What is a Thermistor?
Thermistors (-100°C to +150°C) are typically used for over temperature shutdown purposes. Although not as accurate as some other temperature sensor solutions, thermistors are inexpensive and come in small packages. They are also non-linear and require a temperature compensation look-up table. Thermistors are built with semiconductor materials and can have either a positive (PTC) or negative (NTC) temperature coefficient. NTC devices are typically used for temperature sensing.
Advantages of thermistors include a very high sensitivity to changes in temperature (having a thermal response of up to -100 Ω/°C at 25°C), fast response time and low cost. The main drawback of thermistors is that the change in resistance with temperature is highly non-linear at temperatures below 0°C and greater than 70°C. A conventional fixed gain thermistor amplifier circuit is shown below.
Thermistor Key Features:
- Two-wire measurement
- Variety of packages
- Battery chargers
- Power supplies
- Cold junction compensation
- Fan control
The thermistor application example below shows the thermistor's output is used as a temperature switch, while the TC621 is used as a programmable logic output temperature detector.
Thermistor Applications Circuits
Conventional Fixed Gain Thermistor Amplifier
A simple voltage divider is created with a reference resistor (R1) and the thermistor (RT). A constant voltage source is supplied (VREF) with the output of the voltage divider (VTH) directly correlating to temperature. The output is buffered by an MCP6001 low-power op amp.
The response is shown in the graph of temperature vs. output voltage below. It is fairly linear in the range of 0-70°C, but the accuracy of the circuit is limited without adding additional circuitry.
PGA Circuit Interfaced with a Thermistor
The advantage of the PGA circuit (below) is illustrated by comparing the VOUT slope plots of the conventional circuit with the PGA circuit. The VOUT slope for the PGA circuit has a minimum value of 30 mV for temperatures greater than 35°C, which means that only a 9-bit ADC is required. The programmable gain amplifier we're using in this application is the MCP6S21.
In contrast, a voltage divider with a gain of one will require an 11-bit, or higher, ADC to provide an equivalent temperature resolution. The resolution of a thermistor circuit is important in applications such as overtemperature shutdown circuits.