**Thermistor Calculations** |[home](index.md.html)|[syllabus](syllabus.md.html)|[assignments](assignments.md.html)|[labs](labs.md.html)|[final project](finalproject.md.html)|[flight data](FlightData.md.html)|[getting certified](RocketryCertification.md.html)| # Thermistor The thermistor used on the rockets in E178 is the [TCS10K5][Tk] from [Wavelength Electronics](https://https://www.teamwavelength.com/product/tcs10k5-10-k-thermistor-0-5x3mmsold-in-5-packs/) It is used because of its extremely fast time response. Unfortunately, given its small size and low resistance (10 kΩ instead of 100 kΩ), it would be subject to self heating at normal voltage levels. For that reason, the measurement circuit is a voltage divider with a 0.5 V precision supply voltage. The voltage divider has a 10 kΩ precision resistor on the bottom and the thermistor on the top, so that the voltage goes up as the temperature does, which makes interpreting the raw voltage reading easier. The voltage-divider circuit is amplified by a non-inverting amplifier with a gain of 6.60 ±0.1%, which is fed to the AIM XTRA channels C and D. ## Steinhart-Hart coefficients. The Steinhart-Hart coefficients are given for two different current ranges. Ours is closest to the 100 µA range (you should be able to calculate the actual current at a given temperature), where the coefficients are reported as: Coefficient| Value -----------|---- A | 1.1279E-03 B | 2.3429E-04 C | 8.7298E-08 ## Self Heating When sitting on the launch pad, in fairly still air, it is possible that the current through the thermistor may be causing self heating. However, during flight, the air velocity is high enough that self heating is unlikely to be a problem. The reported Thermal Resistance or Dissipation Constant (in nearly stagnant air) is 0.2 mW/°C, so one would calculate the temperature rise over ambient from self heating by calculating the power dissipation in the thermistor,either $Vi$, $V^2/R$, or $i^2R$, and multiplying by the Dissipation Constant. # Measurement Circuit As mentioned above, the measurement circuit is a voltage divider with a 0.5 V precision supply voltage. The voltage divider has a 10 kΩ precision resistor on the bottom and the thermistor on the top, so that the voltage goes up as the temperature does, which makes interpreting the raw voltage reading easier. The voltage-divider circuit is amplified by a non-inverting amplifier with a gain of 6.60 ±0.1%, which is fed to the AIM XTRA channels C and D. The schematic for the measurement circuit is shown below. ![Figure [circuit]: The Thermistor Conditioning Circuit Schematic](PDF/ThermistorConditionerSchematic.pdf) The main voltage regulator, an MCP1702, can be powered by any voltage between 3.7 V and 13.2 V. It supplies 3.3 V to the dual op amp and the precision 0.5 voltage source. The PC board layout for the measurement circuit is shown below. ![Figure [board]: The Thermistor Conditioning Circuit Board Layout](PDF/ThermistorBoardLayout.pdf) The thermistors are intended to be connected between the +0.5 and in1, and the +0.5 and the in2 inputs. The two outputs are Out1 and Out2. As mentioned above Vin needs to be between 3.7 V and 13.2 V. [Tk]: https://www.teamwavelength.com/download/Datasheets/tcs10k5manual.pdf