* Question
What is the terminology of thermistors?
* Answer
The terminology of thermistors refers to the key terms and parameters used to describe and characterize these temperature-sensitive resistors. Thermistors are widely used in temperature sensing, compensation, and control circuits. Below is a comprehensive list of commonly used terminology relevant to thermistors, particularly NTC (Negative Temperature Coefficient) and PTC (Positive Temperature Coefficient) types.
1. Thermistor
A thermistor is a type of resistor whose resistance varies significantly with temperature. There are two main types:
NTC (Negative Temperature Coefficient): Resistance decreases as temperature increases.
PTC (Positive Temperature Coefficient): Resistance increases as temperature increases.
2. Key Terminology
Term | Definition |
R₀ (Nominal Resistance) | Resistance of the thermistor at a reference temperature (usually 25 °C). |
TCR (Temperature Coefficient of Resistance) | Rate of change of resistance per °C, typically in %/°C or ppm/°C. |
B-Value (Beta Value) | A constant that describes the exponential relationship between resistance and temperature. |
ΔR/R (Resistance Change Ratio) | The relative change in resistance due to temperature change. |
Operating Temperature Range | The range over which the thermistor functions reliably. |
Self-Heating Effect | Temperature rise due to power dissipation in the thermistor itself. |
Thermal Time Constant (τ) | Time required for the thermistor to reach 63.2% of a total temperature change. |
Dissipation Constant (δ) | Power required to raise the thermistor’s temperature by 1 °C. |
Maximum Operating Current | The highest current the thermistor can handle without degradation. |
Failure Rate / Reliability | Often expressed in terms of MTBF (mean time between failures). |
Interchangeability | Ability of thermistors to be swapped without recalibration due to consistent characteristics. |
3. Electrical Behavior
For NTC thermistors, the resistance–temperature relationship follows:
R(T)=R0⋅eB(1/T−1/T0)
Where:
R(T) is resistance at temperature T (in Kelvin)
R0 is nominal resistance at reference temperature T0
B is the beta constant
4. Application-Specific Terms
Application | Key Related Terms |
Temperature sensing | Accuracy, linearization, stability |
Inrush current limiting | Holding current, trip current, I²t |
Temperature compensation | Matching curve, offset voltage |
HVAC & appliances | Response time, hysteresis |
5. Units Commonly Used
Resistance: Ohms (Ω)
Temperature: Celsius (°C) or Kelvin (K)
B-Value: Kelvin (K)
Time Constant: Seconds (s)
Power Dissipation: Milliwatts (mW) or Watts (W)
In summary, understanding thermistor terminology is essential for selecting the right component for sensing, protection, or control applications.
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