The standard IEC 60617 symbol for a varistor is a rectangle bisected by a diagonal line with a distinct "kink" or "hook" at one end, usually labeled "RV" or "VDR". In IEEE/ANSI standards, it appears as a standard resistor rectangle (or zigzag) with the same diagonal non-linear indicator. If you are looking at a schematic and need a physical replacement part for a standard mains input, a 14mm Metal Oxide Varistor (MOV) rated for 130VAC (like the Littelfuse V130LA10A) is the default pick for 120V lines, while a 275VAC rated MOV is the default for 230V lines.

The Varistor Symbol Reference Table (IEC vs. ANSI)

Schematic symbols for passive components vary depending on whether the engineering team follows international (IEC) or North American (IEEE/ANSI) drafting standards. Note that while the NEC (NFPA 70) dictates physical wiring colors and installation rules, it does not govern schematic symbols. Use the table below to identify the varistor symbol on your specific board documentation.

Standard Base Shape Non-Linear Indicator Common Label Primary Region / Usage
IEC 60617 Rectangle Diagonal line with a "kink" or hook at the top end RV, VDR, MOV Europe, Asia, Global standard
IEEE/ANSI Y32.2 Rectangle or Zigzag Diagonal line with a kink, sometimes a small "V" arrow VR, MOV North America, Military/Aerospace
Legacy / Old UK (BS 3939) Rectangle Diagonal line with a kink, enclosed in a circle VDR Older British schematics (pre-1990s)
Manufacturer Specific Rectangle Diagonal line, often with "U" or "V-I" curve drawn next to it Varistor, ZNR Internal OEM service manuals

Rows People Get Wrong: Varistors vs. Similar Components

When scanning a complex power supply schematic, it is easy to misidentify a varistor as another non-linear or protective component. Here is how to tell them apart based on the symbol's diagonal line behavior.

Varistor vs. NTC/PTC Thermistor

The thermistor symbol also uses a rectangle with a diagonal line, but the line breaks and goes completely horizontal (for NTC) or vertical (for PTC) at the end. A varistor's diagonal line simply has a sharp "kink" or bend in it, but it does not terminate in a strict horizontal or vertical axis. If the symbol looks like a hockey stick lying flat, it is a thermistor. If it looks like a bent lightning bolt, it is a varistor.

Varistor vs. Fusible Resistor

A fusible resistor is designed to fail open like a fuse under overcurrent. Its symbol is typically a standard resistor (zigzag or rectangle) with a straight horizontal line striking completely through the middle, or a small box drawn over the center of the resistor body. Varistors do not have a line striking through the body; their indicator is strictly diagonal.

Varistor vs. Standard Zener Diode

While both clamp voltage, a Zener diode symbol is a triangle pointing into a line (the cathode bar), with the ends of the bar bent backward toward the triangle. A varistor symbol never uses a triangle; it is always based on the two-terminal resistor rectangle.

Bench Tip: If you are looking at a physical PCB and the schematic is missing, look at the placement. Varistors are almost always wired in parallel across Line and Neutral (or Line and Ground) to clamp transient spikes. If the component is in series with the load path, it is likely a thermistor (inrush limiter) or a fusible resistor, not a varistor.

Decoding Faded or Blown MOV Markings

MOVs sacrifice themselves to protect downstream electronics. When an MOV absorbs a massive surge, the epoxy coating often chars, cracks, or blows off entirely, taking the printed part number with it. When markings are faded or missing, you must deduce the correct replacement using physical context and standard manufacturer color-coding.

  • Disc Diameter Dictates Energy (Joules): The physical width of the MOV disc directly correlates to its energy absorption capacity. A 10mm disc typically handles 20–30 Joules; a 14mm disc handles 40–70 Joules; a 20mm disc handles 100–150 Joules. Measure the disc with calipers to establish the baseline size class.
  • Color Coding (Voltage Clues): While not strictly standardized across all brands, Bourns and many Asian manufacturers frequently use blue epoxy for standard AC mains MOVs (130VAC–275VAC) and yellow or black for lower voltage DC line suppressors or higher industrial voltage ratings.
  • Lead Pitch and Thickness: A 14mm MOV typically has a lead spacing of 7.5mm and uses 0.8mm thick tinned copper wire. If the PCB footprint has a wider 10mm pitch, you are likely looking at a 20mm MOV footprint.
Safety Warning: Never test a suspected blown MOV with a standard multimeter continuity check while the board is energized. Furthermore, a failed MOV often fails short-circuit. If the board's main fuse is blown, the MOV is the prime suspect. Desolder one leg and test out-of-circuit. A good MOV will read infinite resistance (OL) on a standard DMM. If it reads near zero ohms, it has sacrificed itself.

Decision Tree: Sizing a Replacement MOV

Do not guess when replacing a surge suppression component. Use this decision path to lock in the exact specifications and part number for your repair. This table terminates in concrete, off-the-shelf part numbers available from major distributors like Digi-Key or Mouser.

Condition / Measurement Next Step / Deduction Resulting Specification
Board is connected to 120VAC nominal mains (North America) Select MOV with Maximum Continuous Operating Voltage (MCOV) of at least 1.25x line voltage. Target MCOV: 130VAC to 150VAC
Board is connected to 230VAC nominal mains (Europe/UK/AU) Select MOV with MCOV of at least 1.25x line voltage, accounting for +10% grid tolerance. Target MCOV: 275VAC to 320VAC
Physical disc measures ~14mm (Standard consumer electronics) Match the energy rating to the 14mm class to ensure it fits the PCB footprint and handles standard surges. Target Energy: ~40 Joules, Peak Current: 4.5kA
Physical disc measures ~20mm (Industrial / Power Supplies) Step up to the 20mm class for high-surge environments. Target Energy: ~100 Joules, Peak Current: 10kA
FINAL SELECTION (120VAC Line, 14mm footprint) Order the industry-standard 14mm, 130VAC radial MOV. Default Pick: Littelfuse V130LA10A
FINAL SELECTION (230VAC Line, 14mm footprint) Order the industry-standard 14mm, 275VAC radial MOV. Default Pick: Littelfuse V275LA40A

Safe Interpretation and Installation Rules

When installing a replacement MOV based on the schematic symbol and decision tree above, adhere to these bench and jobsite practices to ensure the protection circuit functions safely.

  1. De-energize and Verify: MOVs sit directly across mains voltage. Before desoldering, unplug the device, discharge large bulk filter capacitors through a high-wattage bleed resistor, and verify the DC bus is below 5V with a multimeter.
  2. Orientation Does Not Matter: Unlike diodes or electrolytic capacitors, standard radial MOVs are non-polarized. You can install them in either direction. However, for neatness and factory-style rework, align the text facing outward, away from adjacent heat sinks.
  3. Keep Leads Short: The clamping effectiveness of an MOV is degraded by lead inductance. During high-frequency transients (like an EFT burst), long leads create a voltage spike ($V = L \frac{di}{dt}$) that adds to the MOV's clamping voltage. Trim the leads so the MOV body sits no more than 2mm to 3mm off the PCB surface.
  4. Pair with a Thermal Fuse: If the original schematic symbol showed a varistor tightly coupled with a thermal cutoff (TCO), do not omit the TCO. Modern thermally protected MOVs (TMOVs) integrate this internally, but if you are using a standard MOV, ensure the upstream fuse or TCO is intact to prevent a fire hazard if the MOV degrades and enters thermal runaway.

By correctly identifying the IEC or ANSI varistor symbol, measuring the physical footprint, and applying the MCOV multiplier rule, you can confidently restore a power supply's transient protection without relying on faded silkscreen text.