Varistor / MOV
Resistor (non-linear)
A voltage-dependent resistor that protects circuits from voltage surges by dramatically increasing resistance when voltage exceeds a threshold.
Schematic symbol
Varistor (VDR)
How varistor / mov appears in a circuit diagram.
Types & variants
Key specs
Voltage rating
Volts (V)Maximum continuous voltage the varistor can handle without conducting.
Clamping voltage
Volts (V)Maximum voltage that will appear across the varistor during a surge event.
Energy absorption
Joules (J)Maximum surge energy the varistor can absorb before failure.
Response time
nanoseconds (ns)How quickly the varistor reacts to a voltage surge.
Markings
Usually printed with voltage rating (e.g., '275' for 275V) and maximum AC voltage (e.g., 'V~'). MOVs often have a color band to indicate voltage rating.
Standard values
Common voltage ratings: 130V, 275V, 440V (for AC applications). Energy ratings from 10J to 10000J depending on size.
How to choose
1) Determine maximum circuit voltage (AC/DC). 2) Select varistor with voltage rating 20-30% above max voltage. 3) Choose energy rating based on expected surge size. 4) Consider physical size for required energy absorption.
Pinout & package
2 leads (non-polar, can go either way). Common packages: disc-shaped (through-hole), SMD (small rectangular packages), and radial leads.
Example circuits
- AC mains input protection
- Surge protection for power supplies
- Protection for relay contacts
- Input protection for data lines
Common failures
Short-circuit failure after absorbing large surges (visible burn marks or cracking). Can degrade over time with repeated surges, becoming less effective.
How to test
Set multimeter to ohms. Out of circuit, should read very high resistance (MΩ range). If it reads near zero, it's shorted and failed. In-circuit testing is unreliable due to parallel components.
Substitutes
TVS diodes for faster response and precise voltage clamping. Gas discharge tubes for very high energy surges. Multiple components can be used in parallel for higher energy handling.