Dual limit temperature automatic controller

In some occasions, a dual-limit automatic temperature control device is often needed. The author has made a dual-limit automatic temperature controller for atmospheric pressure boilers, which has been running for more than two years and has been stable and normal. Here I introduce it to you for reference.

Nowadays, many families or small units use small atmospheric pressure boilers for heating. The hot water circulation of this boiler is forced by the water pump. When the water temperature in the boiler reaches about 85℃, the water pump is manually started to circulate the hot water for heat exchange; as the circulating water temperature slowly drops, when it drops to about 50℃, the water pump circulation is manually stopped, and the boiler continues to burn, and the circulation is resumed when the water temperature rises to 85℃ again. The boiler burner has to check the boiler temperature at all times and then decide whether to start and stop the water pump. In this way, the water in the boiler cannot boil, otherwise there will be a certain pressure in the entire heating system, which will damage some pads of the radiator and cause water leakage. Due to negligence, the water in the boiler is often boiled. In response to this situation, the author designed and produced a control device to solve this problem.

The control current principle diagram is shown in Figure 1. The lower limit of the temperature is set by Rp1 (such as 50°C), and the upper limit of the temperature is set by Rp2 (such as 85°C). When the boiler temperature reaches 85°C, the circulating water pump automatically turns on the power supply to start circulating the water. As the water temperature drops to 50°C during the circulation, the circuit automatically stops the power supply to the circulating water pump and stops the water circulation. Then repeat, and the purpose of automatically controlling the water circulation by using two limit values ​​is achieved.

IC1 is a quad op amp LM324; IC2 is CC4011, which is a quad 2-input NAND gate. IC2-C and IC2-D form a basic RS trigger, which is valid at low level. That is, when the output of IC2-B is low, point F is high, VT is turned on, the relay works, and the power supply of the circulating water pump is turned on through the AC contactor KM, and the water starts to circulate; when the output of IC2-A is low, point F is low, VT is cut off, and the circulation stops. Both IC2-A and IC2-B work in the nonlinear region and compare the voltage. When U+>U-, the op amp output (point D or F) is high, otherwise it is low. Rt uses a glass-sealed NTC negative temperature coefficient thermistor (5K at room temperature). The potential of point C in the figure changes with Rt, that is, the temperature of the boiler. The higher the boiler temperature, the smaller the Rt value, and the larger the UC. It has been measured through actual experiments that when the boiler temperature changes from 45°C to 90°C, Uc changes from 1.58V to 3.01V. The potential of point A, that is, the lower limit, is set by the voltage divider of R2, R3 and RP1. Through Rp1, the potential of UA can be set within the range of 1.74-2.29V. The potential of point B, that is, the upper limit UB can be set between 2.53V-3.00V. As the temperature changes, when UC>UB, point E is high, and point F is also high. Then, due to the action of the RS trigger (hold), only UC