AN-TC11 WAVELENGTH | Alldatasheet
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Technical content
thermistor is used the most. temperature. This range is dependent on the base resistance. Figure 1. Thermistor Symbols very precise temperatures are required.
the resistance of a thermistor changes with temperature. material used in the thermistor. Figure 2. Resistance vs. Temperature Graph data will be covered later in this paper. Table 1. Sensor Comparison
- Long lasting
- Highly sensitive
- Small size
- Lowest cost
- Best for measuring single point temp
- Best response time
- Linear output
- Widest operating temp range
- Best for measuring a range of temp
- Moderately expensive
- Linear output
- Moderately expensive
- Linear output Disadvantages • Nonlinear output
- Limited temp range
- Slow response time
- Expensive
- Low sensitivity
- Limited temp range
- Low sensitivity
- Large size
- Slowest response time
- Limited temp range
- Low sensitivity
- Large size Temperature Range: The approximate overall range of temperatures in which a sensor type can be used. Within a given temperature range, some sensors work better than others. Relative Cost: Relative cost as these sensors are compared to one another. For example, thermistors are inexpensive in relation to RTDs, partly because the material of choice for RTDs is platinum.
TCS651 datasheet, the resistance is 126700 Ω at 20ºC. bias current can be set between 2 μA and 39.5 μA. voltage, which is derived from the thermistor resistance. to temperature is called the Steinhart-Hart equation. Table 2. Thermistor Performance Range
- What are the upper and lower voltage limits of the sensor input of the temperature controller? The voltage limits of the sensor feedback to a temperature controller are speci fi ed by the manufacturer. The ideal is to select a thermistor and bias current combination that produces a voltage inside the range allowed by the temperature controller. Voltage is related to resistance by Ohm’s Law. This equation is used to determine what bias current is needed. Ohm’s Law states that the current through a conductor between two points is directly proportional to the potential difference across the two points and, for this bias current, is written as: V = I BIAS x R, where V is voltage, in Volts (V) IBIAS is the current, in Amperes or Amps (A) IBIAS means the current is fi xed R is resistance, in Ohms (Ω)
© 2013 • Sales & Technical Support: (406) 587-4910 • email: sales@teamWavelength.com • web: www.teamWavelength.com Page 6 WHAT IS THE STEINHART-HART EQUATION? The Steinhart-Hart equation is a model that was developed at a time when computers were not ubiquitous and most mathematical calculations were done using slide rules and other mathematical aids, such as transcendental function tables. The equation was developed as a simple method for modeling thermistor temperatures easily and more precisely. The Steinhart-Hart equation is: 1/T = A + B(lnR) + C(lnR) 2 + D(lnR)3 + E(lnR)4... Where: T is temperature, in Kelvins (K, Kelvin = Celsius + 273.15) R is resistance at T, in Ohms (Ω) A, B, C, D, and E are the Steinhart-Hart coeffi cients that vary depending on the type of thermistor used and the range of temperature being detected. ln is Natural Log, or Log to the Napierian base 2.71828 The terms can go on in fi nitely but, because the error is so small, the equation is truncated after the cubed term and the squared term is eliminated, so the standard Steinhart-Hart equation used is this: 1/T = A + B(lnR) + C(lnR) One of the pleasures of computer programs is that equations that would have taken days, if not weeks, to solve are done in moments. Type “Steinhart-Hart equation calculator” in any search engine and pages of links to online calculators are returned. HOW IS THE STEINHART-HART EQUATION USED? This equation calculates with greater precision the actual resistance of a thermistor as a function of temperature. The more narrow the temperature range, the more accurate the resistance calculation will be. Most thermistor manufacturers provide the A, B, and C coeffi cients for a typical temperature range. REV A
REVISION HISTORY
WHO ARE STEINHART AND HART? John S. Steinhart and Stanley R. Hart fi rst developed and published the Steinhart-Hart equation in a paper called “Calibration curves for thermistors” in 1968, when they were researchers at Carnegie Institution of Washington. Steinhart went on to become Professor of Geology and Geophysics, and Marine Studies at the University of Wisconsin-Madison and Stanley R. Hart became a Senior Scientist at Woods Hole Oceanographic Institution. CONCLUSION Thermistors are temperature-dependent resistors, changing resistance with changes in temperature. They are very sensitive and react to very small changes in temperature. They are best used when a specifi c temperature needs to be maintained, and when monitoring temperatures within 50ºC of ambient. Thermistors, as part of a temperature control system, are the best way to measure and control heating and cooling of a Peltier device. Their ability to adjust in minute increments allows the greatest overall system stability. Thermistors can be embedded in or surface-mounted on the device needing temperature monitoring. Depending on type, they can measure liquids, gases, or solids.