Temperature automatic control circuit

The automatic temperature control circuit is shown in Figure 1-7, which is used for constant temperature control of the thermostat.
(1) Control purposes and methods
    Control object: electric heater EH.
    Control purpose: to keep the temperature in the thermostat constant.
    Control method: Use the thermistor R with negative temperature coefficient. As a temperature detection component, it is controlled by a 555 time base integrated circuit.
    Protection component: fuse FU (heater overcurrent protection).
(2) Circuit composition
    ① main circuit. It consists of switch QS, fuse FU, electric heater EH and bidirectional thyristor V (also used as a control element)
.
    ②Control circuit. It consists of 555 time base integrated circuit A, potentiometer RP, and thermistor R. , a monostable trigger circuit composed of RC components
and a bidirectional thyristor V.
    ③DC power supply. It consists of capacitors Cl, Cz, diode VD and voltage regulator tube vs.
    ④Indicator light. Neon bulb Ne (and current limiting resistor Rs) - electric heater EH heating indication.
(3) Working principle
    ① Preliminary analysis. When the temperature in the thermostat drops below the set value, the heated bidirectional thyristor V should be turned on -
pin 3 of the 555 time base integrated circuit A outputs high level ; when the temperature in the thermostat rises to the set value , heating should be stopped - v turns
off pin 3 of A to output low level.
    ②Follow the analysis. When the power is turned on, the 220V AC power is stepped down by the capacitor Cl, half-wave rectified by the diode VD,
filtered by the capacitor C2 and stabilized by the voltage regulator tube VS, and then provides 12V DC voltage to the 555 time base integrated circuit A. At the same time, the delay circuit composed of 555 time base integrated
circuit A starts timing. If the temperature in the thermostat has dropped below the set value, the resistance of the negative temperature coefficient
thermistor Rt is larger, and the potential of pin 2 of A is passed through RP and R. and R4 divide the voltage, lower than l/3Ee (4V) (Ec is the DC
power supply voltage 1ZV), pin 3 of A outputs high level (about 11V), the bidirectional thyristor V is triggered and turned on, and the electric heater EH is
heated electrically. The neon bulb Ne lights up, indicating that it is heating. Since the monostable circuit enters the transient state at this time, the internal discharge
tube of A is cut off, and its discharge terminal pin 7 is left floating. The power supply charges the capacitor C3 through the resistor Rz. The potential of the threshold terminal pin 6 continues to
increase , and the elapsed time t≈l . 1R2 C3, pin 6 level can rise to 2/3Ec (8V). If the temperature in the thermostat is still
low at this time, that is, the level of pin 2 of the triggering end is still lower than l/3EC, the circuit will remain set and the electric heater EH will continue to be energized
for heating ; if the temperature has risen to the set value, Because of the negative temperature coefficient thermistor R. The resistance decreases as the temperature increases. At this time,
the level of pin 2 is higher than 1/3 Ee, the 555 time base integrated circuit is reset, the monostable trigger flips into a stable state, and pin 3 outputs
a low level (about ov ), the bidirectional thyristor V is turned off, the electric heater EH stops heating, and the fluorine bubble Ne goes out. At this time. At 555,
the internal discharge tube of the base integrated circuit is turned on, and pin 7 is short-circuited to ground, so the charge stored in capacitor C3 is discharged through pin 7 to
prepare for the next heating delay. When the temperature in the thermostat slowly drops over time and drops below the set temperature, the Rt resistance
becomes larger again. The 2-pin level fork of A is reduced to 1/3E. Next, the 555 time base integrated circuit is set again, and the circuit flips into a transient
state. Its pin 3 outputs a high level , the bidirectional thyristor V is turned on, and the electric heater EH starts to heat again. This cycle is repeated to maintain the temperature
in the thermostat . constant.     In the figure, the function of the resistor Ri is to provide a discharge circuit for the capacitor C, so that when the power is cut off, there is no voltage on Cl to prevent the human body from touching the capacitor and receiving an electric shock.