Temperature Measurement Circuits

Temperature measurement circuits convert heat into an electrical signal that can be read on a meter or used to switch a heater, fan or alarm. Several kinds of sensor are common. A thermistor is a resistor whose resistance changes strongly with temperature; the most familiar type has a negative temperature coefficient (NTC), meaning its resistance falls as it gets hotter. Other choices include platinum resistance detectors (RTDs), thermocouples and dedicated integrated-circuit sensors that give a voltage directly proportional to temperature.

Because a thermistor or RTD changes resistance rather than voltage, it is usually placed in a voltage divider or a bridge so the change appears as a measurable voltage. This voltage is then amplified and, if needed, linearised, since the thermistor’s resistance does not vary in a straight line with temperature. A comparator can turn the signal into an on/off output, forming a thermostat that maintains a set temperature with a small amount of hysteresis to avoid rapid switching around the set point.

Thermocouples generate a tiny voltage at the junction of two dissimilar metals and are useful over very wide temperature ranges, but they need a reference (cold-junction) compensation and high-gain amplification. Integrated sensors simplify design by providing a calibrated, linear output, sometimes already in digital form for direct connection to a microcontroller. Accuracy in all these circuits depends on calibration, self-heating of the sensor, and good thermal contact with the object being measured.

Frequently asked questions

What does NTC mean?

It stands for negative temperature coefficient. An NTC thermistor’s resistance decreases as its temperature rises, which is the most common behaviour used in temperature sensing.

How does a thermostat circuit work?

A temperature sensor in a divider feeds a comparator set to a chosen level. When the temperature crosses that level the comparator switches a relay controlling a heater or cooler, with hysteresis added to prevent rapid cycling.

Why are thermistor readings non-linear?

A thermistor’s resistance changes exponentially rather than linearly with temperature, so circuits often add linearising components or use a lookup table to convert the reading accurately.