The thermistor schematic symbol changes depending on whether your design follows North American (IEEE/ANSI) or International (IEC 60617) standards, and whether the component is a Negative Temperature Coefficient (NTC) or Positive Temperature Coefficient (PTC) type. Rather than guessing based on a vague diagram, use the complete reference table below to decode your schematics immediately.
Thermistor Schematic Symbol Reference Table (IEEE vs IEC)
| Component Type | IEEE/ANSI Symbol (US Standard) | IEC 60617 Symbol (International) | Key Identifier Marking | Typical Circuit Use Case |
|---|---|---|---|---|
| NTC Thermistor | Zigzag resistor line with a diagonal arrow pointing down/through, marked -t° |
Rectangle box with a diagonal arrow pointing down/through, marked -t° |
-t° or NTC |
Inrush current limiting, temperature sensing (e.g., 3D printer hotends) |
| PTC Thermistor | Zigzag resistor line with a diagonal arrow pointing up/through, marked +t° |
Rectangle box with a diagonal arrow pointing up/through, marked +t° |
+t° or PTC |
Resettable fuses, overcurrent protection, motor starting |
| Linear PTC (Silistor) | Zigzag line with diagonal arrow and +t°, sometimes with a linear 'L' notation |
Rectangle box with diagonal arrow, +t°, and a straight line segment inside |
+t° + linear indicator |
Temperature compensation in oscillator circuits |
| Varistor (VDR/MOV) (Often confused) | Zigzag line with a diagonal arrow, marked with a voltage curve or U |
Rectangle box with a diagonal arrow passing completely through, marked U or V |
U, V, or non-linear voltage curve |
Transient voltage suppression (surge protection) |
Decoding the Variants: Symbols and Rows People Get Wrong
When reading schematics, engineers and hobbyists frequently misinterpret thermistor symbols due to regional drafting differences and subtle notation overlaps. Here is how to accurately interpret the rows and variants that cause the most confusion on the bench.
The IEEE Zigzag vs. IEC Rectangle
If you are reading a schematic drafted in the US or using older legacy CAD libraries, the base resistor shape will be a zigzag line (IEEE 315 standard). If you are reading a modern European or international design, the base shape is a solid rectangle (IEC 60617 standard). The temperature coefficient is indicated by the arrow and the text next to it: -t° means resistance drops as temperature rises (NTC), while +t° means resistance increases with heat (PTC).
The Varistor Trap
The most common mistake is confusing a PTC thermistor symbol with a Metal Oxide Varistor (MOV). Both feature a resistor base with a diagonal arrow. However, a varistor's arrow typically passes completely through the resistor body or is accompanied by a voltage-dependent notation like U or a non-linear V-I curve symbol. A thermistor's arrow usually breaks at the resistor body or is explicitly tagged with a temperature degree symbol (t°). If you install a 10kΩ NTC thermistor where a 275V AC MOV belongs, the circuit will fail catastrophically upon energization.
If a PCB silkscreen is burned off or faded, never assume a two-legged, disc-shaped component is a standard ceramic capacitor or a fixed resistor. Epoxied NTC thermistors (like the ubiquitous EPCOS B57891M0103K000) look nearly identical to ceramic capacitors. Always verify with a multimeter before applying power or replacing the component.
Practical Bench Identification When Schematics and Silkscreens Fail
Schematics are rarely available when repairing consumer electronics or reverse-engineering legacy control boards. When the thermistor schematic symbol is missing from your documentation and the PCB silkscreen is illegible, you must rely on empirical bench testing to identify the component's type and value.
Step 1: Isolate the Component
Never measure a thermistor while it is fully soldered into a live or complex circuit. Parallel resistance paths from surrounding components will skew your multimeter readings, making a 10kΩ thermistor look like a 2.2kΩ component. Desolder at least one leg of the thermistor to lift it from the PCB pad.
Step 2: Establish the Baseline (25°C)
Set your multimeter to resistance (Ohms) mode. Measure the component at standard room temperature (25°C / 77°F).
- NTC Thermistors: Typically read standard values like 10kΩ, 47kΩ, or 100kΩ. (e.g., The Murata NCP18XH103F03RB reads exactly 10kΩ at 25°C).
- PTC Thermistors (Resettable Fuses): Usually read very low, near 0.1Ω to 2.0Ω.
- Fixed Resistors: Will read their color-coded or SMD-marked value and will not change with temperature.
Step 3: The Heat Gun Test
Apply gentle heat using a hair dryer or a heat gun on a low setting (keep it above 60°C to avoid melting the epoxy coating). Watch the multimeter display:
- If the resistance drops rapidly (e.g., from 10kΩ down to 1.2kΩ), it is an NTC thermistor.
- If the resistance spikes dramatically or goes open-circuit (OL), it is a PTC thermistor.
- If the resistance remains stable (changing by less than 5%), it is a standard fixed resistor or a wirewound inductor.
Thermistor Symbol and Identification FAQ
How do I tell an NTC from a PTC thermistor symbol on an old schematic?
Look at the text annotation next to the diagonal arrow crossing the resistor symbol. If it says -t° (negative temperature), it is an NTC thermistor, meaning its resistance falls as it gets hotter. If it says +t° (positive temperature), it is a PTC thermistor, meaning its resistance rises with heat. On very old, hand-drawn schematics where the degree symbol is missing, an arrow pointing downward toward the resistor generally implies NTC, while an arrow pointing upward implies PTC.
What does the diagonal line and arrow mean on a thermistor symbol?
The diagonal line represents the non-linear relationship between the component's resistance and an external variable. In the case of a thermistor, the arrow specifically denotes temperature dependence. The line breaking through the resistor body indicates that the component's primary operational characteristic is variable, rather than fixed. When the arrow is accompanied by a voltage symbol (U or V), it shifts the meaning from a temperature-dependent resistor (thermistor) to a voltage-dependent resistor (varistor/MOV).
Is a thermistor symbol the same as a varistor (VDR) symbol?
No, though they share a similar visual lineage in CAD libraries. A thermistor symbol will always feature a temperature indicator (t°, -t°, or +t°). A varistor (Voltage Dependent Resistor) symbol will feature a voltage indicator (U or V) or a small non-linear curve drawn next to the arrow. Furthermore, in IEC standards, the varistor arrow often passes completely outside and across the entire rectangle, whereas the thermistor arrow typically originates from the center or breaks at the boundary of the rectangle.
How can I identify a thermistor on a PCB if the silkscreen is burned off?
First, visually inspect the component. Thermistors used for temperature sensing are often small, epoxy-coated beads with thin wire leads, while inrush-limiting NTCs are larger, black or green epoxy discs. If the silkscreen is gone, desolder one leg and measure the resistance at room temperature (25°C). If it reads a standard thermistor value (10kΩ, 47kΩ, 100kΩ) and the resistance drops significantly when you apply heat from a hair dryer, you have confirmed it is an NTC thermistor. You can then source a replacement based on that 25°C baseline resistance and the physical diameter of the disc.
