Constant temperature measurement circuit

(1) The circuit composition control accuracy is ±1°C. The composition of the circuit is shown in the figure. The circuit voltage is supplied by the mains, which
is provided after full-wave rectification and filtering, and is about 9V. In the circuit, the general
operational amplifier F007 (or CF141) is used
as the amplification comparator, the 555 time base circuit is used as the
trigger, the triac VS is used as
the electronic switch of the heater, the thermistor RT is used as the sensor to
control the temperature, and the potential is used The controller RP is adjusted to control the constant
temperature point. LED 1 also serves as a voltage regulator tube, providing
a reference voltage of about 1.8V. LED2 serves as
an indicator light for heating.
  ． (2) Working principle RT has a large resistance at low temperatures and a small resistance at high temperatures. When the ambient temperature is lower than the set temperature,
the voltage of pin ② of the general operational amplifier IJLA741 is higher than the voltage of pin ③, and pin ⑥ outputs low level (required to be less than 3\0.). Therefore, pin ② of the trigger terminal of the 555 time base circuit is low level, Its
output terminal ③ pin outputs a high level, causing LED2 to illuminate and indicate, and at the same time triggering the bidirectional transistor VS to conduct, energizing the heater and heating up. When the
ambient temperature is higher than the set temperature value, the general operational amplifier F007 ② The voltage of pin ③ is lower than the voltage of pin ③, and the output of pin ⑥ is high level (required to be greater than 6V), so the output of pin ③
of the 555 time base circuit output flips to low level , LED 2 goes out, the bidirectional thyristor vs is cut off, and the heater is powered off , stop heating.