Temperature control circuit

The temperature control circuit consists of three parts: temperature control, temperature measurement and power failure alarm, as shown in the figure.

The temperature control part includes a voltage comparator composed of an operational amplifier F007 and a power driver composed of a transistor VT1 with a relay K. Potentiometer RP1 is used to set the temperature. After turning on the power, adjust the potentiometer RP1 to an appropriate size according to the preset temperature. When the temperature drops, the resistance of the negative temperature coefficient thermistor Rt increases. At this time, the voltage at point B is higher than the voltage at point C, that is, Ubc>0, and the output terminal 6 of F007 is a positive voltage. Through the voltage regulator tube VD3, the transistor VT1 is turned on, driving the relay K to attract, and the heating begins. As the temperature gradually rises, the resistance of the thermistor Rt gradually decreases, that is, the potential at point B gradually decreases. When Ubc

The temperature measurement part consists of an unbalanced bridge and a microampere meter. The thermistor is connected to the bridge through leads. As the temperature of the object being measured changes, the potential difference between points AB changes, so the current flowing through the meter also changes. As long as the meter indication is scaled according to the temperature, the temperature of the object being measured can be read.

The power failure alarm part includes an electronic switch composed of a thyristor vs and an audio alarm composed of transistors VT2, VT3, etc. When there is electricity, the voltage between the control electrode and the cathode of the thyristor vs is composed of two parts: one part is the 10V regulated power supply connected to the cathode of the thyristor vs from the voltage regulator tube VD4, and the other part is the 4.5V dry battery power supply connected to the control electrode of the thyristor vs through the resistor R9. For the thyristor, if the control electrode voltage is greater than the cathode voltage, the thyristor is turned on, otherwise it is turned off. When there is electricity, the voltage Ug between the control electrode and the cathode of the thyristor vs is less than 0v, the thyristor is turned off, and the alarm circuit is disconnected. When the city power is off, the voltage between the control electrode and the cathode of the thyristor vs is only powered by a 4.5V battery, Ucg is greater than the trigger voltage of the thyristor, the thyristor is turned on, and the speaker sends an alarm signal.