You are required to design an appropriate circuit for a thermometer containing a
ID: 2084566 • Letter: Y
Question
You are required to design an appropriate circuit for a thermometer containing a thermistor. The relationship between temperature T in degrees C and resistance in Ohms is given by the Steinhart-hart equation. The Steinhart-Hart equation is a model of the resistance of a semiconductor at different temperatures. The equation is: R_T = R_ref middot e^[A + B/T + C/T^2 + D/T^3] Where, T is the temperature in degrees K: Degrees K = 273 + Degrees C R is the resistance at T in Ohms: A, B, C, D are Steinhart-Hart coefficients: A = -9.094: B = 2251.74: C = 229098: D = -2.744820E + 07 The resistance of the thermistor at 25 degrees C is 330 Ohms. The Wheatstone bridge is balanced at 25 degrees C. What is the output voltage at 35 degrees C?Explanation / Answer
Thermistors are temperature sensors that have a resistance proportional to their temperature. This property makes them useful for functioning as safety devices in circuits or as probes that physicians can use during surgery.
What are Thermistors?
Thermistors are semiconductor devices that are used to measure temperature. The name comes from a combination of the words "resistor" and "thermal". Thermistors have an electrical resistance that is proportional to temperature. From a general physics course on electricity and magnetism, you may have learned that this is a property typical for all conductors. For example, devices such toasters, heaters, and light bulbs operate on this principle. Thermistors are different in that they are created to deliberately exploit this effect, and hence are more temperature sensitive than usual.
The Mathematical Model
The basic mathematical model used for thermistors is the Steinhart-Hart equation, discovered by oceanographers I.S. Steinhart and S.R. Hart. In its simplest form it is:
Construction
Thermistors are used in science and engineering applications. They are also useful in medicine as clinical temperature sensors or as probes during surgery. There are two types: PTC (Positive Temperature Coefficient of Resistance) and NTC (Negative Temperature Coefficient of Resistance).
NTC thermistors have temperatures that vary inversely with resistance such that as the temperature increases, the resistance decreases, and vice versa. They are very often used for temperature control and indication, and for current suppression. Common materials used in their construction include oxides of materials such as nickel, manganese, copper, iron, and cobalt. Some are also made from silicon and/or germanium. They are usually packaged in an epoxy, and are the most common type of thermistor.
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