Compact, submersible water pumps are mostly used on air coolers, aquariums, and fountains. If the pump runs out of water and continues to operate — an issue known as dry running — it can become damaged. This circuit protects submersible water pumps from dry running with the help of associated level electrodes. The circuit detects the absence of water and monitors the water level to prevent dry running from occurring.
The water pump guard electronics consist of two level electrodes, a water level detector, an electromagnetic relay, and the relay driver circuitry. Supporting components are necessary to prevent restarting if the pump guard is used in turbulent water. The recommended supply voltage is 5 V. While it is possible to run the unit off of a higher voltage, minor modification is required. The finished electronics can be housed into a minuscule case (the level electrodes, formed from two short-length rigid copper wires, pass out through the case). Don’t forget to waterproof the case (and joints) using any suited epoxy adhesive.
As you can see, the electronics use a readily available and very low-cost 555 IC as the relay driver. Although the 555 IC is normally used as a timer/oscillator, it is also very well suited for relay driving applications. The output (pin 3) can both source and sink current up to 200 mA and the internal flip-flop is triggered between its two states by internal comparators connected to the two sensing inputs on pins 2 and 6. When these pins are taken to a voltage above 2/3 of the supply voltage, the output switches low (0 V); when they are taken below 1/3 of the supply voltage, the output swings high. As the 555 IC can happily work at 5 V, it is good for driving a small 5-V relay coil from a 5-V dc supply voltage.
Dry running is highly dangerous for submersible pumps. Motors of submersible pumps are designed for running under water. They also use water as a heat-transfer medium. If the water level goes down and the pump runs dry, the motor gets overheated and burns out. For protection against dry running, a sensing prod (level electrodes) is usually used. It is immersed in water, slightly above the level of the pump. When the water level drops lower than the sensing prod, it is expected to trip the pump guard. Because this approach has some limitations, the undercurrent detection technique is often used as an alternative. It is possible to offer protection against dry running by sensing the motor current, which changes during dry running conditions. If suitable devices are used to sense the motor current, then it is possible to provide reliable protection against dry running.
The Undercurrent Detection Technique
An electric submersible pump (ESP) is a device which has a hermetically sealed motor close-coupled to the pump body. The whole assembly is submerged in the water to be pumped. Submersible pumps push water to the surface, as opposed to jet pumps, which have to pull water. Note that we can also detect the dry run condition of a submersible pump without using the level electrodes just by measuring the motor current. Since the motor draws little current (undercurrent) in dry running situations, it is not very difficult to rig a circuit to switch the motor off when a preset undercurrent value is reached. The great advantage of this trick is that the design uses the motor itself as a sensor and does not need any additional external sensors like the level electrodes!
With a little tinkering, you can use any ready-made ac/dc current sensor module to follow this “undercurrent” concept. Fortunately, reasonably priced simple current sensor modules and full-fledged current sensors are now available from various vendors. The first one (left) in the next figure is a full-fledged 50-A ac current sensor module with an output relay. The second one (right) is a breakout board for the fully integrated Hall Effect-based linear ACS758 current sensor. The ACS758 outputs an analog voltage (40 mV/A) that varies linearly with sensed current. These types of dedicated current sensors enable you to monitor load current in your water pump guard system for device protection purposes.
However, if you are looking for a cheap ac current measurement for this application, here is a well-documented project based on the MCP6282 dual-rail-to-rail op amp. The Microchip Technology Inc. MCP6282 operational amplifier (op amp) provides wide bandwidth for the current. This chip has a 5-MHz Gain Bandwidth Product (GBWP) and a 65° phase margin. It also operates from a single supply voltage as low as 2.2 V while drawing 450 μA (typ.) quiescent current. Additionally, the MCP6282 supports rail-to-rail input and output swing, with a common mode input voltage range of VDD + 300 mV to VSS – 300 mV.