The SCEL:TM08 circuit will provide modest accuracy and resolution temperature measurements at very low cost. The underlying premise of this design is that very many control applications require temperature measurement over a restricted temperature range. For example, if you are making a washing machine you may be quite happy to measure temperatures between 60°C and 100°C, with a nominal target temperature of 85°C.

Normally you would source your own NTC thermistors. Very likely you will need some special encapsulation or mounting. Naturally, we’ll attempt to suggest suitable thermistors, and we do know of some already packaged industrial type units.

The circuit used to interface the thermistor is quite simple (hence the low cost). We will scale this circuit for your specific needs during the definition phase of your design. Because thermistors are non-linear, you may need to linearise the raw readings to get °C or °F. This is quite a simple matter with SPLat, thanks to the built in floating point arithmetic. There is a tutorial in the SPLat Knowledge Base, and a supporting spreadsheet that generates the linearising polynomial.

This circuit, in various versions, has been used on several SPLat boards, including MMi99, SL99, SPice10200 and AJ18.

The following characteristic data is for two typical applications. When negotiating an implementation of this, or any other circuit element in your custom SPLat controller, you must identify the parameters that are most critical.

Application 1 was very critical around the optimum storage temperature. In such cases it may be necessary to calibrate the system close to the critical temperature.

Application 2 is not at all critical, so no calibration was performed. The basic accuracy then achieved is a function of the grade of thermistor and of circuit errors.

Resolution (the smallest detectable change in temperature) is a function of the analog to digital conversion resolution. For low cost applications we provide 8 bit conversion, and the above figures are based on that. By adjusting circuit parameters we can optimize the circuit to get best resolution around the critical temperature.

Thermistors provide the lowest cost temperature measurement, both because the thermistor devices are very cheap and because the circuitry needed to support them is very simple. Surprisingly, thermistors also provide very good accuracy relative to sensor price. Thermistors are available for around $5 that have <0.1°C inherent accuracy, and for less than $1 more ordinary thermistors still have inherent accuracies of 1°C or so. This compares very favourably with semiconductor sensors that often have accuracies of 2°C or worse.

“Ordinary” thermistors have working temperature ranges from -50°C to +180°C. One company makes units that go up to 700°C.