Thermostat controller circuit using four comparators

Using a four-comparator thermostat using a negative temperature coefficient (NTC) thermistor, a circuit such as Figure 1a can control temperature to 1°C or better accuracy with minimal components, cost, and complexity. The circuit contains protection to prevent the temperature sensor from shorting or opening, and all components are commonly used. The controller is of the PWM type, but it has exponential transfer characteristics rather than linear. This design is based on an LM339 (quad comparator) and includes temperature compensation. Due to the temperature drift of the comparator, Vos changes and causes the oscillator output to change. However, the same change occurs in the comparator that generates the duty cycle, and the two cancel out thereby eliminating the temperature drift of the controller. The core of the controller is an oscillator consisting of IC1a, IC1b and related components. The voltage peak value and minimum voltage value of the oscillator output are the main factors that determine the accuracy of the controller. There are some formulas for this oscillator: PERIOD=[R5×R6/(R5+R6)+R4]×C1×Ln[(Vas-Vmin)/(Vas-Vmax)] seconds DutyCycle=Ln[(Vas-Vtemp )/(Vas-Vmax)] / Ln[(Vas-Vmax)/(Vas-Vmin)] Vmax=Vcc×R3/(R1+R3) Vmin=Vcc×R2×R3/[R2×R3+R1×( R2+R3)] Vas=Vcc×R6/(R5+R6) Vtemp=Vcc×(R7+R8)/(Rtherm+R7+R8) The output of the oscillator is directly connected to the input of the comparator IC1c that generates the duty cycle. . R8 determines the temperature set point. The voltage divider from R8 to Rtherm provides the comparison voltage for the comparator that generates the duty cycle. The output of the comparison drives an optically isolated triac driver. The temperature series of component parameter values ​​shown in Figure 1 is 25 to 115°C. D1 and D2 are used for temperature sensor error and duty cycle indication. R9 and R10 set the level of the inverting terminal of IC1d to detect the open circuit of the temperature sensor.