3.3 Theory of Operation
To implement this system, the first element to consider is the Peltier plate. Peltier plates pump heat from one side to the other in response to current flowing through the plate. This generates a thermal gradient as the top and bottom plates are no longer at the same temperature. As the temperature gradient widens, the amount of heat pumped by the Peltier drops. Eventually, the differential temperature will reach steady state, where the system is held in equilibrium. Figure 3-2 shows the amount of heat pumped from the used Peltier element (P/N: CP85435) as the temperature differential changes.
If the only objective of this system was to cool as much as possible, then only a power supply would be needed. However, since the objective is to regulate the temperature, it should be assumed the plate is not in equilibrium. Instead, the MCU must sense the temperature of the cold plate and use this value to determine when to supply power to the Peltier element. Temperature control is implemented in the firmware with Bang-Bang control. To prevent rapid control oscillation, programmable hysteresis was implemented to keep the temperature within programmable thresholds. An example is discussed below and shown in Figure 3-3.
Assume the cold plate is not at user-set temperature at t = 0. The system will begin to run current through the Peltier element. As the plate cools, the temperature falls to a value TSET − TOVER where the plate becomes overcooled. When this condition is reached, power to the Peltier element is disconnected. The cooled surface then warms up naturally until it reaches the value TSET + TUNDER , at which point power to the Peltier is restarted. This on-and-off cycling allows the MCU to regulate the temperature of the plate.
