How to find the right Varistors AI guide
Written by an AI model from its trained knowledge.
What the AI looks for in Varistors
- Maximum continuous voltage
- Choose the MOV by its maximum AC rms rating with margin over the line. For 120 V mains this is commonly a 150 V rms part, and for 230 V mains a 275 V rms part. Too low a rating makes the MOV conduct on normal swells and overheat. Too high a rating raises the clamping voltage.
- Reading part codes
- Codes like 14D471K mean a 14 mm disc, a 470 V varistor voltage (47 × 10) and ±10% tolerance (K). The varistor voltage in the code is not the operating voltage, which is a frequent cause of wrong replacements.
- Disc size and surge energy
- Larger disc diameters (for example 14 or 20 mm versus 7 mm) give higher surge current and energy capacity. Match or exceed the original size. MOVs degrade with each surge, and smaller discs wear out faster.
- Clamping voltage
- Check the clamping voltage at a stated test current against what the protected equipment can withstand. The clamping voltage is far above the working voltage, and the protected load must survive it.
- Thermal protection and fusing
- On mains, use thermally protected MOVs or add a thermal fuse and a series current fuse. Look for UL recognition for surge-suppression use where it applies. Worn MOVs can fail short and burn.
Brands that matter
- Littelfuse: Wide MOV range, including thermally protected types.
- TDK (EPCOS): SIOV disc and block varistors with thorough datasheets.
- Bourns: Disc MOVs and thermally protected varistors.
- Panasonic: ZNR varistors for mains and electronics.
- Vishay: Disc varistors for general protection.
What to avoid
- Replacing a mains MOV based only on disc color or size without matching its voltage code.
- Mains MOVs installed without a fuse or thermal disconnect.
- Cheap unbranded assortment packs for service-entrance or high-exposure use.
- Assuming an old surge strip still protects you after a large surge, since MOVs wear out silently.
Questions
Which MOV do I need for 120 V or 230 V mains?
Commonly a 150 V rms part (varistor voltage around 240 V) for 120 V lines, and a 275 V rms part (varistor voltage around 430–470 V) for 230 V lines. Always confirm against the datasheet and the original part.