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Proportional thermoregulator synchronizes to line
Jordan D Dimitrov, N Poushkarov Institute of Soil Science and Agroecology, Sofia, Bulgaria -- EDN, 4/13/2000
Whenever the mains voltage crosses zero, a 1-msec positive pulse appears at the collector of Q1. From the resultant series of pulses, dual binary counter IC1, DAC IC2, and R4 generate a sawtooth voltage with a 150-mV amplitude and a 2.56-sec period. Through R4, the circuit subtracts this voltage from the constant setpoint voltage, VS, that R1, R2, R3, and follower IC3A create. Simultaneously, sensing transistor Q2 cooperates with IC3C and associated resistors to create a voltage proportional to the object's current temperature with a sensitivity of 25 mV/°C.
If the difference between the desired and the real temperature of the object exceeds 6°C when the regulator is turned on for the first time, the output of the comparator IC3D is continuously positive. The 4011 gate, IC4A, passes all the triggering pulses from Q1 through the TIL112 optocoupler to the triac's control electrode, and the heater delivers it full power. When the difference in temperatures becomes less than 6°C, the circuit starts working proportionally. In this condition, IC4A passes only part of the triggering pulses generated within one period of the sawtooth voltage to the triac. The higher the object's temperature, the less time that the gate and the lower the average power delivered by the heater. When the real temperature reaches the setpoint temperature, the circuit always keeps the gate closed, and the heater is constantly off.
Thus, during the final stage of regulation, the temperature rises smoothly toward the desired value and is free from the overshoots and oscillations around the setpoint that on/off regulators exhibit. The resolution in the region of proportional regulation is as low as 0.4% of the nominal heater power. If necessary, you can change the width of this zone by simply changing R4. (DI #2510)
