Getting a handle on thermistor temperature measurement
Temperature is one of the most common parameters monitored by an embedded system. There is a wide array of temperature sensors available to achieve this. The sensor types can range from exotic black body detectors to the simpler more mundane resistance type sensors and everything in between. In this article, I will briefly discuss the negative temperature coefficient (NTC) thermistor — one of the most common temperature sensors used in a wide variety of embedded systems.
A thermistor is a resistance element, usually made from a polymer or semiconductor, where the resistance varies with respect to the temperature. This type of device should not be confused with a resistance temperature detector (RTD). Usually RTDs are much more precise, more expensive, and cover a wider temperature range.
There are two types of thermistors, characterized by how the resistance varies with temperature. If the resistance value lowers with increasing temperature, we call this device a Negative Temperature Coefficient (NTC) thermistor. If the resistance rises with increasing temperature, this device is known as a Positive Temperature Coefficient (PTC) thermistor. In general, PTC devices are used as protection devices and NTC devices are used as thermal sensors. A very common application of NTC thermistors is in sensing and controlling the PN junction of a wide-band laser diode.
Another characteristic of the thermistor is the cost. A typical thermistor in low volume is usually priced in the $.05 to $0.10 per piece range. The low cost and ease of interface usually make these devices highly attractive in embedded applications.
A typical thermistor sensing range is from -50 to +125 Celsius. Most thermistor-based applications are used in the -10 to 70 Celsius, or what we know as the typical commercial ambient range.
The typical accuracy of the thermistor is average to fair. Most thermistors are manufactured to yield a ± 5% resistance tolerance. The precision of a thermistor, however, is pretty decent. Typically, we can expect to see precision on the order of ±0.5 to ±1.0%.
The resistance transfer equation for the thermistor is known as the Steinhart-Hart equation. This non-linear equation is shown as Equation 1.
Equation 1. Steinhart-Hart Equation for a thermistor
Figure 1 shows the temperature versus resistance plot for a Panasonic ERTJZET472 NTC thermistor. This graph shows that, on a linear scale, the resistance vs. temperature relationship is very nonlinear!
Figure 1. Resistance vs. Temperature for a Panasonic NTC thermistor
Typically, the thermistors are rated by a parameter known as the R25 value. This is the typical resistance of the thermistor at 25 Celsius. The R25 value for this thermistor is 4700 ohms.
We could easily source the thermistor with a low value current source. We could then read the voltage using an on-chip analog-to-digital converter (ADC) and push the read result through some lookup table to get the temperature reading — or we could attempt to linearize the resistance / temperature characteristic.
On some memory constrained systems, we just may not have the luxury of setting up a lookup table. Therefore, in this application we shall attempt to linearize the thermistor reading.
To a first-order approximation, we can see that the thermistor resistance versus temperature characteristic is roughly inversely proportional. That is the resistance is roughly inversely proportional with respect to temperature in a narrow range. Given this, we can create an inverse proportion circuit to try to linearize the resistance vs. temperature curve. Figure 2 shows how this is done.
Figure 2. An NTC thermistor linearizing circuit
If we really wanted to save money, we could eliminate the voltage reference. Doing this will require some extra filtration to reject any power supply noise. It is important that the ADC reference and the thermistor sensor circuit reference are the same. Doing this allows us to use a ratiometric measurement method for the thermistor versus the ADC reading: That is, the measurement will be independent of the excitation voltage of the thermistor interface circuit.