Beta Value
The Beta value characterizes the resistance-temperature relationship of a thermistor. It is calculated using the formula:
βT2/T1=(1/T2–1/T1)ln(RT2/RT1)
Where temperature is in degrees Kelvin; RT1R_{T1}RT1 is the resistance at temperature T1T1T1; and RT2R_{T2}RT2 is the resistance at temperature T2T2T2.
Steinhart-Hart Equation: The Steinhart-Hart Equation is an empirically developed polynomial that accurately represents the resistance-temperature relationships of NTC thermistors over wide temperature ranges. To solve for temperature when resistance is known:
1T=a+b(lnR)+c(lnR)3\frac{1}{T} = a + b(\ln R) + c(\ln R)^ 3T1=a+b(lnR)+c(lnR)3
To solve for resistance when temperature is known:
R=e[(−α2+(α24+β327))1/3+(−α2−(α24+β327))1/3]R =
e^{\left[\left(\frac{-\alpha}{2} + \sqrt{\left(\frac{\alpha^2}{4} + \frac{\beta^3}{27}\right)}\right)^{1/3} +\left(\frac{-\alpha}{2}-\sqrt{\left(\frac{\alpha^2}{4}+\frac{\beta^3}{27}\right)}\right)^{1/3}\right]}R=e[(2−α+(4α2+27β3))1/3+(2−α−(4α2+27β3))1/3]
Where alpha (α)=(a−1/Tc)(\alpha) = \left(\frac{a-1/T}{c}\right)(α)=(ca−1/T) and beta (β)=bc(\beta) = \frac{b}{c}(β)=cb.
Maximum Temperature Rating / Recommended Operating Ranges: Thermistors can occasionally cycle at temperatures ranging from -50°C to 150°C. The standard storage temperature for optimal stability is 50 °C, and it is continuously operated below 100 °C. Stability is greatly increased when thermistors are used within their specified interchangeable temperature range.
Stability: Our extensive experience in thermistor manufacturing, combined with stringent process controls, ensures the production of highly stable thermistors. Typically, our thermistors exhibit less than 0.02°C thermometric drift per year when stored or operated at temperatures below 50°C. Stability can be affected by environmental factors such as humidity, excessive temperatures, and thermal shock, which should be minimized to maintain optimal performance.
Voltage/Current Requirements
Thermistors used in temperature measurement, control, or compensation applications require low current, typically considered less than 100 mA, to avoid self-heating. The power dissipation should be less than 0.2 mW; this reduces thermal error and keeps the thermal error below 0.1 °C.
Self-heating is considered to be beneficial in some applications, such as airflow measurement and liquid level control. Standard epoxy or phenolic-coated thermistors with a 0.095 O.D. have high power ratings of 30 milliwatts at 25°C and 1 milliwatt at 100°C.
Dissipation Constant
The Dissipation Constant is the power required to raise the temperature of the thermistor by 1°C more than its surrounding environment, expressed in watts. For a thermistor with a 0.095” O.D. coated with epoxy or phenolic, the typical dissipation constant is 13 mW/°C in stirred oil and 2 mW/°C in still air.
Resistance-Temperature(R/T) Curves and Negative Temperature Coefficient
NTC Thermistors are identified by their resistance-temperature (R/T) curves. Nine different types of material are used, and each of them has unique and predictable R/T characteristics. Thermistors are generally distinguished by their R/T curve and their resistance value at 25 °C.
The Negative Temperature Coefficient (NTC) represents the percentage resistance change per degree Celsius. Our thermistors have NTC values of 25°C ranging from –3.9%/°C to –6.4%/°C, with resistance values ranging from 300 ohms to 40 megohms.
Thermistor Definition
Thermistor is derived from “Thermal Sensitive Resistor”. Thermistors are passive semiconductors that produce resistance values dependent on temperature. An NTC or Negative Temperature Coefficient Thermistor decreases in resistance as its temperature increases and shows predictable and significant resistance changes per degree of temperature change, which makes them critically important in various applications.
Manufacturing Process
Thermistor manufacturing is a two-step process involving chip production and assembly. First, metal oxide powders are processed into ceramic sheets to create chips, then these sheets are coated with silver for electrical contact and then diced into individual chips. Each chip undergoes rigorous testing to ensure quality standards. After chip production leads are attached, the chip is trimmed to match exactly with the specified tolerances, and a protective coating is applied. Housing, cables, and connectors can all be customized and performed as per the requirements. Quality assurance is maintained through in-process inspections and Statistical Process Control (SPC) at every manufacturing and assembly step.
The finished products are 100% tested both electrically and mechanically to meet all the specified requirements.