Thermostat Demonstration

The simplest thermostat control scheme is a threshold temperature with a “hysteresis” window so that the switching isn’t too frequent.

A thermocouple is in an enclosure with a heater. The thermostat measures the enclosure with the thermocouple and then sends a signal to a switch that turns a heater on and off. That switch is connected to the energy source that will warm the enclosure.

Schematic

If the thermostat turned on and off at the same temperature, the system would switch very frequently. Instead, there is a window around the desired temperature and the system behaves as below for the switching behavior.

Hysteresis Graph

Thermocouple

The voltage across a thermocouple varies as the temperature varies. A common thermocouple, type K, varies 41 microvolts for each degree Centigrade. Using a sensitive amplifier we can increase this small voltage up to a few volts that can be read by a microcomputer.