The standard schematic symbol for a thermistor is a base resistor shape crossed by a diagonal line with a horizontal bar at the bottom, annotated with -t° for Negative Temperature Coefficient (NTC) or +t° for Positive Temperature Coefficient (PTC). While the IEC 60617 standard uses a rectangle for the resistor base, the IEEE/ANSI standard uses a zigzag line. Below is the complete reference table to identify these symbols, decode physical markings, and select the exact replacement part for your circuit.

Complete Thermistor Symbol & Marking Reference

Use this spec-sheet-table to match the schematic symbol for a thermistor on your blueprint to its physical component and standard. This table assumes standard through-hole and SMD epoxy/ceramic packages.

Symbol Type Standard Base Shape Annotation Physical Appearance Common Use Case
NTC Thermistor IEC 60617 Rectangle Diagonal line, horizontal bar, -t° Black epoxy bead or disc, 2 wire leads Temperature sensing, inrush current limiting
NTC Thermistor IEEE 315 Zigzag Diagonal line, horizontal bar, -t° Black epoxy bead or disc, 2 wire leads Temperature sensing, inrush current limiting
PTC Thermistor IEC 60617 Rectangle Diagonal line, horizontal bar, +t° Orange/blue ceramic disc or green SMD Over-temperature protection, motor winding protection
PTC Thermistor IEEE 315 Zigzag Diagonal line, horizontal bar, +t° Orange/blue ceramic disc or green SMD Over-temperature protection, motor winding protection
Linear PTC (Silistor) IEC / IEEE Rect / Zigzag Diagonal line, horizontal bar, t° (no sign) Cylindrical, often resembles a standard 1/4W resistor Linear temperature compensation in oscillator circuits

Regional Standards: IEC 60617 vs. IEEE 315

When reading schematics, the region where the board was designed dictates the base shape of the symbol. According to the All About Circuits Schematic Symbols Guide, understanding these regional differences prevents catastrophic misidentifications.

Pro-Tip: The diagonal line crossing the base symbol represents the non-linear relationship between resistance and temperature. The horizontal bar at the bottom of the diagonal line indicates that the component is a specific, manufactured semiconductor/ceramic device, not just a theoretical variable resistor.
  • IEC 60617 (Europe, Asia, Global Standard): Uses a solid rectangle as the base resistor symbol. You will see this on schematics from European automotive manufacturers, Asian consumer electronics, and modern global design houses.
  • IEEE 315 / ANSI Y32.2 (North America): Uses a zigzag line as the base resistor symbol. This is prevalent in older US military schematics, legacy American industrial equipment, and textbooks published in the US.

Regardless of the base shape, the modifier (the diagonal line and the -t° or +t° text) remains identical across both standards. The minus sign (-t°) means resistance decreases as temperature rises (NTC). The plus sign (+t°) means resistance increases as temperature rises (PTC).

Rows People Get Wrong (And How to Fix Them)

Misinterpreting the schematic symbol for a thermistor usually happens when designers use shorthand or when technicians confuse physical appearances. Here are the most common errors:

  1. Confusing PTC Thermistors with Resettable Fuses: In power supply schematics, a PTC symbol is frequently used to represent a polymeric PTC (PPTC) resettable fuse (like a Littelfuse PolySwitch). While technically a PTC thermistor, its purpose is overcurrent protection, not temperature measurement. If the symbol is placed in series with a voltage rail input, it is a fuse. If it is placed in a voltage divider network feeding an ADC pin, it is a temperature sensor.
  2. Confusing NTCs with RTDs: Resistance Temperature Detectors (like PT100 or PT1000) are highly linear platinum sensors. Schematics rarely use the standard thermistor symbol for RTDs; instead, they use a standard resistor symbol explicitly labeled 'RTD' or 'PT100'. Do not substitute a 10k NTC for a PT1000—the microcontroller code and bias currents are entirely different.
  3. Misreading SMD NTCs as Resistors: 0603 and 0805 SMD NTC thermistors often have no top marking at all (they are usually blank black or dark green). If you find an unmarked, dark-colored SMD component in a temperature-sensing trace, it is almost certainly an NTC thermistor, not a faded resistor.

Safe Interpretation for Faded or Unmarked SMD Components

When repairing a board where the schematic symbol for a thermistor is present, but the physical component's markings are burned off or missing, you must empirically determine its type and nominal value before ordering a replacement.

Safety Warning: Always de-energize the board and discharge filter capacitors before probing components. Applying external heat to a live board can cause thermal runaway in damaged semiconductors.

The Heat-Gun Identification Test:

  1. Set your multimeter to resistance (Ω) mode and probe the two terminals of the suspect component.
  2. Record the baseline resistance at room temperature (approx. 25°C). Common baseline values are 10kΩ, 47kΩ, or 100kΩ.
  3. Apply mild heat using a hair dryer or a heat gun on its lowest setting (keep it moving to avoid melting solder joints).
  4. If the resistance drops significantly (e.g., from 10kΩ down to 2kΩ), it is an NTC thermistor.
  5. If the resistance spikes sharply (e.g., from 50Ω up to >10kΩ), it is a Ceramic PTC thermistor acting as a temperature switch.

Decision Path: Selecting Your Exact Thermistor Part

Use this decision-tree-table to terminate your search and pick a concrete, readily available part number based on your circuit's requirements.

Circuit Requirement Component Category Concrete Default Pick (2026) Key Specs
Accurate temperature measurement (0°C to 100°C) for a microcontroller ADC NTC Glass/Epoxy Bead Vishay NTCLE100E3103 10kΩ at 25°C, 1% tolerance, Beta 3977K
Inrush current limiting for a 120V AC / 5A switched-mode power supply NTC Power Disc Ametherm SL32 2R015 2Ω cold, 15A steady state, 32mm diameter
Overcurrent protection for a 5V USB-C data/power line Polymeric PTC (Resettable Fuse) Littelfuse 1206L050WR 500mA hold, 1206 SMD package, 6V max
Over-temperature shutoff embedded in a transformer or motor winding Ceramic PTC Sensor EPCOS B59950 Switching temp 130°C, 2.5Ω nominal at 25°C

Worked Example: Calculating NTC Resistance at 85°C

When designing the voltage divider for your microcontroller, you need to know the exact resistance of your thermistor at your target operating temperature. We will use the Beta (β) parameter equation, which is standard for most hobbyist and commercial NTC applications. For deeper precision across wide temperature ranges, refer to the Ametherm NTC Beta Value Calculator and Steinhart-Hart coefficients.

Scenario: You are using the Vishay NTCLE100E3103 (10kΩ nominal at 25°C, Beta = 3977K). What is its resistance at 85°C?

The Beta Equation:
RT = R25 × e^[ β × (1/T - 1/T25) ]

Step 1: Convert temperatures to Kelvin.

  • T (85°C) = 85 + 273.15 = 358.15 K
  • T25 (25°C) = 25 + 273.15 = 298.15 K

Step 2: Calculate the inverse temperature difference.

  • (1 / 358.15) - (1 / 298.15) = 0.0027921 - 0.0033540 = -0.0005619

Step 3: Multiply by Beta and solve the exponent.

  • 3977 × -0.0005619 = -2.2347
  • e^(-2.2347) = 0.1070

Step 4: Multiply by nominal resistance.

  • RT = 10,000 Ω × 0.1070 = 1,070 Ω (or 1.07 kΩ)

At 85°C, your 10kΩ NTC thermistor will read approximately 1.07 kΩ. Use this exact value to calculate your series bias resistor and ADC voltage thresholds in firmware.