Temperature monitoring and control remain among the most critical functions across virtually every electronic device and system in today's world. From medical equipment and automotive systems to household appliances and industrial machinery, the need for accurate, reliable,and cost-effective temperature measurement has never been greater. At the heart of many of these systems lies a seemingly simple yet remarkably versatile component: the NTC thermistor.
This comprehensive technical article explores the fundamental principles, key specifications, and diverse applications of Negative Temperature Coefficient (NTC) thermistors, with a particular focus on Semitec Corporation's advanced offerings. By understanding the physics, specifications, and implementation considerations ofthese components, engineers can make informed decisions that optimize performance, reliability, and cost-effectiveness in their thermal management designs.
NTC thermistors are specialized resistors whose electrical resistance decreases predictably with increasing temperature.The term "thermistor" itself is derived from "thermallysensitive resistor," highlighting their fundamental property. While thermistors come in two variants — Positive Temperature Coefficient (PTC) and Negative Temperature Coefficient (NTC)—this article focuses on NTC types, which constitute the majority of applications in temperature sensing.
NTC thermistors are ceramic semiconductors manufactured by sintering metal oxide compounds at temperatures between 1000°C and 1400°C. The precise composition and manufacturing process directly influence the thermistor's characteristics, allowing for customization to specific applications.
Figure 1: NTCThermistor Microstructure. Cross-sectional view showing the semiconductor grain structure that enables temperature-dependent resistance characteristics. As temperature increases, more electrons gain energy to cross grain boundaries, resulting indecreased electrical resistance.
The negative temperature coefficient behavior stems from the semiconductor properties of the metal oxide materialsused in NTC thermistors. Unlike metals (which increase in resistance with temperature due to increased electron scattering), semiconductors typically decrease in resistance with increasing temperature.
This occurs because higher temperatures provide more thermal energy to electrons in the semiconductor material,enabling them to overcome the energy barrier (band gap) and move from the valence band to the conduction band. With more charge carriers available for conduction, the electrical resistance decreases.
The relationship between resistance and temperature follows an exponential curve, which can be approximated using the Steinhart-Hart equation or the simplified Beta (β) equation. For most practical applications, Semitec utilizes the Beta equation: R_1 =R_2.exp[B(1/T_1 -1/T_2 )]
Where:
This non-linear relationship between resistance and temperature makes NTC thermistors exceptionally sensitive to temperature changes, particularly in the steep portion of their resistance-temperature curve. This high sensitivity is one of the key advantagesof NTC thermistors over other temperature sensing technologies.
Understanding NTC thermistor specificationsis crucial for selecting the right component for a specific application. Let'sexplore the most important parameters:
Zero-power resistance, often designated asR₂₅, refers to the thermistor's resistance at a reference temperature(typically 25°C) when measured with electrical power low enough to preventself-heating. This value serves as the baseline for resistance calculations atother temperatures.
Semitec offers a wide range of resistancevalues, from 500Ω to 1MΩ, with the most common values being 10kΩ (103ATseries), 50kΩ (503AT series), and 100kΩ (104AT series). The resistance value is typically specified with a tolerance, such as ±1%, ±2%, or ±3%.
The B value, sometimes called the Betaconstant, characterizes how rapidly the resistance changes with temperature. Itis calculated using the resistance values at two temperature points: B=(ln(R_1 )-ln(R_2))/(1/T_1 -1/T_2 )
Standard B values for Semitec thermistorstypically range from 3000K to 5000K. The B value is not constant across alltemperatures but varies slightly. For high-precision applications, Semitecspecifies the B value between specific temperature points, such as B₂₅/₈₅(between 25°C and 85°C) or B₀/₅₀ (between 0°C and 50°C).
Common B values for Semitec products include:
A higher B value indicates a more rapid change in resistance with temperature, resulting in higher sensitivity but over a narrower useful temperature range.
The operating temperature range defines the minimum and maximum temperatures within which the thermistor will function reliably. Semitec's product lineup covers a wide spectrum:
For example, the Semitec 103NT-4glass-encapsulated thermistor operates reliably from -50°C to +300°C, making itsuitable for extreme high-temperature environments.
The dissipation factor (δ), measured inmW/°C, indicates the power required to raise the thermistor's temperature by1°C through self-heating in a specified environment. This parameter is crucial for applications where the thermistor might be subjected to significant currentflow.
Typical values for Semitec products rangefrom 0.5 to 10 mW/°C, with the micro-thin film FT series having lower values (approximately 0.3 mW/°C) and the larger AT series having higher values(approximately 3 mW/°C).
The thermal time constant represents the responsiveness of the thermistor to temperature changes. Technically, it's thetime required for the thermistor to change 63.2% of the total difference between its initial and final body temperatures when subjected to a step change in temperature.
Semitec's product range offers various response times:
The time constant is strongly influenced bythe thermistor's size, encapsulation, and mounting method. For applications requiring rapid response, the miniature Semitec Fμ sensor with its 0.5 mm diameter is ideal.
Figure 2: NTC Thermistor Response Time Comparison.Response time comparison showing how quickly each Semitec thermistor seriesresponds to a 60°C temperature step change. Note the dramatic differences: Fμsensor (0.07s), FT series (1.0s), and AT series (15.0s).
Choosing the appropriate NTC thermistor involves balancing various factors including accuracy requirements, environment conditions, space constraints, and cost considerations. Here we present asystematic approach to thermistor selection:
First, identify the minimum and maximum temperatures your application will encounter. This will immediately narrow your options to thermistors capable of operating within this range.
For example:
The required measurement accuracy will influence your choice of thermistor tolerance and linearization approach:
If your application needs to detect rapid temperature changes, the thermal time constant becomes critical:
The thermistor must withstand the environmental conditions of the application:
Space constraints and mounting methods also impact selection:
Finally, consider how the thermistor willintegrate with your circuit:
Figure 3: Application-Specific Thermistor Selection Guide. This decision matrix helps engineers select the optimal Semitec thermistor based on application domain, key requirements, and environmental considerations.
Even the best thermistor will underperformif implemented incorrectly. Here are key considerations for optimizingthermistor performance in your designs:
The non-linear response of NTC thermistorsoften requires linearization for accurate temperature measurement across a widerange:
1/T=A+B ln(R)+C(〖ln(R))〗^3
The coefficients A, B, and C can becalculated from three calibration points.
Self-heating occurs when the measurement current causes the thermistor to warm above ambient temperature, introducing measurement errors:
Temperature measurement circuits can besensitive to electrical noise:
As technology advances, NTC thermistorsfind new applications and implementation methods:
IoT applications demand small, low-power sensors with digital interfaces. Semitec's miniature thermistors (FT and KT series) are ideal for these applications when paired with low-power ADCs. Key design considerations include:
Modern systems often require temperature monitoring at multiple points simultaneously. Semitec offers matched sets and array solutions:
Beyond direct temperature measurement, NTCthermistors excel at compensating for the temperature dependence of othercomponents:
Ultra-low power systems that harvest energyfrom the environment benefit from:
NTC thermistors remain among the most versatile, cost-effective, and reliable temperature sensing technologies available today. The wide range of specialized products from Semitec—from theultra-miniature Fμ sensors to the high-temperature NT series—provides solutions for virtually any temperature monitoring challenge.
By understanding the fundamental principles,key specifications, and implementation best practices outlined in this article, engineers can select and apply the optimal thermistor for their specific application. Whether designing medical devices requiring precise accuracy,automotive systems facing extreme conditions, or consumer electronics balancing performance and cost, there's a Semitec thermistor engineered for the task.
As we have seen, the seemingly simple NTC thermistor is in fact a sophisticated component whose performance depends on material science, manufacturing precision, and thoughtful application. The depth of Semitec's product lineup reflects decades of refinement and specialization to address the evolving needs of temperature measurement across industries.
For assistance selecting the optimal thermistor for your specific application, contact our technical solutions team and we will help you navigate the selection process and optimize your temperature sensing design.
Email: contact@sagacomponents.com
Phone: +46 (0) 8 564 708 00
Web: https://www.semitec-global.com/products/
SEMITEC Part Numbers Quick Reference
103AT-2, 503AT-2, 103FT1005A5P1, 103FT1005A5P, 103FT1005B5P, 103FT1005D5P, 503FT1005A5P, 503FT1005B5P, 364FT1005A5P, 364FT0603A5P, 223Fµ, 223Fµ5183, 223Fµ3122, 103JT, 104JT, 202AP-2, 232AP-2, 502AP-2, 103AP-2, 103AP-2-A, 203AP-2, 503AP-2, 103AT-4-70378, 104AP-2, 204AP-2, 103AT, S103AT-11, 102AT-11, 202AT-11, 502AT-11, 103AT-11, 203AT-11, 102AT-2, 202AT-2, 103AT-3, 203AT-2, 103AT-4, 103AT-5, 103ET, 212ET, 402ET, 582ET, 203ET, 303ET, 403ET, 503ET, 833ET, 104ET, 224ET, 234ET, 103ETB, 503ET-3H, 103NT-4, 502NT-4-R025H39G, 852NT-4-R050H34G, 103NT-4-R025H34G, 103NT-4-R025H41G, 203NT-4-R025H42G, 493NT-4-R100H40G, 503NT-4-R025H42G, 104NT-4-R025H42G, 104NT-4-R025H43G, 204NT-4-R025H43G, 234NT-4-R200H42G, 504NT-4-R025H45G, 105NT-4-R025H46G, 103KT1608T, 503KT1608T, 104KT1608T, 103KT1005T, 103CT, 252CT-4, 512CT-4, 562CT-4, 912CT-4, 103CT-4, 113CT-4, 203CT-4, 473CT-4, 513CT-4, 563CT-4, 104CT-4, 204CT-4, 103CT-01006, 103CT-21048, 503CT-91027, 104CT-90113, S-101T, E-101, S-301T, E-301, S-501T, E-501, S-701T, E-701, S-102T, E-102, S-152T, E-152, S-202T, E-202, S-272T, E-272, S-352T, E-352, S-452T, E-452, S-562T, E-562, S-822T, E-822, S-103T, E-103, S-123T, E-123, S-153T, E-153, S-183T, E-183, S-223T, S-101, S-301, S-501, S-701, S-102, S-152, S-202, S-272, S-352, S-452, S-562, S-822, S-103, S-123, S-153, S-183, 104ET-1, S-223, Z2012, Z2012U, Z2015, Z2015U, Z2018, Z2018U, Z2022, Z2022U, Z2027, Z2027U, Z2033, Z2033U, Z2039, Z2039U, Z2047, Z2047U, Z2056, Z2056U, Z2068, Z2068U, Z2082, Z2082U, Z2100, Z2100U, Z2120, Z2120U, Z2150, Z2150U, Z2180, Z2180U, Z6012, Z6012U, Z6015, Z6015U, Z6018, Z6018U, Z6022, Z6022U, Z6027, Z6027U, Z6033, Z6033U, Z6039, Z6039U, Z6047, Z6047U, Z6056, Z6056U, Z6068, Z6068U, Z6082, Z6082U, Z6100, Z6100U, Z6120, Z6120U, Z6150U, ZD015, ZD018, ZD022, ZD027, ZD033, ZD039, ZD047, ZD056, ZD068, ZS1012, ZS1015, ZS1018, ZS1022, ZS1027, ZS1033, ZS1039, ZS1047, ZS1012U, ZS1015U, ZS1018U, ZS1022U, ZS1027U, ZS1033U, ZS1039U, ZS1047U, ZS1012D, ZS1015D, ZS1018D, ZS1022D, ZS1027D, ZS1033D, ZS1039D, ZS1047D, 5D2-05, 10D2-05, 20D2-05, 5D2-07, 8D2-07, 10D2-07, 12D2-07, 16D2-07, 22D2-07, 5D2-08, 10D2-08, 15D2-08, 20D2-08, 2D2-10, 3D2-10, 5D2-10, 8D2-10, 10D2-10, 12D2-10, 16D2-10, 2D2-11, 3D2-11, 4D2-11, 5D2-11, 8D2-11, 10D2-11, 12D2-11, 15D2-11, 16D2-11, 20D2-11, 1D2-13, 2D2-13, 4D2-13, 4.7D2-13, 5D2-13, 8D2-13, 10D2-13, 12D2-13, 15D2-13, 16D2-13, 2D2-14, 3D2-14, 4D2-14, 5D2-14, 8D2-14, 10D2-14, 12D2-14, 16D2-14, 1D2-15, 1.5D2-15, 2D2-15, 3D2-15, 4D2-15, 4.7D2-15, 5D2-15, 8D2-15, 10D2-15, 12D2-15, 15D2-15, 16D2-15, 4D2-18, 5D2-18, 8D2-18, 10D2-18, 47D2-18, 1D2-22, 3D2-22, 4D2-22, 6D2-22, 10TP583T, 502AT-2, 103AT-4-10228, 103AT-4-70261, 103AT-5-1P-FT, 303ET-1, 402ET-1