Thermistor Schematic Symbol Reference Table
Modern EDA tools like Altium Designer and KiCad 8/9 allow you to toggle between regional symbol libraries. Use this table to decode the symbol for a thermistor regardless of which standard your schematic follows.
| Component Type | IEEE 315 / ANSI (North America) | IEC 60617 (International / EU) | Practical Meaning & Common Use Cases |
|---|---|---|---|
| Generic Thermistor | Zig-zag line crossed diagonally by a straight line ending in t° | Rectangle crossed diagonally by a straight line ending in t° | Used when the specific temperature coefficient is defined in the BOM or text notes rather than the symbol itself. |
| NTC Thermistor | Zig-zag line crossed diagonally, ending in -t° (minus sign indicates inverse relationship) | Rectangle crossed diagonally, ending in -t° | Resistance drops as temperature rises. Used for inrush current limiting (e.g., Ametherm SL32) or precision temperature sensing (e.g., Vishay NTCLE100 10kΩ). |
| PTC Thermistor | Zig-zag line crossed diagonally, ending in +t° (plus sign indicates direct relationship) | Rectangle crossed diagonally, ending in +t° | Resistance spikes as temperature rises. Used for self-regulating heaters, overcurrent protection (resettable fuses), and motor start windings. |
| Linearized Thermistor | NTC symbol drawn in parallel with a standard fixed resistor symbol | NTC symbol drawn in parallel with a standard fixed resistor symbol | An NTC thermistor paired with a parallel resistor to flatten the resistance-to-temperature curve over a specific narrow operating range. |
Regional Standards and "Rows People Get Wrong"
While the IEEE 315 standard dominates US-based legacy schematics and military/aerospace documentation, the IEC 60617 standard is the default for modern European and international consumer electronics. The core difference is purely visual: IEEE uses the jagged zig-zag for resistors, while IEC uses a clean rectangular box. The diagonal line and the t° modifier remain identical across both.
Rows People Get Wrong
- Confusing NTC with a Variable Resistor (Potentiometer): This is the most common bench mistake. A variable resistor symbol features an arrow pointing into the middle of the resistor body, indicating a mechanical wiper. An NTC thermistor symbol features a diagonal line that crosses completely through the resistor body, ending in -t°. If there is no "t", it is not a thermistor.
- Misidentifying PTC Thermistors as Standard Fuses: A PTC used for overcurrent protection (often called a polyfuse or resettable fuse, like the Bourns MF-R series) uses the +t° symbol. However, many CAD libraries mistakenly map PTCs to the standard fuse symbol (a rectangle with a solid line through the center). Always check the BOM part number; if it is a resettable polymer PTC, it should carry the thermistor modifier.
- RTD vs. Thermistor: Resistance Temperature Detectors (RTDs, like PT100 sensors) are technically resistive temperature sensors, but they are not thermistors. RTDs have a highly linear, positive temperature coefficient based on pure metals (platinum, nickel). Schematics will often label an RTD with the text "RTD" or "PT100" next to a standard resistor symbol, rather than using the +t° thermistor modifier.
Practical Schematic Interpretation and Circuit Context
Knowing the symbol for a thermistor is only half the battle; understanding its circuit context tells you how it will behave on the bench. Thermistors are highly non-linear. An NTC thermistor's resistance is defined by its R25 value (resistance at 25°C) and its B-value (the material constant, typically between 3000K and 4500K).
For example, a Vishay NTCLE100E3103 is a 10kΩ NTC with a B-value of 3977K. At 25°C, it reads 10,000Ω. At 100°C, its resistance drops to roughly 670Ω. When you see the NTC symbol in a voltage divider feeding an ESP32 or Arduino ADC pin, it is acting as a sensor. When you see the exact same NTC symbol placed in series with the AC mains line ahead of a bridge rectifier, it is acting as an inrush current limiter (ICL), absorbing the initial capacitor charging surge and then heating up to drop its resistance out of the way.
On high-density power supply boards, the silkscreen designator (e.g., "TH1" or "RT1") and the symbol printed on the PCB often burn off or are obscured by conformal coating. To safely identify an unmarked component:
- Visual ID: Look for a glass-encapsulated axial bead (resembling a small diode) or an epoxy-coated radial teardrop. They are almost always located near heat sinks, power MOSFETs, or transformer primaries.
- Thermal Sweep Test: Set your multimeter to resistance mode. Probe the component. Apply localized heat using a heat gun set to a low 100°C, or carefully warm it with your finger. If the resistance drops smoothly, it is an NTC. If it spikes dramatically, it is a PTC.
- Safety Caveat: Never use a high-temperature soldering iron tip directly on the component body to test it. You risk cracking the glass envelope, delaminating the epoxy, or reflowing adjacent SMD solder joints, which can cause latent shorts.
Frequently Asked Questions
What is the exact symbol for a thermistor on a schematic?
The exact symbol is a standard resistor symbol (zig-zag for IEEE/ANSI, rectangle for IEC) intersected by a diagonal line. The diagonal line must feature a temperature modifier at its end: t° for a generic thermistor, -t° for an NTC thermistor, and +t° for a PTC thermistor. Without the "t" modifier, the symbol represents a variable resistor or potentiometer, not a temperature-dependent component.
How do I tell an NTC symbol from a PTC symbol?
Look at the mathematical sign preceding the "t°" modifier. The NTC symbol features a minus sign (-t°), indicating that resistance decreases (negative coefficient) as temperature increases. The PTC symbol features a plus sign (+t°), indicating that resistance increases (positive coefficient) as temperature rises. In older or hand-drawn schematics, the arrow on the diagonal line may point downward for NTC and upward for PTC, but the +/- text modifier is the modern standard.
Does the symbol for a thermistor indicate its B-value or resistance?
No. The schematic symbol only indicates the type of temperature coefficient (NTC, PTC, or generic). The specific R25 resistance (e.g., 10kΩ, 100kΩ) and the B-value (e.g., 3950K) are never embedded in the symbol itself. These critical parameters are always listed in the component's reference designator value field, the Bill of Materials (BOM), or an adjacent text note on the schematic sheet.
Why does my CAD software show a different thermistor symbol than the datasheet?
This happens because component datasheets often use the IEC 60617 rectangular symbol (as most manufacturers are based in Europe or Asia), while your local CAD library (like Altium or Eagle in the US) defaults to the IEEE 315 zig-zag symbol. Both are electrically and functionally identical. To resolve visual confusion on a team, standardize your EDA workspace to a single library standard (preferably IEC 60617 for modern global designs) and disable mixed-standard symbol generation.
