Thermistor

https://www.teamwavelength.com/thermistor-basics/

What is it?

• A thermal resistor
• Acts as a resistor, where its resistance depends on the temperature
• Made of metallic oxides pressed into a bead or disk and covered in some impermeable material like glass or epoxy
• 2 types of thermistors:
  • Negative Temperature Coefficient (NTC)
    • temp increases, resistance decreases
    • temp decreases, resistance increases
    • The most frequently used kind of thermistor
  • Positive Temperature Coefficient (PTC):
    • Temp increases, resistance increases
    • Temp decreases, resistance decreases
    • Usually used as a fuse
      • Fuse: electrical safety device that prevents overcurrent
        • It's a metal wire/strip that melts when too much current goes through it
    • If it melts, the circuit is interrupted and the current is interrupted

• Thermistor is high precision within a temperature range around 50C around the target temp. the range depends on base resistance
• The arrow shows it's based on temperature

How does it work?

• It doesn't actually read anything
  • It just changes the resistance depending on the temp
    • The resistance depends on the material of the thermistor
• The relationship between resistance and temp is NONLINEAR!!!
  • You can see that from the NTC/PTC graphs above
• The difference between a thermistor and other temperature sensors
  • Benefits/drawbacks of the sensors can be found on the link

Uses

• Good sensor for measuring temperature at a desired base point (very precise!)

  • Not good for extreme temperatures

• When inside a controlled system:

  • A temperature controller is the one monitoring the temperature of the thermistor
    • It takes the sensor info from the thermistor
  • The temperature controller will tell the system when to turn the heater/cooler on/off
  • Uh for more details check that link again
  • But a summary:
    • Thermistor resistance changes depending on temperature
    • The thermistor is connected to some device to measure its temp
    • The thermistor is also connected to a temperature controller so there's some current going through the thermistor (called bias current). The thermistor needs to send info to the controller through this bias
      • Temperature controller is connected to the cooling/heating system. If the temperature read from the thermistor is too high/low, it'll activate the cooling/heating system to change the temperature to the desired temp

• Can be found in:

  • Digital thermometers
  • Cars for measuring oil and coolant temps
  • Oven
  • Fridge

What does ESW use it for?

• We use it for the science mission when we boil water
• It lets us determine the temperature of the boiling water (kinda) so we know when to turn off the heaters once the water's boiled