A small improvement on the double-position automatic control circuit of water pump

Since the pressure of tap water pipes is low, it often cannot meet the water supply needs of high-rise buildings. There are currently two ways to solve this problem:

One is differential pressure two-position control, and the other is constant pressure variable frequency control. Differential pressure control has the advantages of simple control circuit and low cost. This technology is widely used in occasions with low water supply requirements and automatic water replenishment of water heating boilers.
1. Traditional technology and defects
The traditional two-position differential pressure control circuit is shown in the figure below.

This control method often causes the following faults: first, the electric contact PK of the electric contact pressure gauge is frequently burned out; second, the main contact of the contactor is burned out. The electric contact pressure gauge is a relatively expensive instrument, so it will cause certain economic losses.
Since the needle of the electric contact pressure gauge swings very slowly, the moving contact and the static contact driven by it are slow to make and break, and the contact pressure between the contacts of the electric contact pressure gauge is very small, the breaking capacity is poor, and it is not suitable for directly using an AC working voltage of more than 50V. This is the main reason for burning out the electric contact of the electric contact pressure gauge. Specifically, when the set maximum pressure is reached, sparks flash between the moving contact 1 and the upper limit static contact 3, causing the main contact of the AC contactor to burn out, resulting in the inability to shut down. The burning of the moving contact will also lead to poor contact with the lower limit static contact 2, causing the contactor to shake and fail to close normally.

2. Improved motor control circuit
The above figure is a specific solution for solving the above problems.
In the circuit shown in the above figure, CK is a double-position control relay modified by using the shell and part of the internal circuit of the electronic time relay on the market. The internal circuit of the device adopts CMOS integrated gate, and its control signal input terminals A, B, C, and D are well isolated from the mains, and also have the characteristics of low control voltage and small input current.
The device directly drives the AC contactor, eliminating the intermediate relay. The circuit shown in the middle figure can save the expensive electric contact pressure gauge, thereby reducing the cost of the control part and improving the reliability of the system. The circuit can use two control signal devices, one is the electric contact pressure gauge control, as long as the electric contact of the electric contact pressure gauge is connected between the three input terminals A, C, and D, and the two points A and B are short-circuited; the other is the immersion electrode control that directly detects the water level, which is connected to the A, B, and D terminals, and the C terminal is open. The circuits shown in the upper, middle, and lower figures have a time-limited protection function to prevent non-stop failures. In the circuits shown in the middle and lower figures, the step-down rectifier and filter parts are not drawn. G1, C2, and G3 use the CMOS integrated chip CC4001, which is powered by the three-terminal integrated regulator 78L09.

3. The control circuit of the electric contact pressure gauge
is shown in the middle figure: Since the voltage applied to the electric contact of the electric contact pressure gauge is a relatively low DC voltage, and its driving current is very small, the RS trigger in the circuit also has a debounce action, which can effectively protect the electric contact and AC contactor of the electric contact pressure gauge.
The working principle of the circuit is as follows:
Due to the short circuit between points A and B, the input of the OR gate G1 is high level, the output is low level, and the diode Vt is cut off. If the moving contact 1 of the electric contact pressure gauge PK does not contact the static contact at the moment of power on, the charging of the capacitor C5 makes the two input terminals of the OR gate G2 both low level, so C2 outputs a high level, so G3 outputs a low level, V5 is cut off, and the motor is in standby state. This makes the device have a pressure loss protection function.
Assume that when the power is turned on, the moving contact l of PK contacts the lower limit static contact 2, and a high level is applied to the input of the OR gate G:. Since C2 and G3 form an OR-type RS trigger, G: outputs a high level to saturate the transistor V2, and the relay J drives the AC contactor to close, and the motor starts to run. After the motor starts, as the water level rises, the moving contact of the electric contact pressure gauge separates from the lower limit static contact 2. Due to the memory effect of the RS trigger, G3 will maintain the output high level and the motor continues to run.
When the pressure of the water supply system reaches a certain level, the moving contact 1 of the electric contact pressure gauge contacts the upper limit static contact 3, G3 inputs a high level and outputs a low level, the transistor V2 is cut off, J is released, and the motor stops running.
In the event of poor contact between the 1 and 3 contacts of PK, the motor operation timeout protection can play a role. The working principle of the overtime protection is as follows: At the moment of power on, since C4 has not had time to charge, G4 inputs a high level and outputs a low level. As the capacitor C4 charges, the potential of the G4 input terminal gradually decreases. When it drops below the threshold level, C4 outputs a high level, which is added to the input terminal of the G3 gate through the diode V3. The C3 gate outputs a low level, and the motor stops running. After G3 outputs a low level, C2 outputs a high level, and the capacitor C4 discharges through R4 and V2 to prepare for the next circuit action. The time-limited protection can be set by adjusting the potentiometer W. Since the CMOS integrated circuit has a very high input impedance, the time-limited protection time can reach more than 1.5 hours. The user can set it according to the characteristics of the water supply device, which should be slightly longer than the normal motor running time.
Undervoltage protection function: During normal operation, the base potential of the transistor V6 is higher than its emitter, so V.
is cut off. Undervoltage occurs, and the base potential of V6 decreases. When the base potential of V6 drops below its emitter potential, V6 is turned on, making the transistor V7 saturated, forcing the input of C4 to input a low level and output a high level, so G3 outputs a low level, the transistor V5 is cut off, J1 is released, the motor stops running, and undervoltage protection is achieved. The multi-stage amplification effect of V5, V7, G4, G3, and G2 in the circuit can avoid the jitter of the contacts of relay J in the critical conduction state. The purpose of adding undervoltage protection is to prevent the contacts of the contactor from being damaged due to sparking caused by jitter when undervoltage occurs; it also avoids the failure of the contactor's core to attract undervoltage conditions, resulting in overcurrent damage to the attraction coil.

4. Water level direct control scheme
Water level direct control uses the immersed electrode to directly detect the water level for control.
The circuit is shown in the figure below: When the immersed electrode is used for direct control, the C terminal is open. The electrodes connected to the B and D terminals are respectively connected to two electrodes fixed at different heights, with the D terminal electrode on the top and the B terminal electrode on the bottom. The metal rod electrode connected to the A terminal is directly inserted into the liquid, and its position is between the electrodes connected to the B and D terminals. When the liquid level is lower than the position of the electrode connected to the B terminal (lower limit water level), G. inputs a low level and outputs a high level, so C2 outputs a low level, forcing G3 to output a high level, and the motor starts to run.
As the liquid level rises, when the electrode connected to the B terminal is immersed in water, G. inputs a high level and outputs a low level. Due to the memory effect of the RS trigger composed of C2 and G3, G3 still maintains a high level and the motor continues to run. When the liquid level rises to the position of the electrode connected to the D terminal (upper limit water level), G3 inputs a high level and outputs a low level, and the motor stops running.
After the motor stops running, the liquid level begins to drop, the electrode connected to the D end leaves the liquid surface, G2 and G3 both input low level, and when the liquid level drops to above the liquid level corresponding to the electrode connected to the B end, C3 continues to maintain the low level output state. When the liquid level drops below the position of the electrode connected to the B end, the input terminal potential of G1 changes from high level to low level, so Gi outputs high level, forcing G2 to output low level, so G3 outputs high level, and the motor starts again.
The use of immersed electrode control also has timeout protection and undervoltage protection functions.
5. Conclusion
Although the improved double-position automatic control circuit adds a double-position automatic control relay, compared with the control circuit before the improvement, the intermediate relay is omitted, so there is almost no increase in investment. By comparing the previous Figure 1 with Figure 2, it can be seen that after adopting this technology, the double-position control circuit can be simplified and wire materials can be saved. When direct water level control is adopted, the cost of the control circuit can be greatly reduced because the electric contact pressure gauge is replaced by the immersed electrode. Because the device has good isolation and undervoltage protection, its safety and reliability are greatly improved compared with the traditional two-position control method.
The device can also be used for two-position temperature control and two-position control in other situations.