The Metal Oxide Varistor (MOV) symbol in a schematic denotes a nonlinear, voltage-dependent resistor designed to clamp transient voltage spikes and protect downstream electronics. Unlike standard carbon-film resistors that rely on 4- or 5-band color codes, MOVs use alphanumeric body markings (e.g., "471K") to indicate their varistor voltage and tolerance, while their epoxy body color simply denotes the manufacturer's series or material type. Because schematic standards differ globally, identifying the correct MOV symbol requires knowing whether your blueprint follows IEC or IEEE/ANSI conventions.

The MOV Schematic Symbol: IEC vs. IEEE/ANSI Standards

When reading a schematic, the symbol for a varistor (MOV) changes depending on the regional and organizational standard used by the drafting engineer. Below is the complete reference table for identifying the MOV symbol across the two dominant global standards.

Standard Symbol Description Visual Representation Where It Applies
IEEE 315 / ANSI Y32.2 Rectangle with a diagonal line crossing through it, and a small parallel line at the top and bottom edges. [ ▭ ] with diagonal and parallel caps (Nonlinear Resistor) North America, legacy US military specs, older CAD libraries.
IEC 60617 Rectangle with a diagonal line crossing it, often accompanied by the letter "U" or "V" inside or adjacent to the body. [ ▭ ] with diagonal and 'U'/'V' designation Europe, Asia, modern international datasheets, IEC-compliant designs.
Manufacturer Shorthand A simple rectangle or zigzag resistor symbol with "MOV", "VDR", or "ZNR" written next to it. Text-annotated standard resistor Hobbyist schematics, quick reference sheets, application notes.
Bench Tip: If you see a symbol that looks like a standard resistor but has a diagonal line striking through the body and a small parallel line at one end, it is a varistor. If the diagonal line has an arrow pointing away from it, it is a photoresistor (LDR), not an MOV.

MOV Marking Codes & Body Color Reference

MOVs do not use color bands. Instead, they use a 3-digit alphanumeric code printed on the epoxy coating, followed by a tolerance letter. The body color (usually blue, black, or yellow) indicates the zinc-oxide formulation or series, not the electrical value.

Marking Segment Meaning Example Values
Digits 1 & 2 Significant figures of the Varistor Voltage ($V_{1mA}$) "47" = 47, "20" = 20, "10" = 10
Digit 3 Multiplier (Number of zeros to add) "1" = $\times 10^1$, "2" = $\times 10^2$, "3" = $\times 10^3$
Tolerance Letter Acceptable deviation from nominal $V_{1mA}$ K = ±10%, L = ±15%, M = ±20%
Body Color: Blue Standard Zinc Oxide (ZnO) formulation Most common for consumer AC mains (e.g., Bourns MOV series)
Body Color: Black/Yellow High-energy, industrial, or specialized telecom series High surge capacity, low capacitance, or halogen-free epoxy

Worked Example: An MOV marked 471K has significant digits 47, a multiplier of $10^1$ (one zero), and a 10% tolerance. The Varistor Voltage is 470V (measured at 1mA DC test current). An MOV marked 201K is 200V.

Rows People Get Wrong (And How to Avoid Blown Boards)

Replacing an MOV based on a quick glance at the marking code is the most common cause of immediate power-supply failure on the repair bench. Avoid these three critical misinterpretations:

  1. Confusing $V_{1mA}$ with Continuous AC RMS Voltage: A "471" (470V) MOV is not meant for a 470V AC line. The 470V rating is the voltage at which the MOV begins to conduct 1mA of leakage current. For continuous AC operation, you must derate this by roughly 35-40%. A 471K MOV is rated for a maximum continuous 300V RMS (or 385V DC). Putting it on a 400V AC line will cause it to overheat and catch fire.
  2. Assuming Physical Size Dictates Voltage: A 10D471K (10mm disc) and a 20D471K (20mm disc) both clamp at the exact same 470V threshold. The disc diameter dictates the energy absorption (Joules) and peak surge current capacity, not the voltage. The 20mm disc can absorb ~130 Joules, while the 10mm handles ~35 Joules.
  3. Ignoring the Clamping Voltage ($V_C$): The marking code tells you the turn-on voltage, but at a peak surge (e.g., 50A), the voltage across the MOV will spike much higher. A 471K MOV might have a clamping voltage of 775V. Ensure your downstream semiconductors can survive a 775V transient.
Safety Warning: MOVs fail in a "short-circuit" mode when they absorb energy beyond their thermal limit. If an MOV is not backed by a series fuse or thermal cutoff, a shorted MOV will draw massive mains current, leading to violent epoxy casing rupture and fire. Always verify the presence of an upstream protective device when troubleshooting.

Safe Interpretation When Markings Are Faded or Missing

After a severe lightning strike or grid surge, the top of an MOV often chars, blowing the epoxy and erasing the alphanumeric marking. Here is the exact procedure to identify a safe replacement without guessing:

  1. Test for Failure: Set your multimeter to resistance (Ohms) mode. Place probes across the MOV legs. A healthy MOV will read "OL" (Open Loop) because the multimeter's 3V test voltage is far below the MOV's threshold. If it reads near 0Ω, the MOV has sacrificially shorted and must be replaced.
  2. Determine Energy Class (Size): Use digital calipers to measure the disc diameter. Standard sizes are 7mm, 10mm, 14mm, and 20mm. Match the physical footprint of your replacement to ensure it can handle the expected surge energy of the appliance.
  3. Deduce the Voltage Rating: Look at the circuit's nominal input voltage. If the board is a standard US 120V AC appliance, the engineer likely used an MOV rated for 130V or 150V RMS (marking codes 201K or 241K). If it is a European 230V AC appliance, look for a 275V or 300V RMS rating (marking codes 431K or 471K).

MOV Selection Decision Tree

Use this decision path to select the exact replacement part for your next repair or DIY power supply design.

Step Condition / Question Action / Result
1. Identify Line Voltage What is the nominal AC RMS voltage of the circuit? e.g., 120V AC (US) or 230V AC (EU)
2. Apply Safety Margin Multiply nominal AC voltage by 1.25 (25% margin for grid swells). 120V × 1.25 = 150V RMS minimum rating.
3. Select $V_{RMS}$ Rating Choose the next standard MOV $V_{RMS}$ rating above your calculated margin. Select 150V RMS or 130V RMS (if grid is highly stable).
4. Translate to Marking Code Find the corresponding Varistor Voltage ($V_{1mA}$) and 3-digit code. 150V RMS ≈ 240V $V_{1mA}$ → Code: 241
5. Determine Disc Size Is this a low-power SMPS (<50W) or a heavy inductive load/motor drive? Low power = 10mm or 14mm. Heavy load = 20mm.
Final Concrete Pick Standard 120V AC US Appliance Power Supply (14mm disc preferred for margin). Buy: Bourns MOV-14D241K or Littelfuse V150LA10AP

For 230V AC European applications, the math shifts: 230V × 1.25 = 287V. You would select a 300V RMS / 470V $V_{1mA}$ MOV, terminating in the concrete pick of a Bourns MOV-14D471K or Littelfuse TMOV14RP300M (which includes an integrated thermal fuse for added safety).

Real-World Failure Modes & Replacement Rules

When replacing an MOV, never simply swap a standard varistor into a circuit that previously used a Thermally Protected MOV (TFMOV). Standard MOVs suffer from thermal runaway during prolonged overvoltage events (like a neutral-wire disconnect causing 240V on a 120V line). The MOV will slowly cook, eventually cracking and venting plasma.

Manufacturers like Littelfuse and Bourns solve this by integrating a spring-loaded thermal cutoff inside the MOV casing. If you are repairing a high-reliability power supply (like an ATX computer supply or industrial LED driver) and the original part was a TMOV, you must replace it with a TMOV equivalent. Using a standard MOV in its place removes the thermal disconnect, turning a minor grid anomaly into a catastrophic board fire. Always pair standard MOVs with a fast-acting series fuse rated just above the circuit's normal operating current to ensure the MOV can safely sacrifice itself without taking the PCB traces with it.