Varistor Symbol Guide: Schematic Identifiers, Markings & IEC/IEEE Standards

The varistor symbol on a schematic tells you exactly where transient voltage suppression is happening, but the IEEE and IEC standards draw it differently. Here is the complete reference for schematic symbols, physical PCB markings, and how to decode faded metal oxide varistors (MOVs) on the bench.

Varistor Symbol & Standard Reference Table

Before you start tracing tracks or ordering replacements, you need to know which drafting standard your schematic follows. North American designs typically use IEEE/ANSI conventions, while European and global designs default to IEC. Below is the complete reference table for identifying varistors on paper and on the board.

Standard / Context	Symbol Description	Primary Region	Typical Silkscreen Designator
IEEE/ANSI (Std 315)	A capacitor symbol with a diagonal slash through it, or a rectangle with a diagonal line.	North America	VR, MOV, RV
IEC 60617	A rectangle with a diagonal line passing through it, featuring a small perpendicular tick at one end.	Europe / Global	VR, VDR, MOV
SPICE / Netlist	No graphical symbol; defined as a non-linear resistor model (e.g., X1 N1 N2 MOV_MODEL).	Simulation (Global)	X, RV
Physical PCB Silkscreen	A circle with two pads, often matching the physical disc diameter, sometimes with a zigzag or slash inside.	Universal	Matches schematic designator

In practice, the IEEE symbol is the one that trips up most beginners because it closely resembles a standard capacitor. If you see a capacitor symbol with a line striking through it in a US-based power supply schematic, it is almost certainly a Metal Oxide Varistor (MOV) placed across the AC line for surge suppression. The IEC symbol is more distinct; the small perpendicular tick on the diagonal line is meant to represent the non-linear, voltage-dependent resistance characteristic of the component.

Decoding Physical Markings & Faded Components

Schematic symbols only get you half the way there. When you open a chassis and look at the physical component, you will not see an IEEE or IEC symbol printed on the part itself. Instead, manufacturers like Bourns and Littelfuse use an alphanumeric coding system stamped directly onto the epoxy coating.

Let us break down a standard Bourns MOV-14D471K or a Littelfuse TMOV14RP470E. The code is broken into four distinct data rows:

• Diameter (14): The physical disc diameter in millimeters. A 14mm disc typically handles higher surge currents (e.g., 6,500A for an 8/20μs pulse) than a 10mm or 7mm disc.
• Type (D): Indicates a standard disc type. You might also see 'S' for square or 'T' for thermo-protected (which includes an internal thermal fuse).
• Voltage Code (471): This is a three-digit multiplier code, identical to ceramic capacitors. The first two digits are the significant figures, and the third digit is the multiplier (number of zeros). 471 means 47 followed by one zero, equaling 470V.
• Tolerance (K): The clamping voltage tolerance. 'K' stands for ±10%, 'M' stands for ±20%, and 'J' stands for ±5%.

Safe Interpretation When Markings are Missing: If the part is completely illegible, you must determine the operating voltage of the circuit to select a replacement. For a standard 120V AC mains input, the peak voltage is roughly 170V. You need a varistor with an RMS voltage rating (V_RMS) of at least 130V, but 150V or 175V is standard practice to prevent nuisance degradation. For 240V AC mains, look for a V_RMS rating of 275V or 320V. Desolder the component and measure its diameter with calipers to match the surge current capability (e.g., 14mm for heavy appliances, 10mm for standard consumer electronics).

Coding Rows & Symbol Rows People Get Wrong

Even experienced technicians misread specific rows in the varistor datasheet tables or misinterpret the schematic symbols. Here are the most common pitfalls:

1. The Voltage Multiplier Row (Reading 471 as 471V) The most frequent bench mistake is reading the three-digit code literally. A marking of 271 does not mean 271 volts; it means 27 × 10¹ = 270V. Similarly, 431 is 430V, and 561 is 560V. If you order a replacement based on the literal number, you will end up with a component that clamps at the wrong threshold, potentially destroying downstream semiconductors.

2. The IEEE Symbol Confusion (Capacitor vs. Varistor) Because the older IEEE/ANSI symbol for a varistor is literally a capacitor with a line through it, hobbyists often assume the circuit requires a high-voltage ceramic or film capacitor for noise filtering. If you replace an MOV with a standard capacitor across the AC line, you will create a direct short through the capacitor at high frequencies, or worse, a fire hazard if the capacitor is not Y-rated for mains across-the-line use.

3. The AC vs. DC Rating Row Datasheets list both V_RMS (AC) and V_DC (DC) maximum continuous operating voltages. The voltage printed on the disc (e.g., 470V) usually refers to the nominal varistor voltage (V_N) measured at 1mA DC. Beginners often select a varistor where the V_N matches their peak AC voltage. This is wrong. The V_N must be roughly 20% to 30% higher than the peak AC line voltage to prevent the MOV from conducting during normal operation and slowly cooking itself to death.

Frequently Asked Questions

What does the varistor symbol look like on a schematic?

It depends on the drafting standard. In North America (IEEE), it is typically drawn as a capacitor symbol with a diagonal slash through it. In Europe and most international schematics (IEC 60617), it is drawn as a rectangular box with a diagonal line passing through it, usually featuring a small perpendicular tick mark at one end of the line to denote its non-linear resistance. On the physical PCB silkscreen, it is usually represented by a simple circle matching the component's footprint, labeled with a designator like VR1 or MOV1.

How do I test a varistor if the physical markings are burned off?

You cannot determine the exact voltage rating of an unmarked, burned varistor with a multimeter, but you can determine if it has failed. Set your digital multimeter to the highest resistance range (usually 20MΩ or higher). Ensure the component is completely isolated from the circuit (desolder at least one leg, as parallel bleeder resistors or transformer windings will give false readings). A healthy MOV will read 'OL' (Over Limit) or infinite resistance. A failed MOV will typically read as a dead short (less than 1 ohm) or a very low resistance. If it reads shorted, it has done its job sacrificing itself to a surge and must be replaced.

Is a varistor polarized like a diode or electrolytic capacitor?

No. Standard Metal Oxide Varistors (MOVs) are completely bidirectional and non-polarized. The schematic symbol does not include a polarity bar, and the physical component has no stripe or positive/negative indicator. You can solder them in either direction across the AC line or DC bus. The only exception is specialized multi-layer varistor (MLV) arrays or specific TVS (Transient Voltage Suppression) diodes, which are distinct components with their own polarized symbols.

Why did my varistor explode and leave a blackened symbol on the PCB?

MOVs degrade over time. Every time they clamp a transient voltage spike, a tiny amount of energy is absorbed, which microscopically degrades the zinc oxide grain boundaries inside the disc. After many small surges, or one massive surge that exceeds the component's Joule rating, the internal resistance drops. The MOV begins to conduct heavily during normal AC peaks, drawing massive current, overheating, and eventually failing in a short-circuit mode. If the circuit lacks a properly sized upstream fuse or a thermal disconnect (like a TMOV), the short-circuit current will cause the epoxy coating to vent violently or explode. Always replace a blown MOV and verify the upstream fuse is intact.