The symbol for a varistor on a schematic represents a voltage-dependent resistor (VDR), most commonly a Metal Oxide Varistor (MOV), which clamps transient voltage spikes to protect downstream electronics. Below is the complete reference for reading these symbols and decoding their physical markings.
The Complete Varistor Symbol & Marking Reference
Unlike standard resistors, varistors do not use universal color bands. Instead, they rely on standardized schematic symbols and alphanumeric print codes stamped directly on the component housing. Use the tables below to decode both the schematic representation and the physical part markings.
| Standard | Visual Description | Region / Usage |
|---|---|---|
| ANSI/IEEE Std 315 | Rectangle with a diagonal line passing through it, and a parallel 'bent' line (resembling an L or V shape) indicating non-linear resistance. | North America, legacy US schematics |
| IEC 60617 | Rectangle with a diagonal line passing through it, and a parallel straight line featuring a small perpendicular hook or tick at one end. | Europe, International, modern global CAD libraries |
| Code Segment | Example | Meaning in Practice |
|---|---|---|
| Diameter / Size | 14 | Disc diameter in millimeters (e.g., 7, 10, 14, 20, 25). Dictates energy absorption (Joules). |
| Shape / Series | D | 'D' = Disc. 'S' = Square/Chip. 'M' = Metal Oxide specific series depending on brand. |
| Significant Digits | 47 | The first two digits of the varistor voltage ($V_N$). |
| Multiplier | 1 | Number of zeros to add. '1' = $10^1$. (47 + 0 = 470V $V_N$). |
| Tolerance | K | 'K' = ±10% tolerance. 'L' = ±15%, 'M' = ±20%. |
Schematic Standard Variants: ANSI vs. IEC
When tracing a board or reading a datasheet, the region of the engineer who drew the schematic dictates which symbol varistor variant you will see.
The ANSI/IEEE bent-line variant is heavily favored in North American power supply designs and older service manuals. The bent line is meant to visually represent the 'knee' in the V-I curve where the resistance drops exponentially. Conversely, the IEC 60617 standard uses the hooked straight line to denote a non-linear, voltage-dependent component. Modern EDA tools like Altium and KiCad often default to the IEC symbol in international workspaces, but allow toggling to IEEE for US-centric documentation.
Manufacturer marking codes also vary slightly by region and brand. While the '14D471K' format is standard for brands like Bourns and generic Asian manufacturers, European manufacturers like TDK/EPCOS often use a different prefix system (e.g., 'S14K300', where 300 denotes the maximum continuous RMS voltage in volts, rather than the varistor voltage). Always check the specific manufacturer's datasheet if the prefix format deviates from the standard diameter-series-voltage layout.
Rows People Get Wrong: Misread Markings & Tolerances
When ordering replacements or troubleshooting a blown power supply, hobbyists and junior techs frequently misinterpret the marking table. Here are the most common mistakes:
- The 'K' Suffix Confusion: In standard resistor color codes or SMD markings, 'K' means Kilo-ohms. On a varistor, K strictly means ±10% tolerance. A '14D471K' is not a 470 Kilo-ohm resistor; it is a 470V varistor with a 10% manufacturing tolerance.
- Varistor Voltage ($V_N$) vs. Line Voltage: The number calculated from the digits and multiplier (e.g., 471 = 470V) is the Varistor Voltage, measured at a specific DC test current (usually 1mA). It is not the AC line voltage it is meant to protect. To find the maximum continuous AC RMS voltage the part can handle, multiply $V_N$ by roughly 0.64. Therefore, a 470V $V_N$ MOV is rated for roughly 300VAC RMS, making it suitable for 240VAC mains (which can peak around 265VAC under high-line conditions).
- Clamping Voltage ($V_C$) Omission: The marking on the disc never tells you the clamping voltage (the voltage let-through during a massive surge, like a 3kA 8/20µs pulse). A 14D471K might have a $V_N$ of 470V, but its clamping voltage could be as high as 775V. You must consult the datasheet to ensure the let-through voltage won't destroy your downstream bridge rectifier or switching IC.
Safe Interpretation When Markings Are Faded or Burnt
MOVs fail by absorbing catastrophic energy. When they do, the top of the disc often chars, cracks, or blows completely off, obliterating the alphanumeric print. If you are repairing a board and the varistor markings are missing, follow this bench procedure to identify a safe replacement:
- Measure the Physical Diameter: Use digital calipers to measure the disc. Standard through-hole MOVs come in 5mm, 7mm, 10mm, 14mm, 20mm, and 25mm sizes. The diameter dictates the Joule rating and peak current capacity. A 14mm disc is the most common for standard 120VAC appliance power supplies.
- Analyze the Circuit Context: Trace the PCB copper. Is the MOV wired directly across Line and Neutral (L-N) or Line and Earth (L-PE)?
- If across 120VAC L-N, you need an MOV with a $V_{RMS}$ rating of at least 150VAC (which corresponds to a $V_N$ of roughly 240V to 275V, such as a 14D271K).
- If across 240VAC L-N, you need a $V_{RMS}$ rating of at least 275VAC to 320VAC (corresponding to a $V_N$ of 430V to 510V, such as a 14D471K or 14D511K).
- Verify the Fuse: An MOV should always be paired with a series fuse or a thermal cutoff (TCO). If the MOV failed shorted and blew the board's main fuse, replace the MOV with the same physical diameter and the calculated voltage rating, and never bypass the fuse. For high-reliability replacements, look into thermally protected MOVs (like the Littelfuse TMOV series), which have an internal spring-loaded disconnect that opens if the MOV overheats, preventing PCB fires.
Varistor Symbol & Marking FAQ
What does the symbol varistor look like compared to a thermistor?
While both are non-linear resistors, their schematic symbols differ to reflect their trigger mechanisms. The varistor symbol (VDR) features a diagonal line through a rectangle with a parallel bent or hooked line, indicating voltage dependence. A thermistor symbol features a rectangle with a diagonal line passing through it, but the line has a small 'break' or 'hockey stick' curve at the bottom, and is often accompanied by a 't°' or '-t°' label to indicate temperature dependence (NTC or PTC).
Do metal oxide varistors use standard 4-band resistor color codes?
No. You will almost never see a 4-band or 5-band color code on a standard radial-leaded MOV. The blue, yellow, or black epoxy coating on the disc is purely for environmental sealing and dielectric insulation. While some specialized surface-mount (SMD) varistors might use a 3-digit SMD code similar to resistors (e.g., '471' for 470V), through-hole MOVs rely exclusively on printed alphanumeric text. If you see a component with standard color bands that looks like a thick disc, it is likely a high-wattage power resistor or an inductor, not a varistor.
How do I read the manufacturer date code on a varistor?
Many manufacturers stamp a secondary, shorter code on the back or bottom edge of the MOV disc, or print it in smaller text below the main part number. This is typically a 3- or 4-digit date code. For example, '2415' translates to the 15th week of 2024. Some Asian manufacturers use a single letter for the year (e.g., 'R' for 2024) followed by a number for the month. This code is strictly for factory traceability and shelf-life tracking; it does not affect the electrical specifications or the replacement compatibility of the component.
