Varistor
A metal-oxide varistor, or MOV, conducts relatively little current at normal working voltage and much more during an overvoltage. Its clamping voltage rises with surge current. Continuous AC/DC ratings, nominal varistor voltage and surge-clamping voltage are different specifications.
The MOV's nonlinear, bidirectional current-voltage characteristic lets it shunt surge current. Instantaneous dissipation is voltage multiplied by current; pulse energy accumulates over the event. Excessive surges or sustained overvoltage can increase leakage and heating, so failure behaviour and protective coordination are part of the design.
In plain terms
Like a pressure-relief path that conducts more as the pressure rises. Its capacity is limited, and repeated or excessive stress can damage it.
Why designers use it
- Limiting switching- or lightning-induced surges within a designed protection network.
- Handling a documented surge waveform where its current and energy ratings are appropriate.
Best for
- AC mains surge
- DC bus clamping
- Telecom secondaries
Key specifications
- Continuous voltage: Check AC and DC ratings separately
- Clamping voltage: Specified at a stated surge current
- Surge capability: Check waveform, pulse count and derating
- End-of-life protection: Coordinate thermal and overcurrent protection
When not to use it
- When leakage, capacitance, clamping voltage or pulse capability cannot meet the circuit requirements.
- As the sole response to an uncontrolled sustained overvoltage.
- As proof of product-safety compliance without evaluating the complete protection system.
Common mistakes
- Using nominal varistor voltage as the allowed continuous AC mains voltage.
- Ignoring the rise in clamp voltage at higher surge current.
- Omitting evaluation of thermal runaway, end-of-life behaviour and coordinated disconnection.
- Assuming a component's approval establishes compliance of the entire product.
Where you will find it
- A mains-input protection design can combine a suitable MOV with coordinated thermal and overcurrent protection, evaluated against the applicable product requirements.
- Bourns discusses combining MOV and gas-discharge behaviours in protection components. The chosen combination still needs application-specific surge and failure testing.
A short history
Surge protection developed several complementary approaches. A metal-oxide varistor limits voltage with a nonlinear, bidirectional current-voltage characteristic; a gas-discharge tube instead switches into a low-voltage conducting state. Designers can combine these behaviours, but coordination and end-of-life protection are essential parts of the circuit.
Good to know
- An MOV is bidirectional, but its normal AC and DC voltage ratings are still different specifications.
- Clamping voltage is not a perfectly flat ceiling: it changes with current.