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