Temperature Regulator for a Cold Chamber
An electronic control circuit holds a cold chamber at a selectable temperature and lights a warning light if the temperature exceeds a predetermined level above the control temperature. The circuit uses two standard operational amplifiers and two NTC thermistors (B57164K0331K).
The regulator and alarm are shown schematically in the diagram. If the temperature in the cold chamber is greater than TSet, the resistance of the control thermistor is less than that of the shunted TSet potentiometer plus 1000 ohms (R5). Therefore the voltage applied to the inverting input of the control op-amp (IC1) is low, and the output from the op-amp is high. The high voltage turns on the transistor, energizing the relay that turns on the refrigeration unit.
When the refrigeration unit reduces the temperature in the chamber, the resistance of the control thermistor increases, and the output voltage of the control op-amp goes low. This low voltage turns off the transistor, the relay, and the refrigeration unit.
To avoid cycling the refrigeration unit on and off too frequently, 3°C of hysteresis are included in the control circuit. The positive feedback introduced through the R14 and R15 resistors makes it necessary to decrease the thermistor temperature by 3°C below TSet before the control op-amp shuts off the transistor.
The warning portion of the controller works in much the same way: If the warning thermistor temperature is above TWarning, then the thermistor resistance is low; the voltage fed to the inverting input of the warning op-amp (IC2) is low; the op-amp output is high; and the transistor and the warning light are turned on. No hysteresis is built into the warning circuit.
The TSet and TWarning potentiometers are ganged so that for every control temperature selected (TSet), a corresponding warning temperature is selected, a predetermined amount above the control temperature. At TSet = 0°C, TWarning is +8°C; at TSet = -54°C, TWarning is -30°C.
Useful link: B57164K0331K datasheet (pdf)