Symbol for Varistor: IEC vs IEEE Schematics and Physical Codes

The standard symbol for a varistor (specifically a Metal Oxide Varistor, or MOV) depends entirely on the drafting standard your schematic follows. Under the international IEC 60617 standard, the symbol is a rectangle bisected by a diagonal line that terminates in a small horizontal hook. Under the North American IEEE 315 standard, it is typically depicted as a zigzag resistor with a diagonal strike-through and a nonlinear curve hook. In both cases, the component designator is usually RV (Resistor, Voltage-dependent) or VR.

If you are looking at a physical board rather than a schematic, varistors rarely use color bands. Instead, they use a stamped 3-digit alphanumeric code on an epoxy-coated disc (usually blue, orange, or black). Below is the complete reference for interpreting both the schematic symbols and the physical markings.

Schematic Symbols and Physical Markings Reference

Decoding the Symbol and the 3-Digit Physical Code

While the schematic symbol tells you where the varistor is and its theoretical clamping behavior (the hook on the diagonal line represents the nonlinear V-I curve), the physical component tells you its exact limits. Assuming a standard zinc-oxide MOV operating at a 25°C ambient temperature, the physical code breaks down as follows:

Worked Example: Reading a "14471K" Varistor

• 14: Disc diameter in millimeters (14mm). This dictates the physical mass and energy absorption (Joule) rating. A 14mm disc handles significantly more surge energy than a 7mm disc, even if the voltage rating is identical.
• 471: The voltage code. The first two digits are significant figures (47), and the third digit is the multiplier (number of zeros). Therefore, 47 × 10¹ = 470V. This is the nominal varistor voltage (Vn) measured at 1mA DC, not the AC RMS operating voltage.
• K: Tolerance code. K = ±10%. (M = ±20%, L = ±15%).

To find the maximum continuous AC RMS voltage this part can safely sit across, you divide the nominal voltage by roughly 1.35 to 1.4. For a 470V part, the maximum continuous AC RMS voltage is approximately 300V AC, making it a standard replacement for 230V/240V AC mains circuits.

Rows and Symbols People Get Wrong

When cross-referencing schematics to physical boards, hobbyists and junior technicians consistently trip over three specific interpretations:

1. Confusing the IEC Varistor Symbol with a TVS Diode

A Transient Voltage Suppression (TVS) diode serves a similar purpose but reacts in nanoseconds compared to the MOV's microsecond response. The TVS diode symbol looks like a standard Zener diode (a triangle with a bar) but with a small circle or an extra bend at the cathode end. If the schematic shows the IEC rectangle-and-hook symbol, you must use an MOV. Swapping in a TVS diode of the same voltage rating will result in the TVS diode violently exploding during a sustained surge, as TVS diodes have a fraction of the Joule absorption capacity of a 14mm MOV.

2. Misinterpreting the 'V' Designator on the Schematic

In older IEEE schematics, you might see a standard resistor symbol with a diagonal line and a simple 'V' written next to it, lacking the nonlinear hook. Do not confuse this with a variable resistor (potentiometer), which uses an arrow pointing into the resistor zigzag. If the component is placed in parallel across the AC Line and Neutral inputs, it is a varistor, regardless of whether the drafter omitted the hook.

3. Reading the Multiplier as Literal Voltage

The most common bench mistake is reading a "471" stamp and ordering a 471V replacement, or worse, reading a "271" stamp and assuming it is for 271V AC mains. A 271 stamp means 270V nominal DC at 1mA. Its actual AC RMS rating is 175V. If you place a 271K MOV across a 230V AC mains line (which can peak at 325V), the MOV will begin conducting heavily during normal operation, overheat, and catch fire. Always verify the RMS voltage rating in the datasheet, not just the stamped code.

Forensics: Safe Interpretation When Markings are Burnt

MOVs fail by sacrificing themselves. During a massive transient (like a nearby lightning strike), the zinc oxide grains melt together, causing the part to fail into a dead short. This short draws massive current, which eventually blows the upstream fuse or trips the breaker. In the process, the epoxy coating often chars, cracks, or completely vaporizes, taking the markings with it.

If you are reverse-engineering a board with a burnt, unmarked varistor, follow this forensic path:

1. Identify by Topology: If the two-legged component is wired directly between Line and Neutral, or Line and Earth Ground, immediately downstream of the fuse and before the transformer or switching IC, it is an MOV.
2. Measure the Diameter: Use digital calipers to measure the disc. Common sizes are 5mm, 7mm, 10mm, 14mm, and 20mm. This dictates your physical footprint and minimum Joule rating.
3. Determine Circuit Voltage: Look at the input label on the enclosure or the primary bulk capacitor rating. If the bulk cap is rated for 400V DC, the circuit is designed for 230V AC mains. If it is rated for 200V DC, it is a 120V AC circuit.
4. Test for Failure: Set your multimeter to Ohms. A healthy MOV will read "OL" (Open Loop) or infinite resistance. If it reads anywhere below 100kΩ, it is degraded. If it reads near 0Ω, it has failed short and must be desoldered.

Decision Path: Sizing Your Exact Replacement

Do not guess when replacing a mains-rated varistor. Use the decision matrix below to select a concrete, high-quality replacement part from established manufacturers like Littelfuse or TDK Electronics (EPCOS). These selections assume a standard industrial/consumer power supply environment with a 10kA surge requirement.

By matching the schematic symbol to the correct physical topology, decoding the 3-digit multiplier accurately, and sizing the disc diameter to the required Joule absorption, you ensure the circuit survives the next grid transient without turning your PCB into a charcoal briquette.