What Is a Varistor (MOV)?
A varistor (MOV – Metal Oxide Varistor) is a protection component with voltage-dependent resistance. Normally it's high-resistance; when the voltage exceeds a safe limit, its resistance drops sharply and it absorbs the excess energy to protect the circuit.
How Does It Protect?
When the voltage across it exceeds the threshold, the varistor starts conducting and clamps the voltage to a safe level. Once the surge passes it becomes high-resistance again. It's connected in parallel with the line to be protected.
Difference From a TVS Diode
| Property | TVS diode | Varistor (MOV) |
|---|---|---|
| Speed | Very fast | Fast |
| Energy | Medium | High (mains surge) |
| Lifetime | Long | Wears with each surge |
Where Is It Used?
Mains-input surge protection, outlet/power-line protectors and protection against lightning/switching surges.
Energy Rating
When choosing a MOV, besides the clamping voltage, the surge energy (joules) it can absorb matters. The larger the likely surge on the protected line, the higher the energy MOV needed; otherwise it fails permanently on the first big surge.
Common Mistakes
- Forgetting aging: a MOV wears a little with each surge; it may need replacing over time.
- Choosing the wrong voltage: a MOV too close to the working voltage conducts continuously and overheats.
- Using it without a fuse: it can short under overload; a series thermal fuse is recommended.
Frequently Asked Questions
What's the difference between a varistor and a TVS?+
Why does a MOV degrade over time?+
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